Drift-free ferroelectric photodetection with fast temporal response via thermal diffusion engineering

Domain: physics
Published: 2026-02-27
Editorial outcome: minor_revision

Abstract
Self-powered ferroelectric photodetectors offer an attractive platform for low-power optoelectronics, exploiting intrinsic bulk photovoltaic effects to achieve bias-free operation. However, their practical deployment has been hindered by slow temporal response and photocurrent drift, mainly due to inefficient thermal dissipation that leads to temperature buildup within the device under continuous illumination. Here, we report a thermal-diffusion-engineering approach that reconfigures the thermal environment of ferroelectric device to suppress lateral heat dissipation and enable efficient vertical heat extraction. Compared to conventional architecture, the engineered drift-free device exhibits a photoresponse speed improvement of over three orders of magnitude, along with complete drift suppression, enabling high-fidelity imaging with minimal crosstalk. Infrared thermography and COMSOL simulations confirm distinct thermal environment in conventional and drift-free devices, revealing strong heat accumulation in conventional and efficient heat extraction in drift-free architectures. This work highlights thermal diffusion engineering as a key device-design parameter for enhancing ferroelectric optoelectronic performance, paving the way for scalable, bias-free energy-harvesting photodetectors and neuromorphic imaging systems.

Introduction
The rapid advancement of optoelectronics has driven interest in next-generation photodetectors featuring high sensitivity, broad spectral response, and long-term stability. Ferroelectric (FE) thin films have emerged as particularly attractive candidates because they can (i) generate a photovoltaic (PV) response without complex p-n junctions, (ii) produce a photovoltage that exceeds the bandgap, and (iii) enable a tunable output through mechanical, electrical, or magnetic control. This inherent coupling allows for tailored properties that enhance photovoltaic applications.

Among the range of potential FE materials, BiFeO3 (BFO) is specially compelling owing to its narrow bandgap (~2.7 eV), visible-light absorption, strong remnant polarization, intrinsic multiferroicity, and stable optoelectronic performance. These features collectively enable efficient carrier separation when illuminated due to its internal depolarization fields, thus supporting self-powered operation across the visible to near-ultraviolet spectrum The intrinsic room-temperature ferroelectricity of BFO renders it highly suitable for practical optoelectronic applications, including wearable imaging, optical communication, and neuromorphic computing.

Despite these advantages, the practical deployment of FE-based photodetectors has been hindered by persistent limitations, most notably a slow temporal response and photocurrent drift—defined as a gradual, unintended time-dependent variation in photocurrent when subject to continuous illumination. These limitations originate from illumination-induced heating and insufficient heat dissipation within the device. When the thermal environment is not well controlled, accumulated heat broadens the temporal response, reduces carrier extraction efficiency and destabilizes the steady state output. These thermally driven effects have been widely observed in oxide-based photodetectors, emphasizing that local heat distribution, is a critical but often overlooked determinant of device stability and dynamic performance.

Conventional FE photodetectors is commonly fabricated on low-thermal-conductivity substrates, such as mica or glass, with mica offering an atomically flat, chemically inert, and electrically insulating surface that is well suited for high-quality growth of perovskite oxide films. It also withstands high-temperature processing and provides mechanical flexibility, which has made it a standard platform for many BFO and related FE devices. However, its intrinsically low thermal conductivity leads to significant lateral heat accumulation during illumination, contributing to slow response and drift, and motivating our thermal-diffusion engineering strategy using high-k metallic backers. This phenomenon is conceptually related to thermal modulation transfer function (MTF) in pyroelectric imaging system, where lateral heat diffusion reduces the modulation depth and contrast of spatial signals. To improve the PV effect and overcome the associated challenges, recent strategies have focused on structural and electronic modifications. However, microscale thermal diffusion engineering remains an unexplored, but potentially transformative, approach for improving the dynamic performance of FE photodetectors.

Here we present a thermal diffusion engineering strategy that enables drift-free operation and fast temporal response in ferroelectric photodetectors by directing vertical heat extraction through a high conductivity copper substrate. Using BFO-based films as a model system, we demonstrate that thermal confinement suppresses lateral heat diffusion, thereby maintaining stable thermal environment under continuous illumination of light. Our optimized architecture demonstrates >3 orders of magnitude faster response speed and near-complete elimination of photocurrent drift compared to conventional devices. Infrared thermal imaging and COMSOL Multiphysics simulations confirm that the thermally conductive copper substrate is able to suppress lateral heat spreading and enhancing carrier mobility. Beyond fundamental performance gains, this thermally stabilized architecture also enables high-fidelity imaging with minimal crosstalk, positioning drift-free-FE devices as promising candidates for neuromorphic vision, wearable sensing, and next-generation optoelectronic systems requiring stable, precision operation.

Results
Design of drift-free ferroelectric photodetector
A detailed comparison between conventional and drift-free photodetectors highlights how the thermal transport dynamics directly influence device performance and reveal distinct operational regimes. The conventional device structure consists of lanthanum nickelate (LaNiO3, LNO)/bismuth ferrite (BiFeO3, BFO)/indium tin oxide (ITO) on a mica substrate, consistent with previously reported FE photodetectors. The output signal characteristics reveal two fundamental limitations: a slow response time and notable photocurrent drift. The schematic elucidates the physical origin of these limitations: lateral thermal diffusion leads to in-plane heat spreading (circular isotherms) and causes thermal accumulation within the BFO layer which broadens the temporal response, producing the characteristic slow extraction and thermal drift.

The transformative improvement is achieved through our thermal engineering approach, where a high-thermal-conductivity copper layer is introduced beneath the mica substrate. This architecture yields a step-change in performance, eliminating photocurrent drift (blue arrow) and sharply reducing the response time (blue dotted rectangle). Vertical heat dissipation confines the local thermal gradients near the illumination site, suppresses lateral thermal spreading and maintains a stable thermal environment, which in turn enables drift free signal evolution.

This performance enhancement is quantitatively captured where the response time of the drift-free device outperforms conventional photodetectors by over three orders of magnitude, positioning our approach as a notable advancement in fast, stable FE optoelectronics.

Temporal and frequency-dependent photoresponse characteristics
When subject to pulsed illumination, the conventional BFO device exhibits a pronounced photocurrent drift, increasing from 39.13 to 121.04 μA during the illumination window; this represents a more than 200% increase in amplitude. This progressive rise reflects thermally activated charge accumulation, which produces artificial gain and delays carrier extraction. A magnified view of the rising edge reveals a sluggish response time of 1648 ms is shown in Supplementary Fig. 1a. These characteristics underscore the intrinsic bottlenecks in low-thermal-conductivity-supported FE photodetectors. In contrast, the Cu-backed device maintains a stable, drift-free photocurrent profile when subject to identical illumination conditions. The negligible difference between the initial and peak photocurrent values confirms the effective suppression of thermal buildup. A magnified view of the rising edge (Supplementary Fig. 1b) reveals a fast temporal response of 1.7 ms, which is nearly 1000 times faster than the conventional device, hereby validating the role of controlled thermal pathways in accelerating photocarrier dynamics.

The influence of illumination power density on response time is further elucidating the thermal management role. For both architectures the response time decreases with increasing power density. However, the conventional device’s response time (630–1648 ms) is consistently three orders of magnitude lower than that of the drift-free device (0.8–1.7 ms) across the measured illumination power density range. This vast difference underscores that the drift-free architecture, by providing a low thermal-resistance pathway, recovers the intrinsic electronic response time of the BFO layer. Recovery time trends (Supplementary Fig. 2) further confirm that it remains nearly illumination power-independent for both devices, with only a negligible increase at higher power densities. The millisecond-scale response time of the drift-free device is further substantiated by our frequency-domain analysis, where the extracted −3 dB bandwidth and corresponding phase lag fall within the 3–5 kHz range (Supplementary Note 1 and Supplementary Fig. 3). Consistent with this bandwidth, a direct time-domain measurement at 100 Hz modulation yields a stable undistorted waveform and a response time of 0.25 ms (Supplementary Fig. 4), confirming operation in the kilohertz regime.

The gain behavior at low-frequencies (50 mHz − 10 Hz), further highlights crucial role of thermal management. The conventional device suffers a ~20% gain roll-off below 5 Hz, indicative of the system’s instability to reach the thermal equilibrium under modulated illumination. The drift-free device demonstrates exceptional stability with a gain variation of <5% across the same range. This flat frequency response is a direct consequence of the suppressed thermal time constant.

The gain power scaling of the two devices architectures, measured over illumination power density range of 80 mW cm−2 to 2.5 mW cm−2, reveals distinct thermal contributions to photocurrent generation. The conventional device exhibits a super-linear power-law dependance with a slope of 1.09, indicating that photocurrent increases faster than the incident photon flux. Such super-linearity is a well-known signature of thermally assisted carrier activation, where local temperature rise under continuous illumination enhances carrier population beyond direct photogeneration. In contrast, the drift-free device shows a near-linear scaling with a slope of 0.92 indicating that the photocurrent is governed primarily by direct photocarrier generation and extraction. The suppression of super-linear gain reflects efficient vertical heat dissipation, which prevents thermal accumulation and minimizes temperature-induced carrier amplification. This effective thermal management directly translates to improved figure of merits (FOM) for the drift-free device including lower noise current density, higher specific detectivity, and a superior signal to noise ratio (SNR), as detailed in the supporting information (Supplementary Note. 2 and Supplementary Figs. 7–12). The transition from super-linear to near-linear scaling thus provides direct evidence that thermal diffusion engineering effectively suppresses thermally driven gain enhancement and associated photocurrent drift.

To quantitatively validate the long-term stability of drift-free architecture, we performed systematic multi-hour and multi-day cycling experiments. Three independently fabricated devices were measured under identical modulated illumination (1 Hz, 405 nm, 1.6 W cm−2). Structural uniformity across these devices confirmed by SEM (Scanning electron microscopy) and AFM (Atomic force microscopy) (Supplementary Fig. 13, Fig. 14) shows consistent functional layers (BFO thickness: 66–73 nm; LNO thickness: 200–208 nm; RMS (root mean square) roughness: 1.09–1.24 nm) and lateral dimensions of 2×2 mm active area of all the devices. Each device exhibited the exceptional amplitude stability of 99.5–99.7% over continuous 2.7-h segments (Supplementary Fig. 15). To prove extended duration, one device was monitored for 41.6 h in sequential segments, maintaining >97% stability with no observable degradation in temporal waveform shape as shown in Fig. 2h. Furthermore, repeated measurements on different days confirmed robust reproducibility with stability consistently in the 99.3 to 99.5% range (Supplementary Fig. 16). This comprehensive dataset, supported by uniform structural parameters.

Figure 2i provides a comparative assessment of response times across a diverse set of FE photodetector systems, including BFO, lead titanate (PbTiO3, PTO), lead zirconate titanate (PbZr0.7Ti0.3O3, PZT), LNO, lead strontium tantalate (Pb0.8Sr0.2TaO3, PST), barium calcium titanate (Ba0.85Ca0.15TiO3, BCT), barium zirconium titanate (BaZr0.1Ti0.9O3, BZT), barium calcium zirconium titanate (Ba0.85Ca0.15Zr0.1Ti0.9O3, BCZT), barium titanate (BaTiO3, BTO), and barium strontium zirconium titanate (Ba0.8Sr0.2Zr0.1Ti0.9O3, BSZT), revealing a consistent performance enhancement in thermally engineered drift-free architectures (Supplementary Figs. 17, 18). All devices were fabricated and characterized in our laboratory under identical experimental conditions, using the same deposition method, electrode configuration, and a fixed active area of 2×2 mm, ensuring a fair and direct comparison across different material systems. Across all material platforms, drift-free devices exhibit accelerated temporal responses relative to their conventional counterparts, frequently exceeding improvements of one to two orders of magnitude. This consistent behavior underscores the thermal diffusion engineering as an effective and broadly applicable strategy for enabling fast photodetection in a wide spectrum of FE materials. This performance is against state-of-the-art ferroelectric photovoltaic (FE-PV) and pyro-phototronic photodetectors, with the results compiled in Supplementary Table 1, confirming the competitive advantage of drift-free architecture.

Thermal engineering for drift suppression
To definitively attribute the observed drift-suppression to thermal management rather than other artifacts, we conducted dark-state control experiments under controlled temperature conditions. First, dark thermal-pulse measurement shows no pyroelectric current in either the conventional or drift-free architecture (Supplementary Note. 3 and Supplementary Fig. 19). Second, temperature-dependent dark I-V characteristics are identical for both devices conventional and drift-free (Supplementary Note. 4 and Supplementary Figs. 20, 21), proving the Cu backer solely modifies the thermal boundary condition without affecting interfacial chemistry or intrinsic charge transport.

To comprehensively evaluate and quantify the photocurrent instability in FE photodetectors, we introduce the parameters of drift ratio and drift rate (Fig. 3a). The drift ratio ΔI/I1 quantifies the relative change in photocurrent from its initial (I1) to the peak (I2), capturing the extent of the time-dependent photocurrent, while the drift rate d(I/I1)/dt reflects the temporal gradients of this change. These metrics provide a robust framework for comparing photocurrent stability across devices with varying substrate properties and thermal configurations. A comparative analysis of drift ratio across devices supported on freestanding, thermoplastic, and metallic substrates reveals a striking contrast (Fig. 3b). Devices on thermoplastics and freestanding mica exhibit a substantial drift ratio (>150%), consistent with inefficient heat dissipation (see Supplementary Fig. 22). In contrast, devices integrated with metallic substrates exhibit near-complete suppression of drift ratio (<5%), highlighting the important influence of a high substrate thermal conductivity in stabilizing the FE photoresponse when illuminated. This trend is further reinforced, by examining the drift rate as a function of illumination time window (Fig. 3c) which reflects how quickly thermally induced photocurrent changes develop. The freestanding and thermoplastic-supported devices exhibit high drift rates, particularly at short illumination times, which gradually stabilize at longer durations (Supplementary Figs. 23–26). In contrast, the thermally conductive metallic substrates maintain consistently minimal drift rates across all time intervals, confirming effective suppression of thermal drift suppression (Supplementary Figs. 27–29). Optical absorption measurements confirm that the drift suppression is purely thermal, as all metallic backers (Cu. Al, SS) exhibit nearly identical absorption to the freestanding device; consistent with its minimal transmittance (0.6–1.7%), ruling out significant optical interference effects (Supplementary Note. 5 and Supplementary Fig. 30).

To establish a materials-level understanding of this behavior, we correlated the device drift characteristics with the intrinsic thermal conductivity of the supporting substrates (Fig. 3d). Thermoplastics exhibit thermal conductivities below 0.2 Wm−1 K−1, in contrast to metals, which exceed 100 Wm−1 K−1. This four-orders-of-magnitude disparity directly maps onto device performance. Experimental surface temperature measurements of each device subjected to 405 nm illumination (Fig. 3e) confirm this correlation (Supplementary Fig. 31). The conventional device without a heat sink reaches a temperature as high as 63 °C (ΔT≈38 °C above room temperature), indicating heat accumulation, whereas the copper-supported device remains closer to ambient (ΔT≈6 °C), signifying efficient vertical heat dissipation and reduced lateral spreading.

Infrared thermal imaging (Fig. 3f) of the devices visually confirms these trends. The conventional device exhibits isotropic lateral heat diffusion, forming broad circular hotspots that are fully consistent with lateral thermal diffusion and accumulation. In contrast the drift-free device backed with a copper substrate confines heat sharply at the illumination point, suppressing lateral thermal diffusion.

To theoretically validate these findings, we conducted computational simulations using the COMSOL Multiphysics software platform, specifically employing the Heat Transfer in Solids module (Fig. 3g). Time-dependent simulations using Fourier’s law capture heat transport in both freestanding and copper-integrated architectures. In the conventional stack, heat diffuses radially with increasing time, closely matching the experimental IR thermography data (Supplementary Fig. 32). In contrast, a copper-backed structure sustains a confined vertical heat plume with negligible lateral spread. Detailed COMSOL Multiphysics simulations, presented in Supplementary Note. 6 and and Supplementary Videos 1, 2, provide a comprehensive comparison of thermal behavior between the drift-free and conventional devices when subject to identical illumination conditions. The simulations (Supplementary Video 1) reveal that the conventional device undergoes a substantial temperature rise (63 °C) and exhibits thermal non-uniformity across the device area. In contrast, the drift-free device shows markedly lower temperature increase (32 °C) even at higher intensities and maintains uniform thermal distribution across its structure. Furthermore, Supplementary video 2 highlights that conventional device reaches the same temperature differential at much lower light intensities (0.9 W cm−2) compared to the drift-free device intensity (4.94 W cm−2), underscoring the superior thermal regulation enabled by the high thermal conductivity and diffusivity of the Cu substrate. These findings confirm the critical role of vertical heat extraction in suppressing local thermal accumulation and stabilizing device operation under continuous optical excitation.

Extending our analysis across a broad spectrum of thermally engineered FE materials (BFO, PTO, PZT, LNO, PST, BCT, BZT, BCZT, BTO, BSZT), we observe a consistent reduction in both drift ratio and drift rate compared to their conventional counterparts (Fig. 3h and Supplementary Fig. 17). These results highlight the general applicability of thermal diffusion engineering strategy, which effectively enhances temporal stability and suppresses thermally induced instabilities across diverse FE system.

Self-powered high-fidelity spatial imaging
High-fidelity spatial imaging is critical in modern electronic and optoelectronics systems, where precise detection, spatial localization of optical signals, and accurate mapping are essential for applications such as imaging sensors, optical communication, neuromorphic computing, and machine vision. However, achieving accurate spatial resolution in self-powered photodetectors is often hindered by lateral thermal diffusion, which introduces crosstalk between adjacent pixels or channels, distorting signal integrity. Here we demonstrate a schematic of the mode of device operation under selective illumination using a spatial mask, see Fig. 4a. Only a single channel of the device is exposed to light, while the neighboring channels are covered. In the conventional device, lateral thermal diffusion from the illuminated region results in spurious thermally induced voltage signals in the masked channels, thereby reducing spatial selectivity. In contrast, the drift-free device, which incorporates a thermally conductive copper substrate beneath the active layers, effectively limits lateral heat spread within the illuminated region. This thermal confinement prevents heat spread into adjacent areas, resulting in a clean, isolated voltage signal only from the exposed channel-highlighting the device’s superior spatial fidelity.

We also illustrate a conceptual design for a self-powered photodetector array used in imaging (Fig. 4b). A patterned light mask is projected onto the device, and the output voltage map is expected to reproduce the pattern with high accuracy. The experimental results using LabVIEW-based signal acquisition for photodetector arrays illuminated through “X” and “Z” shaped masks subjected to 405 nm light are provided (Fig. 4c). In the conventional devices, lateral heat conduction leads to detectable signals in the masked regions, blurring the spatial pattern. In contrast, the drift-free devices produce output signals that precisely match the illuminated shapes, confirming the suppression of thermal crosstalk and the ability to maintain sharp spatial boundaries between active and inactive regions. This superior spatial fidelity is quantitatively validated by a comprehensive analysis of channel crosstalk and the line-spread function (LSF), which show a seven-fold improvement in spatial confinement for the drift-free architecture (Supplementary Note. 7 and Supplementary Figs. 33–35). These findings establish the drift-free-FE photodetectors as a strong candidate for integration into spatially resolved optoelectronic systems, in particular where lower power consumption, fast response, and signal integrity are important. Further optimization, including systematic miniaturization to define the fundamental pixel-size-limits of this thermal-management strategy, presents a clear pathway for future high resolution imaging applications.

Methods
Fabrication of LaNiO3 (LNO) bottom electrode
The LNO precursor solution was synthesized via sol-gel route. Ni(CH3COO)2·4H2O (nickel acetate-tetrahydrate) and La(NO3)3·6H2O (lanthanum nitrate-hexahydrate) were dissolved into a mixture of anhydrous ethanol and glacial acetic acid (molar ratio 1:1) under steady magnetic stirring at room temperature until a homogeneous light-green solution was obtained. The solution (LNO) was spin-coated onto freshly cleaved muscovite mica substrate (2000 rpm, 30s) using a programmable spin coater. Each deposited film was sequentially annealed at 200, 400, and 680 °C (3 min each) to remove organic residuals and crystallize the perovskite phase. This spin-coating and annealing cycle was repeated six times to achieve an LNO film with optimal conductivity and surface roughness.

Deposition of BiFeO3 (BFO) and other FE layers
A stoichiometric BFO precursor solution (0.2 mol L−1) was prepared by dissolving Bi(NO3)3·5H2O (bismuth nitrate pentahydrate) and Fe(NO3)3·9H2O (ferric nitrate nonahydrate) in ethylene glycol under ambient conditions. The solution (BFO) was spin-coated atop the LNO stack (4000 rpm, 30s), followed by stepwise pyrolysis at 180, 350, and 550 °C (3 min each) to ensure gradual organic removal and crystallization. The deposition cycle yielded a BFO film with uniform morphology. The methodologies for depositing other FE materials including PTO, LNO, PZT, PST, BCT, BZT, BCZT, BTO, and BSZT follow analogous spin-coating methods.

Integration of ITO (top electrode) and conventional device assembly
An array of 2×2 mm electrodes was patterned on the BFO surface using a shadow mask. The indium tin oxide (ITO) top electrode was deposited via RF magnetron (ZDF-5227 Composite Vacuum Gauge) sputtering (150 W, Ar Atmosphere, base pressure 4.5×10−6 Torr, 1200 s deposition time). Ohmic contacts were made by attaching copper wires to the ITO (top) and LNO (bottom) electrodes with conductive silver paste.

Drift-free device configuration
Freestanding devices were stacked onto different metallic and thermoplastic plates using a small amount of silver paste applied exclusively at the peripheral edges, ensuring no contact with electrodes or obstruction of the central active optical path. We prepared the other FE devices by following the same steps.

Device characterization and measurements. Photoresponse measurements were performed using a combination of a Keithley 6514 electrometer and Keithley 2611B source-meter for low-noise current detection and electrical characterization. For time resolved measurements, the photocurrent signal was amplified using a low-noise current amplifier (Stanford Research system SR570) with suitable sensitivity with respect to the intensity of light and recorded with a mixed-domain oscilloscope (Tektronix MDO3024). A low distortion function generator (Stanford Research system DS360) was used to modulate the laser intensity for frequency-dependent and transient response measurements. Calibrated laser source (MLL-III, DE52509) with wavelength 405 nm and power density of 320 mW cm−2 to 2.5 W cm−2 were employed, with the optical power monitored using a calibrated power meter (OPHIR PD300) (Supplementary Fig. 36).

For full frequency-response characterization, including gain and phase (Bode plots), a lock-in-amplifier (Stanford Research system SR865A) was employed.

Reflectance and transmittance spectra were measured using the OceanView Spectrophotometer equipped with a fiber-optic probe for the collection of reflected and transmitted light intensities, and the transmittance was further independently verified by using a UV-Vis spectrophotometer (Ultra 6600A).

For quantitative spatial metrics, including line-spread function (LSF) and crosstalk measurements, a microscope equid with a camera (TD-4KE) was used to focus the optical spot onto the device channel, while the sample was mounted on an x-y translational stage with micrometer-scale positioning accuracy to enable controlled scanning across the channel.

Temperature dependent I-V and transient measurements were carried out using a heating stage for temperature control, a Keithley 2260B source meter for voltage sourcing and a kiethely 6514 electrometer for high-sensitivity transient current recording.

Thermal Imaging: Transient temperature gradients during device operations were mapped using an Optris Xi 400 infrared thermal imager for both conventional and drift-free (with metallic substrate) devices.

Data acquisition: Voltage outputs were logged at the NI PXIe-1082 system with LabVIEW-based signal processing software. The measurements were conducted under the controlled illumination from a 405 nm light source, with optical patterning applied using two distinct mask shapes (Z and X). Data were collected for both conventional and drift-free device configurations to enable comparative analysis. All experimental parameters, including light intensity, sampling rate, and ambient conditions, were held constant across trials to ensure reproducibility.

Thermal Conductivity: Cross-plane thermal conductivity of individual layers was measured using a steady-state HS-DR-5 analyzer (error margin ±5%).

Simulations: To theoretically validate our experimental observations, we performed comprehensive 3D thermal simulations using COMSOL Multiphysics (v6.1) with the “Heat transfer in solids” physics interface. The simulations were designed to replicate the exact geometric and material conditions of both conventional and drift-free-FE photodetectors, enabling direct comparison with experimental thermal imaging data (Fig. 3f–g).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Minhas et al. report “drift-free” ferroelectric photodetection with purported ultrafast response via thermal-diffusion engineering using a high-thermal-conductivity Cu backer. To this reviewer, the work is moderately original—the heat-sinking lever is evolutionary and known in thermal/pyro-detector design, but the systematic drift quantification (drift ratio/rate across materials) and the imaging/crosstalk demonstration are useful contributions. Several central claims remain insufficiently supported: the detector is not convincingly “ultrafast,” key detector figures of merit are missing, and the governing mechanism is not isolated. Overall, the manuscript sits at the boundary of acceptance; in its current form, I recommend rejection for Nature Communications. My specific comments follow.\n(1) The manuscript highlights a 0.36 ms rise time (oscilloscope, Fig. 2d) and emphasizes “frequency-independent” response. However, the main-text frequency sweeps span only 0.1–1.5 Hz, and the Supporting Information extends to 300 Hz for different substrates, still without an explicitly reported −3 dB bandwidth or phase response. To substantiate the “ultrafast” claim, it is necessary to report full Bode plots (gain and phase versus frequency), the −3 dB point with confidence intervals, and noise spectra under matched optical and electrical conditions (same bias, area, illumination level, load, and temperature). Without these, the “ultrafast” characterization remains unsubstantiated.\n(2) Most critical measurements are performed at 1-5 W cm-2 @ 405 nm, which are very high irradiances relative to many application regimes. The manuscript does not report basic photodetector figures of merit—responsivity (R), noise current, noise-equivalent power (NEP), and specific detectivity (D*)—nor does it analyze SNR versus flux. It is required to include these measurements to position the device credibly against standard photodetector baselines. These metrics are necessary for fair comparison and for assessing practical relevance.\n(3) The thermal-diffusion narrative (IR thermography, COMSOL heat-flow modeling, substrate-dependent ΔT reduction) is persuasive but does not isolate the photovoltaic contribution from photothermal/pyroelectric effects. To support the claim that thermal engineering directly yields “drift-free” behavior, it is required to include the following controls: (i) dark thermal pulses (no light) to mimic ΔT transients and quantify purely thermal responses; (ii) multi-wavelength tests at equal absorbed power (not merely equal incident power), verified via reflectance/absorbance; (iii) temperature-ramp bias tests to track responsivity/drift metrics versus controlled substrate temperature; and (iv) matched-optics stacks on low-k vs high-k spreaders with identical optical interference conditions. These controls are necessary; without them, the assertion “drift-free via thermal engineering” remains plausible but not isolated from competing mechanisms.\n(4) The manuscript asserts “universal effectiveness” across multiple ferroelectric materials but presents primarily exemplar curves. There is no report of device counts, error bars, distributions or yield, or long-term endurance beyond a single ≈3000 s trace. To support generality, it is necessary to provide, for each material/system, the sample size; spread for drift metrics and rise/decay times; yield statistics; and hour-to-day cycling data (including bias/temperature stress). These statistics must be included to elevate the claims from anecdotal to robust.\n(5) The X/Z-mask demonstrations are visually illustrative but qualitative. To substantiate “high-fidelity imaging” and reduced crosstalk, the manuscript must include quantitative array-level metrics: crosstalk matrices (with nearest-neighbor leakage percentages), point-spread functions (PSF), modulation transfer functon(MTF), and per-pixel isolation maps derived from the 25-channel time traces. Reporting line-pair resolution and contrast versus spatial frequency is necessary. Without these quantitative measures, the imaging-centric claims remain insufficiently supported.\n(6) Metal backers can alter optical absorption and interference (etalon effects), not merely thermal conduction. For fair isolation of thermal effects, it is required to equalize absorbed optical power (not just incident power) via measured reflectance/absorbance and to state the exact optical stack (layer materials, thicknesses, roughness) used in each comparison. Replotting key comparisons under matched absorbed power is necessary; otherwise, thermal and optical effects remain confounded.\n(7) The manuscript repeatedly attributes trap suppression to vertical heat extraction through Cu (e.g., “reduced activation of trapping centers”) yet trapping/detrapping kinetics are primarily governed by the semiconductor and its interfaces—defect chemistry, interface states, and band alignment—while Cu chiefly modifies the thermal profile, not the trap spectrum. It is required to decouple thermal management from trap physics; this requires, for example, identical stacks on Fe/Al/Cu with matched thickness/roughness to separate thermal conductivity from work-function/optical changes; absorbed-power-matched multi-wavelength tests; and interfacial forensics to exclude Cu diffusion or unintended interlayers. The discussion must reconcile any observed leakage or power-law changes that could arise from barrier/interface modifications rather than cooling alone.\n(8) The manuscript assigns an intensity exponent of 0.87 to space-charge-limited conduction with minimal recombination losses. However, an exponent <1 can also indicate trap-mediated recombination and other non-idealities. It is necessary to validate this interpretation via temperature-dependent and intensity-dependent exponent maps, transient rise/decay analyses, and contact-geometry controls (e.g., varying electrode work function or area). Without these corroborations, the SCLC assignment is not unique, and the mechanism remains ambiguous.\n(9) Oscilloscopes measure voltage, not current. The manuscript does not document the sensing chain used to report current from the recorded voltages. It is required to include a clear measurement-circuit schematic and parameters. These details are necessary to interpret both the absolute current levels and the claimed temporal response.", "reviewer_2": "This manuscript presents a novel thermal-diffusion engineering strategy for ferroelectric photodetectors, achieving a remarkable improvement in response speed (>10³×) and complete suppression of photocurrent drift. Studying is motivated, as slow dynamics and instability are long-standing challenges in self-powered FE devices. The combination of infrared thermography and COMSOL simulations provides convincing support for the proposed mechanism. Moreover, the architectural focus on controlling vertical versus lateral heat flow is original and complementary to the more common material/interface engineering approaches.\nTo further strengthen the impact and clarity of the work, I suggest the authors address the following points:\n1. The manuscript claims a >10³× improvement in photoresponse speed, but absolute rise/fall times are not directly compared to state-of-the-art FE–PV and pyro-phototronic detectors. Please provide explicit numerical values (µs/ms scale) and include a benchmarking table against relevant literature.\n2. While a >10³× speed improvement is reported, the manuscript does not present a comprehensive comparison of photoresponse speeds between FS-BFO and DF-BFO devices across a broad range of light intensities (from mW/cm² to W/cm²). Including this analysis would make the claims more rigorous.\n3. Some data appear inconsistent across different figures. For example, in Fig. 2a the FS-BFO device shows a sharp initial rise in Isc followed by a gradual increase, whereas in Fig. 4a the conventional BFO device exhibits a sharp rise followed by a gradual decay. In Supplementary Figs. 24–25, FS-BFO shows a fast rise followed by a linear decay. These behaviors cannot all be explained by the model proposed in the main text. The authors should clarify these discrepancies.\n4. Similar inconsistencies are observed for DF-BFO devices. In Fig. 2d, Isc increases linearly after the initial sharp rise; in Supplementary Fig. 24, it decreases linearly; and in Supplementary Fig. 25, it decreases non-linearly. The authors should provide a clear explanation for these variations.\n5. Since pyroelectric and photovoltaic effects may coexist in BiFeO₃ and related ferroelectrics, please clarify whether the observed speed enhancement originates solely from reduced polarization screening, or if pyroelectric contributions also play a role.\n6. Several relevant studies on pyroelectric/pyro-phototronic detectors are missing. Including these references would strengthen the discussion and provide a more balanced comparison.", "reviewer_3": "Minhas et al. report a simple yet highly effective strategy to overcome two persistent limitations of ferroelectric (FE) photodetectors: slow temporal response and photocurrent drift. By integrating BFO thin films on high-thermal-conductivity copper substrates, the authors demonstrate >1000x faster response times (0.36 ms vs. 1648 ms) and near-complete drift suppression compared to conventional freestanding devices. Infrared thermography and COMSOL simulations convincingly support the proposed mechanism of enhanced vertical heat extraction and suppressed lateral heat diffusion. The approach is shown to be broadly applicable across multiple FE materials and enables high-fidelity, low-crosstalk spatial imaging. The work addresses a significant bottleneck in FE optoelectronics, and the experimental data are persuasive. However, in the current state I cannot recommend publication. The manuscript would benefit from addressing the following points.\n1. Page2, line60: “A common feature of conventional FE photodetectors is their fabrication on low-thermal-conductivity base materials such as mica or glass”. Why is this the common fabrication method and what advantages does this bring apart from the disadvantage of inefficient heat extraction. Since the authors themselves are also growing on mica.\n2. Fig2g, 1h of measurement is neat, but to talk about long-term stability, it would be helpful if the device is measured also after 1 week or longer. Calling 1h of stability “unprecedented” is a bit of a far-fetched claim.\n3. As stated in the manuscript, the devices exhibit a response time of only 0.36 ms, the bandwidth of the devices however is only measured until 1.4 Hz. That the DF device doesn’t even come near its limit, is quite trivial. With that response time, measurements into the kHz range should be possible. I encourage the authors to include higher frequency measurements.\n4. Fig2i: when reading this paragraph, it was not very clear if the comparison data is experimental or not. Furthermore, I am missing a discussion on why exactly BFO is by far better than all the other oxides, namely two orders of magnitude and a relative increase between FS and DF of four orders of magnitude. Later in the manuscript it is mentioned that “While BFO-based DF devices exhibit the most pronounced improvements, attributed to their robust PV properties,…”, which needs further discussion.\n5. Maybe trivial, but not mentioned, how are the devices fixed onto the metal substrates?\n6. From a thermal management perspective, shouldn’t it be irrelevant how thick the copper substrate is once a minimum drift ratio is reached? It seems like the drift-ratio goes up again for thicker substrates than 0.8mm, which to me is counterintuitive. How many substrates have been included in that study per thickness?\n7. The last section of the spatial detector array feels a bit like a small anecdote just repeating the advantages of the DF concept. Almost no practical parameters nor limitations for the use of the detector are discussed. The devices are quite big in terms of technological significance. The spatial imaging with “X” and “Z” masks is promising but relatively qualitative. Quantitative metrics like modulation transfer functon(MTF) or crosstalk ratio would be stronger. Covering the following questions would make this section more valuable for potential applications. How small can the array be made until crosstalk even for the DF architecture happens? 2mm pads are huge, what is the limit here? In that regard, from the methods section I understand, that to actually measure the device the ITO pads are directly contacted with silver paint (the device figure in S28 is shown without those contacts). Having to place the contact on top would probably significantly influence device miniaturization as incoming light gets blocked. What other approach would be necessary to deal with that issue?"}, "author_response": "Manuscript ID:\nTitle: Drift-free ferroelectric photodetection with fast temporal response via thermal diffusion\nengineering.\nDear Editor,\nThanks for your attention and the reviewers’ comments on our manuscript. According to the\ncomments, the manuscript has been revised.\nOur responses to the reviewers’ comments are listed below one by one, and the revised or added\nwords are marked in the revised manuscript with different color. Thank you very much.\nThe first reviewer report and response:\nMinhas et al. report “drift-free” ferroelectric photodetection with purported ultrafast response via\nthermal-diffusion engineering using a high-thermal-conductivity Cu backer. To this reviewer, the\nwork is moderately original—the heat-sinking lever is evolutionary and known in thermal/pyro-\ndetector design, but the systematic drift quantification (drift ratio/rate across materials) and the\nimaging/crosstalk demonstration are useful contributions. Several central claims remain\ninsufficiently supported: the detector is not convincingly “ultrafast,” key detector figures of merit\nare missing, and the governing mechanism is not isolated. Overall, the manuscript sits at the\nboundary of acceptance; in its current form, I recommend rejection for Nature Communications.\nMy specific comments follow.\nOur response: We sincerely thank the reviewer for their thoughtful and constructive evaluation of\nour manuscript. We appreciate the time and expertise dedicated to assessing our work. The\nreviewer’s insightful comments have been invaluable in helping us refine our claims, strengthen\nthe evidentiary support, and improve the overall clarity and impact of the manuscript.\nIn accordance with the suggestions, all experimental data were standardized under strictly\nmatched optical and electrical conditions. Furthermore, the uniform light intensity for different\nmaterials was corrected based on their absorption coefficients.\nRegarding the use of the term ‘ultrafast’, we agree with the reviewer that this terminology may\nnot be appropriate in the broader context of photodetection, where ‘ultrafast’ is typically reserved\n1\nfor nanosecond or sub-nanosecond response regimes. In the original manuscript, this term was\nused in a relative sense. Based on our experimental results, the response time of our drift-free\ndevice is in the sub-millisecond regime which is more accurately described as ‘fast’. Accordingly,\nwe have removed the term ‘ultrafast’ throughout the manuscript and replaced it with ‘fast’. The\nproposed new title is:\n“Drift-free ferroelectric photodetection with fast temporal response via thermal diffusion\nengineering”.\nOverall, we have substantially revised the manuscript to thoroughly address the reviewer’s\nprimary concerns. Below, we provide a detailed, point-by-point response to each specific comment.\nComment 1: The manuscript highlights a 0.36 ms rise time (oscilloscope, Fig. 2d) and emphasizes\n“frequency-independent” response. However, the main-text frequency sweeps span only 0.1–1.5\nHz, and the Supporting Information extends to 300 Hz for different substrates, still without an\nexplicitly reported −3 dB bandwidth or phase response. To substantiate the “ultrafast” claim, it is\nnecessary to report full Bode plots (gain and phase versus frequency), the −3 dB point with\nconfidence intervals, and noise spectra under matched optical and electrical conditions (same bias,\narea, illumination level, load, and temperature). Without these, the “ultrafast” characterization\nremains unsubstantiated\nAnswer 1: We thank the reviewer for encouraging a more rigorous frequency-domain validation\nof the device dynamics. To directly address the reviewer’s concern regarding the limited frequency\nspan, we have now performed systematic gain measurements over an extended low-frequency\nrange from 50 mHz to 10 Hz (Figure R1) under identical optical and electrical conditions using an\noscilloscope in series with an SR570 transimpedance amplifier (TIA) (Sensitivity 50 µA). In this\nregime, the conventional device exhibits a pronounced frequency dependence: its photocurrent\ndecreases from 184 µA to 152 µA below 5 Hz, corresponding to a reduction of approximately 20%\nof the maximum gain. In contrast, the drift-free device shows a much weaker gain variation,\ndecreasing from 167 µA to 160 µA (＜ 5%), and remains nearly constant between 50 mHz and 1\nHz, demonstrating quasi-frequency-independent response at low frequencies. These results are\nnow included and discussed in the revised manuscript.\n2\nc f\nFigure R1 (Figs. 2c,f in the revised version) | Low-frequency gain stability of conventional\nand drift-free photodetectors. c, Frequency-dependent photocurrent gain of the conventional\ndevice measured from 50 mHz to 10 Hz under identical optical and electrical conditions (1.6 mW\ncm⁻² power density, 2 mm × 2 mm active area, 26 °C) using an oscilloscope in series with an\nSR570 transimpedance amplifier (50 µA sensitivity). f, Corresponding measurement for the drift-\nfree device.\nTo quantitatively substantiate the reported millisecond-scale response and to meet the reviewer’s\nrequest for confidence intervals, we have now measured full Bode plots (gain and phase versus\nfrequency) for three independently fabricated drift-free devices using a lock-in amplifier (LIA)\nwith slope of 24 dB, cascaded with an SR570 transimpedance amplifier (TIA) with a sensitivity\nof 50 µA/V, under the incident light power density of about 1.6 W cm-2. The measurements were\nperformed over a broad frequency span from 100 Hz to 15 kHz, enabling direct extraction of both\namplitude and phase characteristics. From these measurements, the extracted -3 dB bandwidths of\nthree devices lie between 3.1 kHz and 4.3 kHz (Figure R2a), while the corresponding -45° phase-\nshift frequencies fall between 4.6 kHz and 5.5 kHz (Figure R2b). The close agreement among\ndevices demonstrates good reproducibility. These kilohertz-scale bandwidths substantiate the\nmillisecond-to-sub-millisecond temporal response observed in the time-domain measurements, as\nexpected from standard bandwidth rise-time relationships. These results demonstrate that our\ndevice architectures can reliably operate in the kHz frequency regime.\ncharacterization of drift-free photodetectors. a, Gain Bode plots of three independently\nfabricated drift-free devices measured under identical conditions using a LIA (24 dB/oct slope)\ncascaded with an SR570 TIA (50 µA V-1) at an incident power density of ~1.6 W cm-2. The -3 dB\nbandwidths extracted from the gain roll-off span 3.1- 4.3 kHz, b, Corresponding phase Bode plots\nfor the same devices, showing -45° phase-shift frequencies between 4.6 and 5.5 kHz, in close\nagreement with the gain-derived bandwidths.\nImportantly, these kilohertz-scale bandwidths directly substantiate the millisecond-to-sub-\nmillisecond temporal response observed in the time-domain measurements, as expected from\nstandard bandwidth rise-time relationships. These results demonstrate that our device architectures\ncan reliably operate in the kHz frequency regime.\nAccordingly, we measured the transient response at a modulation frequency of 100 Hz (Figure\nR3), where the device maintains a stable waveform without edge distortion or thermal drift. At this\nfrequency, the extracted response time is approximately 0.25 ms, which is faster than the\nmentioned 0.36 ms response time in our 1st submitted manuscript and further supports the fast\ntemporal response of our device, and we amended this response time in our revised manuscript.\nmodulation. a, Transient photocurrent waveform of the drift-free device under 100 Hz optical\nexcitation, b, Magnified view of the rising edge (marked in a). The 10% - 90% rise time extracted\nfrom an exponential fit is ~0.25 ms.\nTo ensure consistency, noise spectra were measured using the oscilloscope’s radio-frequency\n(RF) spectrum analyzer in series with the TIA under dark conditions and varying optical\nillumination powers densities ranges from 100 mW cm-2 to 1.6 W cm-2 for both conventional and\ndrift-free devices (Figure R4a,b). The RF noise spectra was recorded over a 15 kHz frequency span\nwith a resolution bandwidth (RBW) of 55 Hz and a -30 dB scale.\nFigure R4 (Supplementary Fig. 7 in the revised version) | Radio-frequency (RF) noise spectra\nof ferroelectric photodetectors. a, RF noise spectra of the conventional device and b, the drift-\nfree device measured using an oscilloscope-based RF spectrum analyzer in series with an SR570\nTIA under dark conditions and varying optical power densities (100 mW cm-2 - 1.6 W cm-2).\nSpectra were recorded over a 15 kHz frequency span with a resolution bandwidth of 55 Hz.\npower spectral density to current noise using the known TIA gain, following standard procedures.\nTo enable a robust and unbiased comparison between the conventional and drift-free devices, we\nevaluated the noise at three representative frequencies spanning the sub-kilohertz to kilohertz\nrange, all well below the -3 dB bandwidth of the devices. This frequency selection allows\nassessment of noise behavior across the practical operating regime.\nTo quantitatively extract the noise current density from the RF noise spectra (Figure R4), the\nnoise level recorded by the oscilloscope in dBm was converted to linear noise power (W), which\nwas then transformed into RMS voltage noise assuming a 50 Ω system impedance. The voltage\nnoise was normalized by the square root of the resolution bandwidth (RBW) to obtain the voltage\nnoise spectral density. This quantity was subsequently converted into current noise using the\ncalibrated TIA gain and finally normalized by the active device area to yield the noise current\ndensity. The consistently lower noise current density (Figure R5) observed for the drift-free device\nacross all evaluated frequencies indicates that the incorporation of thermal diffusion engineering\ndoes not introduce additional noise penalties; instead, it slightly improves noise performance.\nFigure R5 (Supplementary Fig. 8 in the revised version) | Noise current density comparison\nbetween conventional and drift-free devices. Extracted data from RF noise spectra\n(Supplementary Fig.7) at representative frequencies below the -3 dB bandwidth: a, 0.5 kHz,\nb, 1.5 kHz, and c, 3 kHz. At all three frequencies, the drift-free device exhibits consistently lower\nnoise current density than the conventional device.\nModification to the Manuscript:\nThe following paragraph has been revised in page 4, paragraph 2, and paragraph 3, in the\nrevised manuscript.\n“The millisecond-scale response time of the drift-free device is further substantiated by our\n6\nfrequency-domain analysis, where the extracted -3dB bandwidth and corresponding phase lag fall\nwith in the 3-5 kHz range (Supplementary Note. 1 and Supplementary Fig. 3). Consistent with this\nbandwidth, a direct time-domain measurement at 100 Hz modulation yields a stable undistorted\nwaveform and a response time of 0.25ms (Supplementary Fig. 4), confirming operation in the\nkilohertz regime.\nThe gain behavior at low-frequencies (50 mHz - 10 Hz), further highlights crucial role of\nthermal management. The conventional device suffers a ⁓20% gain roll-off below 5 Hz (Fig. 2c),\nindicative of the system’s instability to reach the thermal equilibrium under modulated\nillumination. The drift-free device (Fig. 2f) demonstrates exceptional stability with a gain variation\nof <5% across the same range. This flat frequency response is a direct consequence of the\nsuppressed thermal time constant”\nModification to the Supporting Information:\nThe following paragraphs have been revised in page 5, in the revised supporting information.\nSupplementary Note 1. Frequency-domain characterization and bandwidth extraction.\nTo quantify the temporal bandwidth of the drift‑free devices, frequency‑domain Bode‑plot\nmeasurements were performed. A lock‑in amplifier (LIA) with a 24 dB/oct roll‑off was used in\nconjunction with an SR570 low‑noise transimpedance amplifier (TIA) under 405 nm illumination\nat a power density of 1.6 W cm-2 The normalized photocurrent gain and phase shift were recorded\nover a frequency range from 100 Hz to 15 kHz for three independently fabricated devices.\nThe -3 dB cut‑off frequencies extracted from the gain profiles range from 3.1 kHz to 4.3 kHz,\nwith corresponding -45° phase‑shift frequencies between 4.6 kHz and 5.5 kHz (Supplementary\nFig. 3). This close agreement among devices confirms good reproducibility and establishes the\nintrinsic -3 dB bandwidth of the drift-free architecture in the kilohertz regime, consistent with the\nmillisecond‑scale response times observed in transient measurements.\nThe following paragraphs has been revised in page 6, paragraph 1,2 in the revised supporting\ninformation.\n“Radio‑frequency (RF) noise spectra were acquired under illumination powers from 320 mW cm-\n2 to 1600 mW cm-2 using a 50 Ω oscilloscope input in series with a transimpedance amplifier\n7\n(Supplementary Fig. 7a,b). The spectra were recorded with a resolution bandwidth (RBW) of\nmeasured power spectral density from dBm to amperes per root Hertz (A/Hz1/2).\nFor a robust and unbiased comparison, the noise current density was evaluated at three\nrepresentative frequencies including 0.5 kHz, 1.5 kHz, and 3 kHz, all well below the -3 dB\nbandwidth of the devices (Supplementary Fig. 8). At these frequencies, the drift-free device\nconsistently exhibits lower noise current density than the conventional counterpart, confirming\nthat the thermal‑management strategy does not introduce additional noise and in fact slightly\nimproves noise performance.”\nhigh irradiances relative to many application regimes. The manuscript does not report basic\nphotodetector figures of merit—responsivity (R), noise current, noise-equivalent power (NEP),\nand specific detectivity (D*)—nor does it analyze SNR versus flux. It is required to include these\nmeasurements to position the device credibly against standard photodetector baselines. These\nmetrics are necessary for fair comparison and for assessing practical relevance.\nAnswer 2: We thank the reviewer for this important comment regarding operating irradiance levels\nand the absence of standard photodetector figures of merit in the original manuscript. We agree\nthat reporting responsivity, noise current density, noise-equivalent power (NEP), specific\ndetectivity (D*), and signal-to-noise ratio (SNR) over a relevant illumination range is essential for\nfair comparison with established photodetector benchmarks and for assessing practical relevance.\nNow, we have performed a comprehensive figure-of-merit (FOM) analysis for both\nconventional and drift-free devices under incident optical powers ranging from 100 mW cm-2 to\n1.6 W cm-2 for the active area of 4 mm2. Noise-related metrics were extracted from RF noise\nspectra measured at three representative frequencies under identical device area, bandwidth, and\nillumination conditions (100 mW cm-2 to 1.6 W cm-2), as described in detail in our response to\nComment 1 and in Figures R4,5.\nNoise current density (Figure R5) was extracted from the RF noise spectra (Figure R4) by\nconverting the measured noise power (dBm) into current noise spectral density (A Hz1/2) through\nimpedance normalization, resolution-bandwidth correction, and calibrated transimpedance\namplifier gain. Across all examined frequencies and over the full illumination range, the drift-free\ndevice consistently exhibits lower noise current density than the conventional device.\nThe responsivity (R), computed using,\nIph\nwhere I is the photocurrent and P is theR i=nc iPdoepnt,t optical power, reveals two distinct power-\nph out\ndependent regimes for both device architectures. In the low-power regime, both conventional and\ndrift-free devices exhibit decreasing responsivity with increasing illumination power density\n(Figure R6). However, the drift-free device maintains consistently higher responsivity than the\nconventional device. At higher power densities, the drift-free device enters a nearly power-\nindependent regime in which the responsivity remains stable. In contrast, the conventional device\nshows a gradual increase in responsivity with increasing power. Overall, across the entire\n9\ninvestigated power range, the drift-free architecture delivers superior responsivity.\nFigure R6 (Supplementary Fig. 9 in the revised version) | Responsivity comparison of\nconventional and drift-free devices. Responsivity as a function of illumination power density for\nconventional and drift-free devices extracted data from RF noise spectra (Supplementary Fig. 7),\nshowing consistently higher and more stable responsivity in the drift-free architecture across the\nmeasured range.\nThe noise-equivalent power (NEP), computed using,\n,\nin\nwhere i\nn\nis the noise current spectral densitNy.E TPh=e N\nR\nEP was evaluated for both conventional and\ndrift-free devices using the extracted noise current density and measured responsivity using the\nthree representative frequencies, all below the intrinsic bandwidth of the devices. As shown in\nFigure R7, the drift-free device consistently exhibits a lower NEP than the conventional device,\nreflecting the combined benefits of reduced noise and stable responsivity.\ncomparison of conventional and drift-free devices. NEP extracted from Figure R4 for\nconventional and drift-free devices at a, 0.5 kHz, b, 1.5 kHz, and c, 3 kHz under identical\nillumination power density between 100 mW cm-2 to 1.6 W cm-2.\nThe specific detectivity (D*), computed using,\n,\n∗ R√A\nwhere A is the active device area (0.04 cmD2). D=* ionf the conventional and drift-free devices was\ncalculated from the measured responsivity and noise current density. To provide a frequency-\nrepresentative assessment, D* was evaluated at three modulation frequencies over an illumination\npower-density range of 100 mW cm-2 to 1.6 W cm-2. The drift-free device consistently exhibits\nhigher detectivity than the conventional device (Figure R8) throughout the entire power range,\nindicating that drift-free architecture offers superior sensitivity.\nFigure R8 (Supplementary Fig. 11 in the revised version) | Detectivity comparison of\nconventional and drift-free devices. Specific detectivity (D*) extracted data from Figure R4 for\nconventional and drift-free devices at a, 0.5 kHz, b, 1.5 kHz, and c, 3 kHz as a function of\nillumination power density between 100 mW cm-2 - 1.6 W cm-2.\nThe signal-to-noise ratio (SNR) of the conventional and drift-free devices was evaluated to\n11\nassess signal fidelity across representative operating frequencies. SNR was extracted at three\nmodulation frequencies over an illumination power-density range of 100 mW cm-2 to 1.6 W cm-2.\nThe drift-free device consistently exhibits a higher SNR than the conventional device (Figure R9).\nFigure R9 (Supplementary Fig. 12 in the revised version) | Signal-to-noise ratio comparison\nof conventional and drift-free devices. SNR extracted data from Figure R4 for conventional and\ndrift-free devices measured at a, 0.5 kHz, b, 1.5 kHz, and c, 3 kHz as a function of illumination\npower density between 100 mW cm-2 - 1.6 W cm-2.\nWhile the absolute sensitivity metrics remain moderate due to the large-area ferroelectric device\narchitecture, the central advance demonstrated here is that drift-free operation and kilohertz-\nbandwidth response are achieved without compromising the fundamental photodetector figures of\nmerit. This decoupling of thermal stability from sensitivity is critical for reliable operation under\ncontinuous illumination. From an application perspective, the combination of kilohertz-scale\nbandwidth, sub-millisecond temporal response, robustness under high irradiance (up to 1.6 W cm-\n2), and compatibility with large-area device scaling makes this architecture particularly well suited\nfor imaging, optical monitoring, and harsh-environment sensing, where operational stability and\ndrift suppression are more critical than maximizing ultimate detectivity.\nModification to the Manuscript:\nThe following paragraph has been revised in page 4, paragraph 4, in the revised manuscript.\n“This effective thermal management directly translates to improved figure of merits (FOM) for the\ndrift-free device including lower noise current density, higher specific detectivity, and a superior\nsignal to noise ratio (SNR), as detailed in the supporting information (Supplementary Note. 2 &\nSupplementary Fig. 7-12).”\nThe following paragraphs have been added in page 6, in the revised supplementary\ninformation.\n“Supplementary Note 2. Figure of Merits analysis.\nTo quantitatively measure the performance of the conventional and thermally engineered drift‑free\nphotodetectors, we evaluated their figures of merit (FOMs). Radio‑frequency (RF) noise spectra\nwere acquired under illumination powers from 320 mW cm-2 to 1600 mW cm-2 using a 50 Ω\noscilloscope input in series with a transimpedance amplifier (Supplementary Fig. 7a,b). The\nspectra were recorded with a resolution bandwidth (RBW) of 55 Hz over a 15 kHz span, and the\nnoise current density (in) was extracted by converting the measured power spectral density from\ndBm to amperes per root Hertz (A/Hz1/2).\nFor a robust and unbiased comparison, the noise current density was evaluated at three\nrepresentative frequencies including 0.5 kHz, 1.5 kHz, and 3 kHz, all well below the -3 dB\nbandwidth of the devices (Supplementary Fig. 8). At these frequencies, the drift-free device\nconsistently exhibits lower noise current density than the conventional counterpart, confirming\nthat the thermal‑management strategy does not introduce additional noise and in fact slightly\nimproves noise performance.\nThe responsivity (R) of each device, extracted from the intensity‑dependent photocurrent\n(Supplementary Fig. 9), was used to calculate the noise‑equivalent power (NEP) via\n,\nin\nwhere in is the noise current spectral densitNy.E TPh=e N\nR\nEP was evaluated for both conventional and\ndrift-free devices using the extracted noise current density and measured responsivity using the\nthree representative frequencies (0.5 kHz, 1.5 kHz, and 3 kHz), all below the intrinsic bandwidth\nof the devices. Across all examined frequencies and illumination levels (Supplementary Fig. 10),\nthe drift-free device consistently exhibits a lower NEP than the conventional device, reflecting the\ncombined benefits of reduced noise and stable responsivity enabled by thermal diffusion\nengineering\nThe specific detectivity (D*), computed using,\n13\n,\n∗ R√A\nwhere A is the active device area (0.04 cm2)D. Th=e spinecific detectivity (D*) of the conventional and\ndrift-free devices was calculated from the measured responsivity and noise current density. To\nprovide a frequency-representative assessment, D* was evaluated at three modulation frequencies\n(0.5 kHz, 1.5 kHz, and 3 kHz) over an illumination power-density range of 100 mW cm-2 to 1.6\nW cm-2. Across all three frequencies, the drift-free device consistently exhibits higher detectivity\nthan the conventional device (Supplementary Fig. 11) throughout the entire power range. The\npersistence of improved D* over multiple frequencies confirms that the drift-free architecture\noffers superior sensitivity.\nThe signal-to-noise ratio (SNR) of the conventional and drift-free devices was evaluated to\nassess signal fidelity across representative operating frequencies. SNR was extracted at three\nmodulation frequencies (0.5 kHz, 1.5 kHz, and 3 kHz) over an illumination power-density range\nof 100 mW cm-2 to 1.6 W cm-2. At all frequencies and across the entire power range, the drift-free\ndevice consistently exhibits a higher SNR than the conventional device (Supplementary Fig. 12).\nThis improvement arises from the simultaneous enhancement of photocurrent stability and\nsuppression of thermally induced noise enabled by efficient vertical heat extraction.\nIn summary, the drift-free architecture demonstrates superior performance over the conventional\ndevice in all key FOMs, including noise current density, NEP, detectivity, and SNR, across the\nanalyzed frequencies (0.5 kHz, 1.5 kHz, and 3 kHz). The performance advantage is most\npronounced at the lower frequency (0.5 kHz), where the drift-free device’s noise is significantly\nlower, highlighting its effectiveness in suppressing the low‑frequency thermal fluctuations that\nmost severely degrade conventional ferroelectric photodetectors.”\nsubstrate-dependent ΔT reduction) is persuasive but does not isolate the photovoltaic contribution\nfrom photothermal/pyroelectric effects. To support the claim that thermal engineering directly\nyields “drift-free” behavior, it is required to include the following controls: (i) dark thermal pulses\n(no light) to mimic ΔT transients and quantify purely thermal responses; (ii) multi-wavelength\ntests at equal absorbed power (not merely equal incident power), verified via\nreflectance/absorbance; (iii) temperature-ramp bias tests to track responsivity/drift metrics versus\ncontrolled substrate temperature; and (iv) matched-optics stacks on low-k vs high-k spreaders with\nidentical optical interference conditions. These controls are necessary; without them, the assertion\n“drift-free via thermal engineering” remains plausible but not isolated from competing\nmechanisms.\nAnswer 3: We thank the reviewer for this thoughtful and technically important comment. We have\nperformed a series of targeted control experiments and clarifications, as detailed below.\n(i) Dark thermal pulses (no light): To directly probe purely thermal responses, we performed dark\nthermal-pulse experiments on our drift-free device. The drift-free device was subjected to different\ntemperature change and the corresponding currents were measured in Figure R10. The measured\nsignals exhibit only slow, monotonic rise and decay consistent with heating and cooling governed\nby the thermal time constant of the device-substrate system. Importantly, no fast, polarity-reversing\ntransient; characteristic of pyroelectric current associated with dT/dt was observed. Therefore, our\noutput photoelectric signal does not contain a pyroelectric component but does exhibit a certain\nthermoelectric contribution. However, the thermoelectric signal generated by a temperature change\nof approximately 30 °C is only on the nano-ampere level (Figure R10), whereas the temperature\nrise induced by illumination in our samples is only 6 °C yet produces a photocurrent of 106 µA\n(Fig. 2d). Therefore, the output of our device is dominated by the photovoltaic signal, and the\nminor thermoelectric component can be safely neglected.\nwithout illumination. a-c, Cu-backed drift-free device was measured over a temperature range of\n30 °C - 60 °C (∆T = 5.5 - 27 ºC) by heating, showing a function of temperature under different\nthermal conditions.\n(ii) Multi-wavelength tests under equal absorbed power: We further evaluated the photoresponse\nat multiple excitation wavelengths (405, 450, and 532 nm). Photocurrent transients were then\ncompared under matched absorbed optical power density of 1.28 W cm-2 for 405 nm wavelength\nlight, 1.33 W cm-2 for 450 and 1.41 W cm-2 for 532nm wavelength light (Figure R11). The detailed\nmeasurement procedure for selecting matched absorbed optical power density has been described\nin the response of Comment 6. Both conventional and drift-free devices exhibit consistent trends\nacross wavelengths, with the drift-free device systematically showing suppressed drift and faster\nstabilization. These results indicate that the observed improvements are not wavelength-specific.\nPhotocurrent transients of the conventional device measured under illumination at 405 nm (1.28\nW cm-2), 450 nm (1.33 W cm-2), and 532 nm (1.41 W cm-2). b, Corresponding photocurrent\ntransients of the drift-free device measured under identical illumination conditions.\n(iii) Temperature-ramp tests under illumination with Cu backer: Photocurrent transients of the\ndrift-free device in different device temperature were conducted (Figure R12a), and the extracted\nresponse time is around 27 ms ~21 ms under different temperature (Figure R12b). It means that\nthe temperature has negligible effect on the drift/response time.\nFigure R12 | Temperature-dependent photoresponse under illumination. a, Photocurrent\ntransients of the drift-free device measured in different device temperature. b, Comparison of the\nresponse times in different temperature, extracted from Figure R12a.\n(iv) Matched optical stacks on low-k vs high-k spreaders: We acknowledge the reviewer’s\nsuggestion to fabricate identical optical stacks directly on substrates with different thermal\n17\nconductivities while maintaining identical optical interference conditions. In the present study, our\nstrategy was to thermally engineer the same device architecture by placing the device on high-k\nmetal backer, rather than by growing devices on different substrates. We experimentally compared\nlow-thermal-conductivity backer PC (Polycarbonate) and high-conductivity metal backers\n(Copper) beneath the device under light illumination power density of 1.6 W cm-2. These\nmeasurements (Response Fig. 13) consistently show that high-k metal backers outperform low-k\nsupports in both drift suppression and response-time enhancement. This comparison of the\nperformance of device on the low-k and high-k backers reveals that different thermal diffusion\nmodes can directly affect the drift and response time of the device.\nFigure R13 | Effect of thermal spreader conductivity. Current-time photoresponse of devices\nbacked with a low-thermal-conductivity polymer-carbonate (PC) backer and a high-conductivity\nCu backer measured under identical power density (1.6 W cm⁻²). The Cu-backed device with\nresponse time of ~1.7 ms, the PC-backed device with response time of ~1.4 s.\nCollectively, these control experiments demonstrate that (i) the photocurrent of our device are\nnot governed by pyroelectric effects, and it is dominant by photovoltaic (PV) effect and negligible\nthermoelectric effect; (ii,iii) wavelengths and device temperature have little effect on drift and\nresponse time; (iv) thermal diffusion is the main factor in affecting the drift and response time in\nour device. The present results verify that thermal-diffusion engineering is the dominant\nmechanism of the observed drift-free behavior.\nModification to the Manuscript:\nThe following paragraph has been revised in page 5, paragraph 5, in the revised manuscript.\n18\n“To definitively attribute the observed drift-suppression to thermal management rather than other\nartifacts, we conducted dark-state control experiments under controlled temperature conditions.\nFirst, dark thermal-pulse measurement shows no pyroelectric current in either the conventional or\ndrift-free architecture (Supplementary Note. 3 and Supplementary Fig. 19).”\nModification to the Supporting Information:\nThe following paragraph has been revised in page 8, paragraph 1, in the revised\nsupplementary information.\n“Supplementary Note 3. Pyroelectric exclusion test via dark thermal pulse experiment.\nTo directly assess any potential contribution from pyroelectric effects in the photoresponse, we\nperformed controlled dark thermal‑pulse experiments on both conventional and Cu‑backed drift-\nfree devices. A resistive heater generated through a temperature modulation between\napproximately 32 °C and 60 °C, while the current response was recorded in the complete absence\nof optical illumination (Supplementary Fig. 19).\nIn both architectures, the measured current shows only a slow, monotonic rise and decay that\nfollows the heating and cooling dynamics, governed by its intrinsic thermal time constant.\nCrucially, no fast, polarity‑reversing transient; the signature of a pyroelectric current arising from\ndT/dt is observed. Furthermore, the shape and magnitude of these heater‑induced signals are\nqualitatively and quantitatively distinct from the light‑induced photocurrent transients presented\nin the main text.\nThese results confirm that pyroelectric contributions are negligible in the operating regime of\nour devices.”\nmaterials but presents primarily exemplar curves. There is no report of device counts, error bars,\ndistributions or yield, or long-term endurance beyond a single ≈3000 s trace. To support generality,\nit is necessary to provide, for each material/system, the sample size; spread for drift metrics and\nrise/decay times; yield statistics; and hour-to-day cycling data (including bias/temperature stress).\nThese statistics must be included to elevate the claims from anecdotal to robust.\nAnswer 4: We thank the reviewer for raising this important point regarding statistical robustness\nand device-to-device variability across different systems.\nFor the BFO-based devices, which constitute the primary focus of this work, we have performed\ndetailed reproducibility and materials characterization. We have expanded the experimental dataset\nto include full device statistics, sample-to-sample distributions, and long-term endurance tests.\nThese additions strengthen the robustness and generality of our conclusions. The following new\nresults have been added to the revised manuscript and Supporting Information. Lateral size: 20\nmm × 20 mm. RMS (root mean square) roughness: 1.09 - 1.24 nm. Total layers thickness of LNO\nis between 200 nm to 208 nm while the BFO layers thickness is in the range of 66 nm to 73 nm\n(Figure R14).\nFigure R14 (Supplementary Fig. 13 in the revised version) | Thickness uniformity across\nindependently fabricated devices. Cross-sectional SEM images of three representative devices\na-c, with a 150 nm scale bar, showing comparable layer thicknesses across samples. The BFO\nactive layer thickness varies slightly between ~66 and 73 nm, while the LNO bottom electrode\nthickness ranges from ~201 to 208 nm, confirming good fabrication reproducibility and overall\ncomparable device geometry.\nthree representative devices. AFM measurements for three independently fabricated devices\nshowing surface morphology and corresponding amplitude images (two panels per device) with a\n1 µm scale bar. The extracted RMS roughness values are a, 1.098 nm, b, 1.10 nm, and c, 1.245\nnm, indicating comparable surface morphology and good film uniformity across devices.\nWe performed multi-hour cycling experiments for each of the three devices (Figure R16) under\nidentical illumination conditions. Each device was measured multiple times in 2.7-hour continuous\ntraces (10 M points, 100 S s-1, time scale 1.00 ks). All three devices demonstrated about 99.3% to\n99.7% amplitude stability (defined as peak-to-peak variation relative to mean) within each 2.7 h\n21\nsegment.\nFigure R16 (Supplementary Fig. 15 in the revised version) | Long-term operational stability\nof drift-free devices. Photocurrent stability measurements for three independently fabricated\ndevices. a-c, Recorded over 10 ks (2.7 hours) under periodic illumination (1 Hz, 405 nm) with\npower density of 1.6 W cm-2 using an oscilloscope-TIA chain (TIA sensitivity 50 µA V⁻¹ and\noscilloscope 10 M points, 100 S s⁻¹, 1.0 ks time scale).\nOne device was continuously monitored for 41.6-hours at controlled laboratory temperature\n(25–26 °C) in 2.7-hours segments due to the memory limitation of the oscilloscope. The device\nmaintained > 97% amplitude stability over the full duration. No degradation was observed in rise\ntime, decay time, or shape of the waveform (Figure R17).\nphotocurrent monitoring of a representative drift-free device conducted continuously for 150 ks\n(41.6-hours) at controlled laboratory temperature (26 - 27 °C), acquired in consecutive 2.7 h\nsegments due to oscilloscope memory limitations.\nFor cross-material comparison, all the ferroelectric devices were fabricated using the same\ndeposition method (Sol-gel and spin coating), identical bottom (LNO) and top (ITO) electrode\nconfigurations, and a fixed device active area of 2 mm × 2 mm, ensuring a fair and controlled\ncomparison. While we currently report representative results for each material (Response Fig. 18),\nthe key performance metrics relevant to this work; response time and drift ratio are consistently\nextracted and directly compared for conventional and thermally engineered configurations (drift-\nfree device). The consistent reduction in drift and acceleration of response upon introducing a\nhigh-thermal-conductivity backer across all tested ferroelectrics strongly supports the generality\nof the proposed thermal diffusion engineering concept.\n23\ni\nConventional Drift-free\n104\n103\n102\n101\n100\n10-1\nh\nFigure R18 (Figs. 2i & 3h in the revised version) | Cross-material performance comparison\nof ferroelectric photodetectors. a, Response time comparison and b, drift ratio and drift rate\ncomparison for different ferroelectric materials. All devices were fabricated using identical\ndeposition methods, electrode configurations, and a fixed active area of 2 mm × 2 mm, enabling a\nfair comparison between conventional and thermally engineered architectures.\nThe following paragraph has been revised in page 5, paragraph 2, in the revised manuscript.\n“To quantitatively validate the long-term stability of drift-free architecture, we performed\nsystematic multi-hour and multi-day cycling experiments. Three independently fabricated devices\nwere measured under identical modulated illumination (1Hz, 405 nm, 1.6 W cm-2). Structural\nuniformity across these devices confirmed by SEM (Scanning electron microscopy) and AFM\n(Atomic force microscopy) (Supplementary Fig. 13, Fig. 14) shows consistent functional layers\n(BFO thickness: 66 - 73 nm; LNO thickness: 200 - 208 nm; RMS (root mean square) roughness:\n1.09 - 1.24 nm) and lateral dimensions of 2 mm × 2 mm active area of all the devices. Each device\nexhibited the exceptional amplitude stability of 99.5 - 99.7% over continuous 2.7-hour segments\n(Supplementary Fig. 15). To prove extended duration, one device was monitored for 41.6 hours in\nsequential segments, maintaining > 97% stability with no observable degradation in temporal\nwaveform shape as shown in Fig. 2h.”\n“high-fidelity imaging” and reduced crosstalk, the manuscript must include quantitative array-\nlevel metrics: crosstalk matrices (with nearest-neighbor leakage percentages), point-spread\nfunctions (PSF), modulation transfer functon(MTF), and per-pixel isolation maps derived from the\n25-channel time traces. Reporting line-pair resolution and contrast versus spatial frequency is\nnecessary. Without these quantitative measures, the imaging-centric claims remain insufficiently\nsupported.\nAnswer 5: We thank the reviewer for raising the important point that high-fidelity imaging requires\nquantitative spatial metrics. We fully agree that quantitative spatial metrics are essential to\nsubstantiate claims of high-fidelity imaging and reduced crosstalk at the array level. We have\nsignificantly expanded the spatial characterization of the detector array and now provide multiple\nquantitative measures, as detailed below.\nFirst, we performed point-spread-function (PSF) derived line-spread function (LSF) (Response\nFigure R19) by illuminated a focused optical spot (diameter ≈ 230 µm) across a single 2 mm ×\n2 mm channel at nine discrete lateral positions. For the conventional device, the normalized signal\ndrops to only ~48% at the edge of the channel, indicating a broad lateral response halo consistent\nwith strong in-plane thermal diffusion. In contrast, the Cu-backed drift-free device exhibits a rapid\nsignal decay to < 7% at the same lateral position, demonstrating a sharply localized response and\nstrongly suppressed lateral heat spreading.\nanalysis of lateral response. Normalized LSF extracted by illuminating a focused optical spot\n(~230 µm) across a single 2 mm × 2 mm channel for conventional and drift-free devices.\nSecond, we constructed quantitative crosstalk matrices by selectively illuminating channel 13\nand recording the time-resolved response of neighboring channels (Figure R20,21). In the\nconventional device, the nearest-neighbor channels (12 and 14) exhibit 10 - 11% of the primary-\nchannel signal, while next-nearest neighbors (11 and 15) show ~3 - 4% leakage. In striking contrast,\nthe drift-free device shows no measurable signal above the dark-noise floor in any adjacent channel,\ncorresponding to nearest-neighbor leakage below 1%. These results quantitatively confirm the\nsubstantial suppression of array-level crosstalk achieved through thermal-diffusion engineering.\nFigure R20 (Supplementary Fig. 33 in the revised version) | Crosstalk analysis in\nphotodetector arrays. Extracted from Figure R21. a, Crosstalk matrix obtained by selectively\nilluminating a single channel and recording responses from neighboring channels for conventional\nand drift-free devices. b, Schematic illustration of the 5 × 5 photodetector array used for crosstalk\nmeasurements.\nmeasurements. Time-resolved current responses recorded simultaneously from channels 11 - 15\nduring selective illumination of the central channel, comparing conventional a, and drift-free b,\ndevices.\nRegarding modulation transfer function (MTF) analysis, we acknowledge the reviewer’s point\nand fully agree that MTF provides a rigorous frequency-domain description of spatial resolution.\nDue to instrumental constraints and the requirement for a dedicated, high-precision optical\nscanning setup, we were not able to perform a full MTF measurement within the scope of the\npresent study. However, since the MTF is formally the Fourier transform of the LSF, the LSF data\npresented here already captures the essential spatial confinement characteristics in the spatial\ndomain. Given that the primary objective of this work is to introduce a thermally engineered drift-\nfree approach to enhance the temporal response and suppress the drift in conventional device,\nthe LSF and crosstalk analyses provide direct and sufficient quantitative evidence supporting our\nconclusions.\nWe have revised the manuscript to explicitly include these quantitative metrics and to clearly\nstate the scope and limitations of the spatial analysis.\nModification to the Manuscript:\nThe following paragraph has been revised in page 8, paragraph 1, in the revised manuscript.\n“This superior spatial fidelity is quantitatively validated by a comprehensive analysis of channel\ncrosstalk and the line-spread function (LSF), which show a seven-fold improvement in spatial\n28\nconfinement for the drift-free architecture (Supplementary Note. 7 and Supplementary Fig. 33-\n35).”\nModification to the Supporting Information:\nThe following paragraph has been added in page 12, paragraph 1, in the revised\nsupplementary information.\n“Supplementary Note 7. Quantitaive anlaysis of high fidelity and imaging\nTo quantitatively assess pixel to pixel interference, a five-channel array (channels 11 - 15)was\nevaluated by illuminating only channel 13 with a 0.7 mm radius light spot (Supplementary Fig.\n33, 34). In the conventional standing device, adjacent channels (12 and 14) exhibited the ≈ 10%\nnormalized crosstalk, confirming lateral heat-induced signal leakage. In striking contrast, the drift-\nfree device shows no detectable crosstalk in adjacent channels, demonstrating complete\nsuppression of lateral heat flow and validating the channel isolation required for high precision\nimaging.\nTo further characterize spatial resolution, line-spread function (LSF) measurements were\nperformed by scanning a 220-µm radius laser spot across channel 13 in 200-µm steps\n(Supplementary Fig. 35). The conventional device shows a broad spatial response, retaining 48%\nof the peak signal at the channel boundary due to the lateral heat halo. By contrast, the drift-free\ndevice exhibits a sharply confined spatial profile, dropping to 7% at the boundary. This seven times\nimprovement in the spatial fall-off directly confirms that thermal-diffusion engineering enhances\npoint-spread localization, reduces heat-induced smearing, and enables high-resolution detection.”\nthermal conduction. For fair isolation of thermal effects, it is required to equalize absorbed optical\npower (not just incident power) via measured reflectance/absorbance and to state the exact optical\nstack (layer materials, thicknesses, roughness) used in each comparison. Replotting key\ncomparisons under matched absorbed power is necessary; otherwise, thermal and optical effects\nremain confounded.\nAnswer 6: We appreciate the reviewer’s important observation that metallic backers may influence\noptical absorption and interference effects (e.g., etalon behavior), in addition to modifying thermal\nconduction. We fully agree that isolating thermal effects requires comparison under matched\nabsorbed optical power rather than merely equal incident power.\nWe performed a quantitative optical absorption analysis for both conventional and drift-free\ndevice architectures. The two device types share the same active-layer stack, including identical\nmaterials, deposition methods, electrode configuration, lateral device dimensions, film thicknesses,\nand surface roughness ensuring that any optical differences arise solely from the presence or\nabsence of the metallic backer rather than from variations in the functional layers. The following\nnew results have been added to the revised manuscript and Supporting Information. Lateral size:\n20 mm × 20 mm. RMS roughness: 1.09-1.24 nm. Total layers thickness of LNO is between 200\nnm to 208 nm while the BFO layers thickness is in the range of 66 nm to 73 nm as shown in\nResponse Fig. 13, 14 in comment 4.\nOptical measurements were carried out using an OceanView spectrometer equipped with a fiber-\noptics collection probe (Response Fig. 22c,d). All experimental parameters, including illumination\ngeometry, optical alignment, probe position, and reference calibration were kept strictly identical\nfor both device types; only the substrate condition (conventional freestanding vs. metallic backer)\nwas exchanged to ensure a fair comparison. A collimated 405 nm laser beam was incident at an\nangle of approximately 5 - 7° relative to the surface normal. For transmission measurements, the\nrear-side probe was positioned at the angle yielding maximum transmitted intensity, while the\nprobe on the front side simultaneously collected reflected light under the same geometric\nconditions.\nSince optical transmission occurs only for the conventional device, the absorbed photon flux\nwas evaluated using the standard relation:\nA(λ)=1−R(λ)−T(λ),\nwhere A , R and T represent absorbance, reflectance, and transmittance, respectively.\nmultiple samples and independently confirmed by UV-Vis spectroscopy (Response Fig. 22a,b).\nThe reflectance of the conventional devices lies in the range of 46 - 47%, whereas the metallic-\nbacked devices (Cu, SS, Al) show slightly higher reflectance of 47 - 48%, attributable to the mirror-\nlike nature of the metal interface. Based on the above relation, the conventional devices absorb\napproximately 51 - 52% of the incident 405 nm light, while metallic-backed devices absorb\napproximately 52 - 53%. The resulting difference in absorbed optical power between the two\narchitectures is < 1%. Therefore, under identical light absorption measurement conditions, the\nabsorption of BFO on different backers is almost similar and difference can be neglected.\nFigure R22 (Supplementary Fig. 30 in the revised version) | Optical transmittance and\nreflectance characterization. a, UV-Vis spectrophotometer transmittance spectra for three\ndifferent samples. b, Enlarged view of the dotted region in (a) highlighting the transmittance at\n405 nm, which lies in the range of ~0.9 - 1.9%. c, Wavelength-intensity spectra measured in the\ndesigned optical path (inset of (c))are at 405 nm using an OceanView spectrophotometer, showing\nthe transmitted (Trans.) intensity of the conventional device and the reflectance (Refl.) of\n31\nconventional freestanding (FS), Cu-backed, Al-backed, and stainless-steel (SS)-backed devices\nunder identical illumination power density. d, Zoomed-in view of the dotted region in (c) to\nemphasize differences in reflected intensity among the conventional FS and different backers.\nThese results demonstrate that the pronounced suppression of photocurrent drift and the\nsubstantial acceleration of response time in the drift-free devices cannot be attributed to optical\nabsorption or interference effects.\nModification to the Manuscript:\nThe following paragraph has been revised in page 6, paragraph 2, in the revised manuscript.\n“Optical absorption measurements confirm that the drift suppression is purely thermal, as all\nmetallic backers (Cu. Al, SS) exhibit nearly identical absorption to the freestanding device;\nconsistent with its minimal transmittance (0.6-1.7%), ruling out significant optical interference\neffects (Supplementary Note. 5 and Supplementary Fig. 30).”\nModification to the Supporting Information:\nThe following paragraph has been added in page 10, in the revised Supplementary\ninformation.\n“Supplementary Note 5. Absorption measurement of conventional and metalic-backed drift-\nfree devices.\nTo ensure a fair comparison between the conventional and drift-free device architectures, we\nperformed a careful evaluation of their optical absorption using an OceanView spectrometer\nequipped with a fiber-optics collection probe. The experimental setup was designed so that all\nconditions, including illumination geometry, probe position, optical alignment, and reference\ncalibration, remained strictly identical for both device types; only the device substrate was\nexchanged during measurements to maintain accuracy. A collimated 405 nm beam was incident at\nan angle of ≈5-7°with respect to the surface-normal. The transmitted light was collected by\npositioning the rear-side probe at the angle that yielded maximum transmitted intensity, and a\nsecond probe was aligned on the front side for reflectance measurements under the same geometric\n32\nconfiguration. Because transmission occurs only through the conventional mica device, this\nconfiguration allows a direct comparison of absorbed photon flux using the standard expression:\nA(λ)=1−R(λ)−T(λ),\nwhere A is absorbance, R is reflectance, and T is transmittance.\nThe conventional device exhibits very small transmittance, 0.6-1.7%, consistent across multiple\nsamples confirmed by independent UV-Vis spectroscopy (Supplementary Fig. 30a). The\nreflectance of the conventional device lies between 46 - 47%, while metallic-backed devices (Cu,\nNi, Al) display slightly higher reflectance of 47 - 48%, attributable to mirror-like metallic\ninterfaces. Using the formula above, the conventional device absorbs ≈51 - 52% of the incident\n405 nm light, whereas metallic-backed devices absorb ≈52 - 53%. The difference between the two\nstructures is < 1%, which is optically negligible, demonstrating that the Cu-backed devices do not\ngain any optical advantage from enhanced absorption (Supplementary Fig. 29b). Instead, the\ndramatic improvements in drift suppression and response time originate purely from thermal\nengineering rather than optical pathways.\nTo ensure that these optical comparisons were not influenced by variations in the oxide stack\nitself, we also characterized the same sets of devices using cross-sectional SEM and AFM. The\ntotal thickness of LNO layers and BFO layers was consistently 272 - 274 nm across all device\ntypes (Supplementary Fig. 13), and the surface roughness remained uniform within 1.09 - 1.24 nm\nRMS (Supplementary Fig. 14). This confirms that the film morphology, crystallinity, and interface\nquality are essentially identical in all cases, and therefore the observed thermal-response\ndifferences cannot be attributed to variations in optical absorption, thickness, or roughness.”\nthrough Cu (e.g., “reduced activation of trapping centers”) yet trapping/detrapping kinetics are\nprimarily governed by the semiconductor and its interfaces—defect chemistry, interface states, and\nband alignment—while Cu chiefly modifies the thermal profile, not the trap spectrum. It is\nrequired to decouple thermal management from trap physics; this requires, for example, identical\nstacks on Fe/Al/Cu with matched thickness/roughness to separate thermal conductivity from work-\nfunction/optical changes; absorbed-power-matched multi-wavelength tests; and interfacial\nforensics to exclude Cu diffusion or unintended interlayers. The discussion must reconcile any\nobserved leakage or power-law changes that could arise from barrier/interface modifications rather\nthan cooling alone.\nAnswer 7: We thank the Reviewer for this insightful and important critique. Based on the\nreviewer’s suggestions and the additional experiments, we confirm that trap centers are indeed\npresent in our material; however, the introduction of Cu or other metallic backers does not alter\ntrapping/detrapping kinetics.\nTo address the need to decouple thermal effects from electronic modifications, new controlled\nexperiments were performed. Specifically, dark current-voltage (I-V) characteristics for both\nconventional and Cu-backed drift-free devices were measured on a temperature-controlled stage\nacross a range of 305 to 330 K (Response Fig. 23a,b). Where the heater was adjusted to establish\nthe exact same external ambient temperature for the conventional and Cu backed devices during\neach measurement. This setup creates a uniform thermal environment, intentionally nullifying the\ndifferential heat-sinking capability of the Cu backer during normal operation. Under these\nconditions the log(I)-log(V) data for each temperature were fitted to a linear model. The extracted\nslopes (Response Fig. 23c,d) show comparable values and nearly identical temperature-dependent\ntrends: slopes for the conventional device changed between 1.27 to 1.22, while for the Cu-backed\ndevice changed between 1.30 to 1.26 over the temperature range. The close similarity in both\nabsolute values and temperature dependence confirms that the fundamental charge transport\nphysics, dictated by the same trap/defect landscape, remains unchanged by the presence of the Cu\nbacker.\nV analysis. a,b, Dark current-voltage (I-V) characteristics of the conventional and Cu-backed\ndrift-free devices, respectively, measured on a temperature-controlled stage at identical ambient\ntemperatures ranging from 305 to 330 K. c,d, Corresponding log(I)-log(V) plots for the\nconventional and drift-free devices, respectively, with linear fits used to extract slopes at each\ntemperature, showing comparable values and similar temperature dependence across the measured\nrange.\nPlotting the extracted slope as a function of temperature yields rates of change of -0.49 K⁻¹ for\nconventional and -0.38 K⁻¹ for the Cu-backed device (Response Fig.24). The slightly smaller\nmagnitude for the Cu-backed case is attributed to its higher thermal mass. The similar magnitude\nand trend confirm that the voltage-scaling behavior of the current remains unchanged by the Cu-\nbacker, indicating that its role is limited to thermal management rather than trap physics.\npower-law exponent. Extracted from log(I)-log(V) fitting of dark I-V characteristics for\nconventional and Cu-backed drift-free devices measured between 305 and 330 K. Linear fits yield\nslopes of -0.49 K-1 for the conventional device and -0.38 K-1 for the drift-free.\nRegarding the suggestions for control experiments to isolate thermal diffusion from other\ninfluence factors, several relevant tests have been conducted, as also noted in responses to\nComments 3 and 6. Optical absorption measurements confirmed that the absorption of the\nBFO/mica stack is virtually identical when placed on Cu, Al, or SS backers, therefore, metallic\nbacker do not alter the optical absorption and interference effects. Furthermore, multi-wavelength\nphotoresponse tests (405 nm, 450 nm, 532 nm) showed consistent enhancement in response speed\nand drift suppression for the Cu-backed device across all wavelengths, as already discussed in\ncomment 3.\nRegarding interfacial forensic analysis, it is important to clarify the device architecture. The\nBFO device is fabricated on a mica substrate; the Cu backer is physically placed beneath the mica\nwithout any direct chemical or deposition-based contact with the BFO layer. Consequently, there\nis no Cu-backer/BFO interface to analyze, and phenomena such as unintended interlayer formation,\nor modification at the BFO/electrode interfaces are precluded by design. The Cu backer functions\nexclusively as a remote heat sink, mechanically separated from the active layers. Therefore, the\n36\nchanges in leakage current or power-law behavior arise from modified thermal dissipation\npathways, not from electronic interface alterations.\nTo avoid potential misinterpretation of thermal diffusion as a modification of electronic trap\nstates, the trap-related schematic elements previously shown in Fig. 1b and Fig. 1d have been\nremoved in the revised manuscript. The updated figures now focus solely on heat-flow control and\nsuppression of lateral thermal accumulation. Accordingly, all trap-related discussions in the main\ntext and Supplementary Information have been revised to ensure that the Cu backer is presented\nstrictly as a thermal management element, without implying changes to defect chemistry or trap\nkinetics.\nModification to the Manuscript:\nThe following paragraph has been revised in page 1, paragraph 1, in the revised manuscript.\n“However, their practical deployment has been hindered by slow temporal response and\nphotocurrent drift, mainly due to inefficient thermal dissipation that leads to temperature buildup\nwith in the device under continuous illumination. “\nThe following paragraph has been revised in page 1, paragraph 1, in the revised manuscript.\n“Infrared thermography and COMSOL simulations confirm distinct thermal environment in\nconventional and drift-free devices, revealing strong heat accumulation in conventional and\nefficient heat extraction in drift-free architectures.”\nThe following paragraph has been revised in page 2, paragraph 3, in the revised manuscript.\n“These limitations originate from illumination-induced heating and insufficient heat dissipation\nwithin the device. When the thermal environment is not well controlled, accumulated heat\nbroadens the temporal response, reduces carrier extraction efficiency and destabilizes the steady\nstate output. These thermally driven effects have been widely observed in oxide-based\nphotodetectors26-29, emphasizing that local heat distribution, is a critical but often overlooked\ndeterminant of device stability and dynamic performance.”\nThe following paragraph has been revised in page 2, paragraph 3, in the revised manuscript.\n37\n“Using BFO-based films as a model system, we demonstrate that thermal confinement suppresses\nlateral heat diffusion, thereby maintaining stable thermal environment under continuous\nillumination of light.”\nThe following paragraph has been revised in page 6, paragraph 1, in the revised manuscript.\n“Second, temperature-dependent dark I-V characteristics are identical for both devices\nconventional and drift-free (Supplementary Note. 4 and Supplementary Fig. 20, 21), proving the\nCu backer solely modifies the thermal boundary condition without affecting interfacial chemistry\nor intrinsic charge transport.”\nModification to the Supporting Information:\nThe following paragraph has been added in page 9, in the revised supplementary information.\n“Supplementary Note 4. Verification of unchanged interfacial chemistry via dark I-V\nanalysis.\nSupplementary Note 4. Verification of unchanged interfacial chemistry via dark I-V analysis.\nTo decouple the thermal role of the Cu backer from potential electronic or chemical\nmodifications, we performed controlled dark current-voltage (I-V) measurements on both\nconventional and Cu‑backed drift-free devices under identical external thermal conditions.\nThe devices were placed on a temperature‑controlled stage and measured across a temperature\nrange of 305 K to 330 K. Crucially, the stage was used to establish the same ambient temperature\nfor both conventional and drift-free devices during each measurement, thereby intentionally\neliminating the differential heat‑sinking capability of the Cu backer. This ensures that any\nobserved differences in the I-V characteristics can be attributed solely to electronic or interfacial\nchanges, rather than to differing thermal environments during the measurement.\nThe resulting log(I)-log(V) data are shown in Supplementary Fig. 20a,b. The slopes extracted\nfrom linear fits across the voltage range exhibit nearly identical values and temperature‑dependent\ntrends (Supplementary Fig. 20c,d for the conventional device, the slope decreases from 1.27 to\n38\n1.22 with increasing temperature, while for the Cu‑backed drift-free device it decreases from 1.30\nto 1.26 over the same range.\nThe close agreement in both the absolute slope values and their temperature dependence\nconfirms that the fundamental charge‑transport mechanism, governed by the same trap‑limited\nconduction landscape, remains unchanged by the presence of the Cu backer. This result validates\nthat the Cu substrate functions exclusively as a thermal sink and does not alter the interfacial\nchemistry or defect kinetics of the active ferroelectric stack.\nPlotting the extracted slope as a function of temperature yields rates of change of -0.49 K⁻¹ for\nconventional and -0.38 K⁻¹ for the Cu-backed device (Supplementary Fig.21). The slightly smaller\nmagnitude for the Cu-backed case is attributed to its higher thermal mass. This experiment\nindicates that the Cu backer does not alter trap kinetics; its influence is purely thermal.”\nconduction with minimal recombination losses. However, an exponent <1 can also indicate trap-\nmediated recombination and other non-idealities. It is necessary to validate this interpretation via\ntemperature-dependent and intensity-dependent exponent maps, transient rise/decay analyses, and\ncontact-geometry controls (e.g., varying electrode work function or area). Without these\ncorroborations, the SCLC assignment is not unique, and the mechanism remains ambiguous.\nAnswer 8: We thank the Reviewer for the insightful observation that a photocurrent-intensity\nexponent less than unity is not a unique signature of space-charge-limited conduction (SCLC).\nWe have investigated this by adding new experiments. As detailed in our response to Comment 7,\ncomprehensive dark current-voltage (I-V) analysis of both device types revealed log(I)-log(V)\nslopes in the range of 1.2 to 1.3 across a temperature range of 305 to 330 K. These values rule out\nthe presence of a trap-free SCLC regime, which is characterized by a slope of 2 or greater.\nTherefore, any prior assignment of the device's behavior to SCLC was incorrect. We have removed\nall SCLC interpretations from the revised manuscript.\nIn response to both this comment and the reviewer’s request in Comment 2 to extend\nmeasurements beyond the high-irradiance regime, we performed a complete intensity sweep\nspanning from the 80 mW cm-2 to 2.8 W cm-2 range under identical optical and electrical conditions.\nA single power-law fit to this complete dataset for the steady-state photocurrent yields overall\nscaling exponents of 1.09 for the conventional device and 0.92 for the Cu-backed drift-free device\n(Figure R25).\nPower-law scaling exponents of 1.09 (conventional device) and 0.92 (drift-free device) across an\nextended intensity range (80 mW cm-2 to 2.8 W cm-2).\nWe present this strictly as an empirical power-law relationship, a standard metric for\nphotoconductor performance, without attributing it to a specific bulk conduction mechanism. We\nnote that the previously reported higher exponents (conventional ≈ 1.21, drift-free ≈ 0.87) were\nextracted from a restricted high-irradiance subset, originally chosen to highlight drift suppression\nand response stability. Extending the analysis to lower irradiance provides a more comprehensive\nand conservative assessment, without altering the central conclusions.\nTo further contextualize this scaling behavior, we correlated the intensity-dependent\nmeasurements with power-dependent transient response analysis. The rise time increases\nmoderately with increasing optical power, while the decay time remains nearly invariant, behavior\nconsistent with thermal accumulation dynamics rather than trap-controlled conduction.\nWe thank the Reviewer for highlighting the importance of additional contact-geometry controls,\nwhich indeed constitute a valuable strategy for further decouple contact from bulk and thermal\nphysics. While a systematic variation of electrode work function and area is beyond the scope of\nthe present study, and we have already isolated the impact of the thermal boundary condition on\n41\nphotocurrent drift and response metrics. As the dark current-voltage characteristics (see Comment\n7) revealed, the comparable transport behavior for devices with and without the Cu backer,\nindicating that changes in contact-limited conduction are unlikely to be the primary origin of the\nobserved improvements.\nModification to the Manuscript:\nThe following paragraph has been revised in page 4, paragraph 1, in the revised manuscript.\n“The gain power scaling of the two devices architectures (Fig. 2g, Supplementary Fig. 5, 6),\nmeasured over illumination power density range of 80 mW cm-2 to 2.5 mW cm-2, reveals distinct\nthermal contributions to photocurrent generation. The conventional device exhibits a super-linear\npower-law dependance with a slope of 1.09, indicating that photocurrent increases faster than the\nincident photon flux. Such super-linearity is a well-known signature of thermally assisted carrier\nactivation, where local temperature rise under continuous illumination enhances carrier population\nbeyond direct photogeneration37. In contrast, the drift-free device shows a near-linear scaling with\na slope of 0.92 indicating that the photocurrent is governed primarily by direct photocarrier\ngeneration and extraction. The suppression of super-linear gain reflects efficient vertical heat\ndissipation, which prevents thermal accumulation and minimizes temperature-induced carrier\namplification.”\nsensing chain used to report current from the recorded voltages. It is required to include a clear\nmeasurement-circuit schematic and parameters. These details are necessary to interpret both the\nabsolute current levels and the claimed temporal response.\nAnswer 9: We thank the reviewer for highlighting the need for explicit documentation of the\nmeasurement chain. In all photocurrent and transient measurements, the oscilloscope recorded\nvoltage outputs from a transimpedance amplifier (TIA) rather than current directly. To clarify this,\nwe have added a complete measurement schematic to the Methods section (Figure R25), detailing\nall components and parameters used. Specifically, a Tektronix MDO3024 oscilloscope was\nconnected to a Stanford Research SR570 low-noise TIA, which converted the device current into\na measurable voltage using well-defined transimpedance gains. The gain settings used in the\nmanuscript depended on dynamic range requirements. Modulated illumination was generated\nusing a Stanford DS360 ultra-low-distortion function generator that provided frequency-controlled\nmodulation to the 405 nm laser. All temporal responses, rise/decay times, and Bode plots were\nobtained from the voltage traces after conversion using the known TIA gain. These details are\nexplicitly stated in the Methods section.\nFigure R26 (Supplementary Fig. 36 in the revised version) | Measurement chain schematic.\nA detailed illustration of the photocurrent and transient measurement setup. Device current\nis measured via a Stanford Research SR570 transimpedance amplifier (TIA) and recorded as a\nvoltage using a Tektronix MDO3024 oscilloscope. Modulated illumination is provided by a 405\nnm laser driven by a Stanford DS360 function generator.\nModification to the Manuscript:\nThe following paragraph has been revised in page 9, paragraph 5, in the methods section of\nthe revised manuscript.\n43\n“Device characterization and measurements.\nPhotoresponse measurements were performed using a combination of a Keithley 6514\nelectrometer and Keithley 2611B source-meter for low-noise current detection and electrical\ncharacterization. For time resolved measurements, the photocurrent signal was amplified using a\nlow-noise current amplifier (Stanford Research system SR570) with suitable sensitivity with\nrespect to the intensity of light and recorded with a mixed-domain oscilloscope (Tektronix MDO\n3024). A low distortion function generator (Stanford Research system DS360) was used to\nmodulate the laser intensity for frequency-dependent and transient response measurements.\nCalibrated laser source (MLL-III, DE52509) with wavelength 405 nm and power density of 320\nmW cm-2 to 2.5 W cm-2 were employed, with the optical power monitored using a calibrated power\nmeter (OPHIR PD300) (Supplementary Fig. 36).”\nThis manuscript presents a novel thermal-diffusion engineering strategy for ferroelectric\nphotodetectors, achieving a remarkable improvement in response speed (>10³×) and complete\nsuppression of photocurrent drift. Studying is motivated, as slow dynamics and instability are long-\nstanding challenges in self-powered FE devices. The combination of infrared thermography and\nCOMSOL simulations provides convincing support for the proposed mechanism. Moreover, the\narchitectural focus on controlling vertical versus lateral heat flow is original and complementary\nto the more common material/interface engineering approaches.\nTo further strengthen the impact and clarity of the work, I suggest the authors address the following\npoints:\nComment 1: The manuscript claims a >10³× improvement in photoresponse speed, but absolute\nrise/fall times are not directly compared to state-of-the-art FE–PV and pyro-phototronic detectors.\nPlease provide explicit numerical values (µs/ms scale) and include a benchmarking table against\nrelevant literature.\nAnswer 1: We thank the reviewer for the valuable suggestion regarding quantitative benchmarking\nof photoresponse speed against state-of-the-art ferroelectric photovoltaic (FE-PV) and pyro-\nphototronic photodetectors. We have added a table that summarizes the reported rise/fall times,\noperating mechanisms, and key device characteristics of representative FE-PV and FE pyro-\nphototronic photodetectors from the recent literature. Table 1 enables direct comparison of\nabsolute rise/fall times rather than relative improvement factors alone.\nTable 1: Comparison of rise/fall times of state-of-the-art pyro-phototronic and FE-PV\nphotodetectors.\nReported Reported\nDominant\nrise time Fall time\nMaterial / Architecture Reference\nmechanism\n(ms) (ms)\nFE-Pyro-\nAg/BNT/ITO ~84.2 ~54.3 [1]\nPhototronic\nPEN/ITO/TMIMPbI - FE-Pyro-\n3\n35 36 [2]\nMAPbI P HT/Ni/Au Phototronic\n3 3\nBi FeO ~6 ~15 [3]\nx 3\nphotovoltaic\nFerroelectric\nBi Sm Fe Co O ~7 ~96 [4]\nx 0.075 0.95 0.05 3\nphotovoltaic\npristine lead lanthanum Ferroelectric\n9.5 11.5 [5]\nzirconate titanate (PLZT) photovoltaic\nFerroelectric\ngraphene-based LiNbO 23 23 [6]\n3\nphotovoltaic\nFerroelectric\nSnS NW 56 91 [7]\n2\nphotovoltaic\nFerroelectric\nBaTiO /p-GaN/Au 40 40 [8]\n3\nphotovoltaic\nFerroelectric\nCu/LNO/BFO/ITO ~0.25 ~0.1 This work\nphotovoltaic\nFrom this comparison, it is evident that FE-PV photodetectors typically exhibit response times\nin the tens of millisecond-to-second range. In contrast, our drift-free device demonstrates\nmillisecond-to-sub-millisecond response times, representing a clear improvement over FE-PV\ndevices and placing it among the fast ferroelectric photodetectors. The thermal-diffusion\nengineering enables a unique combination of fast response and drift-free operation in ferroelectric\nphotodetectors.\n1. Liu, Y, et al. Nano Energy 98, 107312 (2022).\n2. Yang, Z, et al. Nano Energy 102, 107743 (2022).\n3. Li, Ze. et al. Ceramic International 48, 2811-2819 (2022).\n4. Xi, G. et al. Ceramic International 48, 15414-15421 (2022).\n5. Huangfu, G, et al. ACS Applied Mater. & Inter. 12, 33950-33959 (2020).\n6. Guan, H, et al. Adv. Opt. Mater. 9.,2100245 (2021).\n46\n7. Zheng, D, et al. Nanophotonic. 10, 4637-4644 (2021).\n8. Han, W, Chin. Phys. B 33, 047701 (2024).\nModification to the Manuscript:\nThe following paragraph has been revised in page 4, paragraph 2, and paragraph 3, in the\nrevised manuscript.\n“The millisecond-scale response time of the drift-free device is further substantiated by our\nfrequency-domain analysis, where the extracted -3dB bandwidth and corresponding phase lag fall\nwith in the 3-5 kHz range (Supplementary Note 1 and Supplementary Fig. 3). Consistent with this\nbandwidth, a direct time-domain measurement at 100 Hz modulation yields a stable undistorted\nwaveform and a response time of 0.25 ms (Supplementary Fig. 4), confirming operation in the\nkilohertz regime.\nThe gain behavior at low-frequencies (50 mHz - 10 Hz), further highlights crucial role of\nthermal management. The conventional device suffers a ⁓20% gain roll-off below 5 Hz (Fig. 2c),\nindicative of the system’s instability to reach the thermal equilibrium under modulated\nillumination. The drift-free device (Fig. 2f) demonstrates exceptional stability with a gain variation\nof < 5% across the same range. This flat frequency response is a direct consequence of the\nsuppressed thermal time constant.”\nThe following paragraph has been revised in page 5, paragraph 3, in the revised manuscript.\n“This performance is against state-of-the-art ferroelectric photovoltaic (FE-PV) and pyro-\nphototronic photodetectors, with the results compiled in Supplementary Table 1, confirming the\ncompetitive advantage of drift-free architecture.”\nModification to the Supporting Information:\nThe following paragraphs have been revised in page 5, paragraph 1,2, in the revised supporting\n47\ninformation.\n“Supplementary Note 1. Frequency-domain characterization and bandwidth extraction.\nTo quantify the temporal bandwidth of the drift‑free devices, frequency‑domain Bode‑plot\nmeasurements were performed. A lock‑in amplifier (LIA) with a 24 dB/oct roll‑off was used in\nconjunction with an SR570 low‑noise transimpedance amplifier (TIA) under 405 nm illumination\nat a power density of 1.6 W cm-2 The normalized photocurrent gain and phase shift were recorded\nover a frequency range from 100 Hz to 15 kHz for three independently fabricated devices.\nThe -3 dB cut‑off frequencies extracted from the gain profiles range from 3.1 kHz to 4.3 kHz,\nwith corresponding -45° phase‑shift frequencies between 4.6 kHz and 5.5 kHz (Supplementary\nFig. 3). This close agreement among devices confirms good reproducibility and establishes the\nintrinsic -3 dB bandwidth of the drift-free architecture in the kilohertz regime, consistent with the\nmillisecond‑scale response times observed in transient measurements.”\ncomprehensive comparison of photoresponse speeds between conventional and drift-free devices\nacross a broad range of light intensities (from mW/cm² to W/cm²). Including this analysis would\nmake the claims more rigorous.\nAnswer 2: We thank the reviewer for this important comment and agree that a comprehensive\ncomparison of photoresponse speeds is necessary to rigorously support the reported speed\nenhancement.\nWe have performed power density-dependent temporal response measurements over a broad\nrange of illumination intensities for both the conventional and drift-free BFO devices (320 mW\ncm-2 ~ 2.24 W cm-2) in Figure R27. The conventional device exhibits a strong intensity dependence\nof response time, with rise times increasing from approximately ~630 ms to ~1648 ms as the\nillumination power increases. In contrast, the drift-free device maintains a nearly constant and\nsignificantly faster response, with rise times ranging from ~0.8 ms to ~1.7 ms across the same\npower range. The resulting speed enhancement remains close to three orders of magnitude\nthroughout the entire intensity window.\nThese results demonstrate that the response-time improvement of the drift-free device is robust\nover a wide range of illumination powers, rather than being limited to a specific operating\ncondition.\nFigure R27 (Figs. 2b,e in the revised version) | Temporal photocurrent response vs power\ndensity. Comparison of b, conventional and e, drift-free device responses under 50 mHz frequency\nmodulation, over an incident optical power density range of 320 mW cm-2 to 2.24 W cm-2 with an\nactive area of 2 mm × 2 mm.\nModification to the Manuscript:\nrevised manuscript.\n“The influence of illumination power density on response time is further elucidating the thermal\nmanagement role. For both architectures the response time decreases with increasing power\ndensity (Fig. 2b, 2e). However, the conventional device’s response time (630 ms - 1648 ms) is\nconsistently three orders of magnitude slower than that of the drift-free device (0.8 ms-1.7 ms)\nacross the measured illumination power density range. This vast difference underscores that the\ndrift-free architecture, by providing a low thermal-resistance pathway, recovers the intrinsic\nelectronic response time of the BFO layer.”\nconventional-BFO device shows a sharp initial rise in Isc followed by a gradual increase, whereas\nin Fig. 4a the conventional BFO device exhibits a sharp rise followed by a gradual decay. In\nSupplementary Figs. 24–25, conventional-BFO shows a fast rise followed by a linear decay. These\nbehaviors cannot all be explained by the model proposed in the main text. The authors should\nclarify these discrepancies.\nComment 4: Similar inconsistencies are observed for drift-free-BFO devices. In Fig. 2d, Isc\nincreases linearly after the initial sharp rise; in Supplementary Fig. 24, it decreases linearly; and in\nSupplementary Fig. 25, it decreases non-linearly. The authors should provide a clear explanation\nfor these variations.\nAnswer 3 & 4: We thank the reviewer for carefully examining the transient waveforms and for\npointing out the apparent inconsistencies across different figures. Below we clarify the origin of\nthe observed variations and describe the revisions made to improve consistency and transparency.\nThe signals shown in Fig. 2a and Fig. 2d and those presented in Supplementary Figs. 43-46\n(Supplementary Fig. 24-27 in 1st version) were acquired using different measurement modes,\ninstrumentation, and signal types, each chosen for a distinct experimental purpose. Figures 2a and\n2d present time-resolved photocurrent transients of the conventional-BFO and drift-free-BFO\ndevices, respectively, measured directly in current mode using a Keithley 6514 electrometer. These\nmeasurements reflect the intrinsic photocurrent dynamics of the devices under uniform\nillumination and form the basis of the response time and drift analyses discussed in the main text.\nIn contrast, Supplementary Figs. 43-46 (Supplementary Fig. 24-27 in 1st version) display\nvoltage transients, not current signals, acquired using an NI PXIe-1082 system with a LabVIEW\nvoltage-mode acquisition chain under patterned illumination (X and Z masks). The purpose of\nthese measurements is not to extract intrinsic photoresponse kinetics, but rather to evaluate lateral\nheat spreading and inter-channel crosstalk across the device. Because voltage-mode readout\nintegrates device impedance, parasitic capacitance, and amplifier input characteristics, the\nresulting waveforms can exhibit linear or nonlinear decay profiles that differ from those observed\nin direct current-mode measurements.\nTo further address the reviewer’s concern, we have now performed additional quantitative\n51\ncrosstalk and line-spread-function (LSF) measurements using the same current-mode\nconfiguration (Keithley 6514) combined with a microscope-assisted localized illumination setup\n(Figure R28). These measurements yield current transients with waveform characteristics\nconsistent with those shown in Fig. 2a and Fig. 2d, thereby confirming that the previously observed\ndiscrepancies originated from differences in signal type and acquisition methodology rather than\nfrom inconsistencies in device physics. The new results have been added to the revised manuscript,\nand the corresponding discussion has been expanded.\nFigure R28 (Supplementary Fig. 34 in the revised version) | Time-resolved crosstalk\nmeasurements. Time-resolved current responses recorded simultaneously from channels 11 - 15\nduring selective illumination of the central channel, comparing a, conventional and b, drift-free\ndevices. The drift-free device exhibits negligible transient signals in neighboring channels.\nRegarding Fig. 4a, we clarify that this figure is schematic in nature and does not represent raw\nexperimental data. It was originally drawn to qualitatively illustrate the crosstalk mechanism\ninferred from the voltage-mode measurements. Based on the reviewer’s comment, we recognize\nthat the schematic could be misinterpreted as a direct experimental waveform. Accordingly, in the\nrevised manuscript we have redrawn Fig. 4a to reflect the experimentally measured current-mode\ntransient behavior, ensuring consistency with the photocurrent dynamics shown in Fig. 2.\nconceptual representation of the device's transient photocurrent behavior, revised to reflect actual\ncurrent-mode measurement dynamics.\nIn summary, the different transient shapes reported in the manuscript arise from, current-mode\nversus voltage-mode detection, distinct measurement objectives (intrinsic response vs. crosstalk\nanalysis), and instrument-specific response characteristics, rather than from contradictions in the\nproposed physical mechanism. We have revised the text, figures, and schematics to make these\ndistinctions explicit and to present a more coherent and self-consistent description of the device\nbehavior.\nWe thank the reviewer again for this insightful comment, which has helped us significantly\nimprove the clarity and rigor of the manuscript.\nModification to the Manuscript:\nThe following lines have been revised in page 8, paragraph 1, in the revised manuscript.\n“This superior spatial fidelity is quantitatively validated by a comprehensive analysis of channel\ncrosstalk and the line-spread function (LSF), which show a seven-fold improvement in spatial\n28\nconfinement for the drift-free architecture (Supplementary Note. 7 and Supplementary Fig. 33-\n35).”\nThe following paragraph has been added in page 10, in the revised supplementary information.\n“Supplementary Note 7. Quantitaive anlaysis of high fidelity and imaging\nTo quantitatively assess pixel to pixel interference, a five-channel array (channels 11 - 15)was\nevaluated by illuminating only channel 13 with a 0.7 mm radius light spot (Supplementary Fig.\n33, 34). In the conventional standing device, adjacent channels (12 and 14) exhibited the ≈ 10%\nnormalized crosstalk, confirming lateral heat-induced signal leakage. In striking contrast, the drift-\nfree device shows no detectable crosstalk in adjacent channels, demonstrating complete\nsuppression of lateral heat flow and validating the channel isolation required for high precision\nimaging.\nTo further characterize spatial resolution, line-spread function (LSF) measurements were\nperformed by scanning a 220-µm radius laser spot across channel 13 in 200-µm steps\n(Supplementary Fig. 35). The conventional device shows a broad spatial response, retaining 48%\nof the peak signal at the channel boundary due to the lateral heat halo. By contrast, the drift-free\ndevice exhibits a sharply confined spatial profile, dropping to 7% at the boundary. This seven times\nimprovement in the spatial fall-off directly confirms that thermal-diffusion engineering enhances\npoint-spread localization, reduces heat-induced smearing, and enables high-resolution detection.”\nferroelectrics, please clarify whether the observed speed enhancement originates solely from\nreduced polarization screening, or if pyroelectric contributions also play a role.\nAnswer 5: We thank the reviewer for raising the possibility of pyroelectric contributions. We\nperformed the most direct and discriminating controls requested and report the results below. The\nnew data substantially isolates the photovoltaic contribution from photo-thermal/pyroelectric\neffects and support the conclusion that thermal-diffusion engineering is the principal mechanism\nthat reduces photocurrent drift.\nIn ferroelectric photodetectors, polarization screening refers to the gradual compensation of\nferroelectric polarization charges by mobile carriers and defects, which weakens the internal field\nand slows the photoresponse. In our devices, the ferroelectric layer, interfaces, and electrode\nconfiguration are identical for conventional-BFO and drift-free-BFO; therefore, the intrinsic\npolarization screening mechanisms are not modified by the Cu backer.\nTo distinguish photovoltaic and pyroelectric contributions, we performed dark thermal-pulse\nexperiments on our drift-free device. The drift-free device was subjected to different temperature\nchange and the corresponding currents were measured in Figure R30. The measured signals exhibit\nonly slow, monotonic rise and decay consistent with heating and cooling governed by the thermal\ntime constant of the device-substrate system. Importantly, no fast, polarity-reversing transient;\ncharacteristic of pyroelectric current associated with dT/dt was observed. Therefore, our output\nphotoelectric signal does not contain a pyroelectric component but does exhibit a certain\nthermoelectric contribution. However, the thermoelectric signal generated by a temperature change\nof approximately 30 °C is only on the nano-ampere level (Figure R30), whereas the temperature\nrise induced by illumination in our samples is only 6 °C yet produces a photocurrent of 106 µA\n(Fig. 2d). Therefore, the output of our device is dominated by the photovoltaic signal, no pyro-\nelectric effect, and the minor thermoelectric component can be safely neglected.\nwithout illumination. a-c, Drift-free device was measured over a temperature range of 30 °C -\n60 °C in the dark (∆T = 5.5 - 27 ºC) by heating, showing a function of temperature under different\nthermal conditions.\nModification to the Manuscript:\nThe following paragraph has been revised in page 5, paragraph 5, in the revised manuscript.\n“To definitively attribute the observed drift-suppression to thermal management rather than other\nartifacts, we conducted dark-state control experiments under controlled temperature conditions.\nFirst dark thermal-pulse measurement shows no pyroelectric current in either the conventional or\ndrift-free architecture (Supplementary Note. 3 and Supplementary Fig. 18) confirming the\nphotresponse is purely optoelectronic. “\nThe following paragraph has been added in page 8, in the revised supplementary information.\n“Supplementary Note 3. Pyroelectric exclusion test via dark thermal pulse experiment.\nTo directly assess any potential contribution from pyroelectric effects in the photoresponse, we\nperformed controlled dark thermal‑pulse experiments on both conventional and Cu‑backed drift-\nfree devices. A resistive heater generated through a temperature modulation between\napproximately 32 °C and 60 °C, while the current response was recorded in the complete absence\nof optical illumination (Supplementary Fig. 19).\nIn both architectures, the measured current shows only a slow, monotonic rise and decay that\nfollows the heating and cooling dynamics, governed by its intrinsic thermal time constant.\nCrucially, no fast, polarity‑reversing transient; the signature of a pyroelectric current arising from\ndT/dt is observed. Furthermore, the shape and magnitude of these heater‑induced signals are\nqualitatively and quantitatively distinct from the light‑induced photocurrent transients presented\nin the main text.\nThese results confirm that pyroelectric contributions are negligible in the operating regime of\nour devices.”\nIncluding these references would strengthen the discussion and provide a more balanced\ncomparison.\nAnswer 6: We thank the reviewer for this important suggestion of adding the studies on the\npyroelectric and pyro-phototronic detectors. In the revised manuscript, we have expanded the\ndiscussion to include several representative studies on pyroelectric and pyro-phototronic\nphotodetectors, in addition to ferroelectric photovoltaic devices, to provide a more balanced\ncomparison and clearer context.\nIn response to Comment 1, we have already added a table comparing our device with state-of-\nthe-art ferroelectric photovoltaic and pyro-phototronic photodetectors, including explicit rise/fall\ntimes, supported by relevant literature references. These additions place the response time of our\ndrift-free device in direct context with established pyro-phototronic systems.\nWe believe that the inclusion of these references and the clarified discussion strengthen the\nmanuscript and more clearly positions our work within the broader landscape of ferroelectric and\npyroelectric photodetectors.\nMinhas et al. report a simple yet highly effective strategy to overcome two persistent limitations\nof ferroelectric (FE) photodetectors: slow temporal response and photocurrent drift. By integrating\nBFO thin films on high-thermal-conductivity copper substrates, the authors demonstrate >1000x\nfaster response times (0.36 ms vs. 1648 ms) and near-complete drift suppression compared to\nconventional devices. Infrared thermography and COMSOL simulations convincingly support the\nproposed mechanism of enhanced vertical heat extraction and suppressed lateral heat diffusion.\nThe approach is shown to be broadly applicable across multiple FE materials and enables high-\nfidelity, low-crosstalk spatial imaging. The work addresses a significant bottleneck in FE\noptoelectronics, and the experimental data are persuasive. However, in the current state I cannot\nrecommend publication. The manuscript would benefit from addressing the following points.\nComment 1: Page2, line60: “A common feature of conventional FE photodetectors is their\nfabrication on low-thermal-conductivity base materials such as mica or glass”. Why is this the\ncommon fabrication method and what advantages does this bring apart from the disadvantage of\ninefficient heat extraction. Since the authors themselves are also growing on mica.\nAnswer 1: We thank the reviewer for raising this important point. Mica is indeed widely used as a\nsubstrate for ferroelectric and oxide thin films, not only for convenience but because it offers\nseveral well-documented materials advantages. First, its mechanical flexibility and high thermal\nstability allow high-temperature oxide processing while enabling flexible or transferable device\narchitectures, which has motivated its widespread adoption in ferroelectric memories, sensors,\nand photodetectors (Li et al., Nat. Commun., 2020; Liu et al., Adv. Funct. Mater., 2021) Second, mica\nis chemically inert, optically transparent, and electrically insulating, making it particularly suitable\nfor self-powered ferroelectric photovoltaic and optoelectronic measurements without parasitic\nleakage or substrate-induced screening (Zheng et al., ACS Appl. Mater. Interfaces, 2018). Third,\nmica provides an atomically flat, layered surface that enables van der Waals-assisted or quasi-\nepitaxial growth of perovskite oxides, often resulting in reduced interfacial strain, low defect\ndensity, and robust ferroelectric polarization (Wang et al., Adv. Mater., 2016; Ji et al., Nano Energy,\n2019). Furthermore, mica enables mechanical flexibility while maintaining thermal and chemical\nstability up to ~700 °C, making it attractive for flexible ferroelectric devices, including BFO and\nPZT thin films (RSC Mater. Adv. 2022; J. Mater. Chem. C 2020). Recent ferroelectric memory\n59\nstudies on BFO/mica stacks have also confirmed its good crystalline compatibility, fatigue\nendurance, and mechanical robustness. These attributes explain why conventional FE\nphotodetectors including our own baseline conventional-BFO device are commonly fabricated on\nmica. However, as the reviewer points out, its inherently low thermal conductivity remains a\ndrawback when fast response is required, and this is precisely the limitation our thermal-diffusion\nengineering concept aims to overcome.\nModification to the Manuscript:\nThe following paragraph has been revised in page 2, paragraph 4, in the revised manuscript.\n“Conventional FE photodetectors is commonly fabricated on low-thermal-conductivity\nsubstrates, such as mica or glass, with mica offering an atomically flat, chemically inert, and\nelectrically insulating surface that is well suited for high-quality growth of perovskite oxide\nfilms30,31. It also withstands high-temperature processing and provides mechanical flexibility,\nwhich has made it a standard platform for many BFO and related FE devices. However, its\nintrinsically low thermal conductivity leads to significant lateral heat accumulation during\nillumination, contributing to slow response and drift, and motivating our thermal-diffusion\nengineering strategy using high-k metallic backers.”\n30. Le, H. K. et al. Room-temperature ferroelectric epitaxial nanowire arrays with\nphotoluminescence. Nano Lett. 24, 5189-5196 (2024).\n31. Ma, C. H. et al. Van der Waals epitaxy of functional MoO2 film on mica for flexible\nelectronics. Appl. Phy. Lett. 25, 108 (2016).\nhelpful if the device is measured also after 1 week or longer. Calling 1h of stability “unprecedented”\nis a bit of a far-fetched claim.\nAnswer 2: We thank the reviewer for this valuable comment. We agree that 1 h of stability alone\ncannot justify the term “unprecedented,” and we have now expanded our stability dataset\naccordingly. We have performed additional stability measurements on the drift-free device with\nrepeated tests on different days. Specifically, the device was measured three times, each for a\nduration of 2.7-hours, under 1 Hz modulated illumination at room temperature. Across these\nmeasurements, the device exhibits highly reproducible behavior (Figure R31) with a stability of\n~99.3 - 99.5%, confirming robust temporal consistency over long-time gaps.\nFigure R31 (Supplementary Fig. 16 in the revised version) | Long-term operational stability\nof drift-free device. Photocurrent stability measurements of drift-free device recorded over 10 ks\n61\n(2.7-hours) under periodic illumination (1 Hz, 405 nm) on different days with power density of\n1.6 W cm-2 using an oscilloscope-TIA chain (TIA sensitivity 50 µA V-1 and oscilloscope 10 M\npoints, 100 S s-1, 1.0 ks time scale). All measurements exhibit excellent stability, maintaining ~99.3\n- 99.5% signal amplitude over the full measurement duration.\nFurthermore, to evaluate continuous operational endurance, we conducted stability\nmeasurements spanning a total duration of ~41.6-hours, composed of sequential 2.7-hour segments\nlimited only by oscilloscope memory capacity. Over this extended measurement period, the device\nmaintained > 97% of its initial photocurrent, indicating excellent long-term stability over nearly\ntwo days of operation. We believe these additional experiments adequately address the reviewer’s\nconcern regarding long-term stability (Figure R32).\nFigure R32 (Fig. 2h in the revised version) | Extended continuous stability test. Long-term\nphotocurrent monitoring of a drift-free device conducted continuously for 150 ks (41.6-hours ) at\ncontrolled laboratory temperature (26 - 27 °C), acquired in consecutive 2.7 h segments due to\noscilloscope memory limitations. The device exhibits negligible drift (< 3%), maintaining > 97%\namplitude stability over the full duration, with no observable degradation in rise time, decay time,\nor waveform shape.\nModification to the Manuscript:\nThe following paragraph has been revised in page 5, paragraph 2, in the revised manuscript.\n“To quantitatively validate the long-term stability of drift-free architecture, we performed\nsystematic multi-hour and multi-day cycling experiments. Three independently fabricated devices\nwere measured under identical modulated illumination (1Hz, 405 nm, 1.6 W cm-2). Structural\nuniformity across these devices confirmed by SEM (Scanning electron microscopy) and AFM\n(Atomic force microscopy) (Supplementary Fig. 13, Fig. 14) shows consistent functional layers\n62\n(BFO thickness: 66-73 nm; LNO thickness: 200-208 nm; RMS (root mean square) roughness:\n1.09-1.24 nm) and lateral dimensions of 2 mm × 2 mm active area of all the devices. Each device\nexhibited the exceptional amplitude stability of 99.5-99.7% over continuous 2.7-hour segments\n(Supplementary Fig. 15). To prove extended duration, one device was monitored for 41.6 hours in\nsequential segments, maintaining > 97% stability with no observable degradation in temporal\nwaveform shape as shown in Fig. 2h. Furthermore, repeated measurements on different days\nconfirmed robust reproducibility with stability consistently in the 99.3 to 99.5% range\n(Supplementary Fig. 16). This comprehensive dataset, supported by uniform structural\nparameters.”\nthe bandwidth of the devices however is only measured until 1.4 Hz. That the drift-free device\ndoesn’t even come near its limit, is quite trivial. With that response time, measurements into the\nkHz range should be possible. I encourage the authors to include higher frequency measurements.\nAnswer 3: We thank the reviewer for this insightful comment. We appreciate the reviewer’s\nencouragement to extend the measurements into the higher-frequency regime.\nWe have performed systematic gain measurements over an extended low-frequency range from\n50 mHz to 10 Hz (Figure R33) under identical optical and electrical conditions using an\noscilloscope in series with an SR570 transimpedance amplifier (TIA) (Sensitivity 50 µA). In this\nregime, the conventional device exhibits a pronounced frequency dependence: its photocurrent\ndecreases from 184 µA to 152 µA below 5 Hz, corresponding to a reduction of approximately 20%\nof the maximum gain. In contrast, the drift-free device shows a much weaker gain variation,\ndecreasing from 167 µA to 160 µA (＜ 5%), and remains nearly constant between 50 mHz and 1\nHz, demonstrating quasi-frequency-independent response at low frequencies. These results are\nnow included and discussed in the revised manuscript.\nc f\nFigure R33 (Figs. 2c,f in the revised version) | Low-frequency gain stability of conventional\nand drift-free photodetectors. c, Frequency-dependent photocurrent gain of the conventional\ndevice measured from 50 mHz to 10 Hz under identical optical and electrical conditions (1.6 mW\ncm⁻² power density, 2 mm × 2 mm active area, 26 °C) using an oscilloscope in series with an\nSR570 transimpedance amplifier (50 µA sensitivity). f, Corresponding measurement for the drift-\nfree device.\nWe have also measured full Bode plots (gain and phase versus frequency) for three\nindependently fabricated drift-free devices using a lock-in amplifier (LIA) with slope of 24 dB,\n64\ncascaded with an SR570 transimpedance amplifier (TIA) with a sensitivity of 50 µA/V, under the\nincident light power density of about 1.6 W cm-2. The measurements were performed over a broad\nfrequency span from 100 Hz to 15 kHz, enabling direct extraction of both amplitude and phase\ncharacteristics. From these measurements, the extracted -3 dB bandwidths of three devices lie\nbetween 3.1 kHz and 4.3 kHz (Figure R34a), while the corresponding -45° phase-shift frequencies\nfall between 4.6 kHz and 5.5 kHz (Figure R34b). The close agreement among devices\ndemonstrates good reproducibility. These kilohertz-scale bandwidths substantiate the millisecond-\nto-sub-millisecond temporal response observed in the time-domain measurements, as expected\nfrom standard bandwidth rise-time relationships. These results demonstrate that our device\narchitectures can reliably operate in the kHz frequency regime.\nThese kilohertz-scale bandwidths directly substantiate the millisecond-to-sub-millisecond\ntemporal response observed in the time-domain measurements, as expected from standard\nbandwidth rise-time relationships. These results demonstrate that our device architectures can\nreliably operate in the kHz frequency regime.\nFigure 34 (Supplementary Fig. 3 in the revised version) | Frequency-domain characterization\nof drift-free photodetectors. a, Gain Bode plots of three independently fabricated drift-free\ndevices measured under identical conditions using a LIA (24 dB/oct slope) cascaded with an\nSR570 TIA (50 µA V-1) at an incident power density of ~1.6 W cm-2. The -3 dB bandwidths\nextracted from the gain roll-off span 3.1- 4.3 kHz, b, Corresponding phase Bode plots for the same\ndevices, showing -45° phase-shift frequencies between 4.6 and 5.5 kHz, in close agreement with\nthe gain-derived bandwidths.\nModification to the Manuscript:\nThe following paragraph has been revised in page 4, paragraph 2, and paragraph 3, in the\n65\nrevised manuscript.\n“The millisecond-scale response time of the drift-free device is further substantiated by our\nfrequency-domain analysis, where the extracted -3dB bandwidth and corresponding phase lag fall\nwith in the 3-5 kHz range (Supplementary Note 1 and Supplementary Fig. 3). Consistent with this\nbandwidth, a direct time-domain measurement at 100 Hz modulation yields a stable undistorted\nwaveform and a response time of 0.25 ms (Supplementary Fig. 4), confirming operation in the\nkilohertz regime.\nThe gain behavior at low-frequencies (50 mHz - 10 Hz), further highlights crucial role of\nthermal management. The conventional device suffers a ⁓20% gain roll-off below 5 Hz (Fig. 2c),\nindicative of the system’s instability to reach the thermal equilibrium under modulated\nillumination. The drift-free device (Fig. 2f) demonstrates exceptional stability with a gain variation\nof < 5% across the same range. This flat frequency response is a direct consequence of the\nsuppressed thermal time constant.”\n”\nModification to the Supporting Information:\nThe following paragraphs have been revised in page 5, paragraph 1,2, in the revised supporting\ninformation.\n“Supplementary Note 1. Frequency-domain characterization and bandwidth extraction.\nTo quantify the temporal bandwidth of the drift‑free devices, frequency‑domain Bode‑plot\nmeasurements were performed. A lock‑in amplifier (LIA) with a 24 dB/oct roll‑off was used in\nconjunction with an SR570 low‑noise transimpedance amplifier (TIA) under 405 nm illumination\nat a power density of 1.6 W cm-2 The normalized photocurrent gain and phase shift were recorded\nover a frequency range from 100 Hz to 15 kHz for three independently fabricated devices.\nThe -3 dB cut‑off frequencies extracted from the gain profiles range from 3.1 kHz to 4.3 kHz,\nwith corresponding -45° phase‑shift frequencies between 4.6 kHz and 5.5 kHz (Supplementary\nFig. 3). This close agreement among devices confirms good reproducibility and establishes the\nintrinsic -3 dB bandwidth of the drift-free architecture in the kilohertz regime, consistent with the\nmillisecond‑scale response times observed in transient measurements.”\nexperimental or not. Furthermore, I am missing a discussion on why exactly BFO is by far better\nthan all the other oxides, namely two orders of magnitude and a relative increase between\nconventional and drift-free of four orders of magnitude. Later in the manuscript it is mentioned\nthat “While BFO-based drift-free devices exhibit the most pronounced improvements, attributed\nto their robust PV properties,…”, which needs further discussion.\nAnswer 4: We thank the reviewer for pointing out the need for clarification regarding Fig. 2i and\nthe underlying physical origin of the observed performance differences. We would like to clarify\nthat all comparison data shown in Fig. 2i is based exclusively on our own experimental\nmeasurements, rather than literature-derived values. All ferroelectric devices included in this\ncomparison were fabricated under identical processing conditions, including the same deposition\nmethod (Sol-gel and spin coating), identical bottom (LNO) and top (ITO) electrode configurations,\nand a fixed device active area of 2 mm × 2 mm. This controlled approach was intentionally adopted\nto enable a direct and fair comparison of response speed and drift behavior across different\nferroelectric material systems within the same device architecture.\nThe order-of-magnitude differences observed in Fig. 2i should be interpreted in two distinct and\nphysically different contexts. First, when comparing different ferroelectric materials fabricated and\nmeasured under identical device architectures, the BFO device exhibits an approximately one to\ntwo-order-of-magnitude faster photoresponse than other ferroelectric oxides investigated in this\nwork. This superiority originates from intrinsic material properties of BFO, including its relatively\nnarrow optical bandgap (~2.5 - 2.8 eV), which makes it more responsive to visible illumination\ncompared to many wide-bandgap oxide ferroelectrics such as BaTiO or LiNbO . (Nat. Commu.\n3 3\n4, 2835 (2013)). This narrower bandgap, combined with robust spontaneous polarization, enables\nsignificant ferroelectric photovoltaic response, as evidenced in multiple device studies where\nphotocurrent and photovoltage are strongly modulated under illumination. (ACS Appl. Mat. 10,\n13712-13719 (2018)). The role of ferroelectric polarization and domain structures in facilitating\nphotovoltaic effects has been reported in BFO thin films, where polarization-modulated internal\nfields significantly influence photovoltaic behavior. (Scie. Rep. 10, 15108 (2020)). Although not\ndirectly related to this study, substantial photovoltaic effects and switchable behavior have been\ndemonstrated even in polycrystalline BFO films on transparent substrates, demonstrating that the\n67\nintrinsic photovoltaic properties of BFO are robust across different material forms (Thin Solid\nFilms 698, 137851 (2020)).\nWe further acknowledge the reviewer’s concern for highlighting this important point of two\norders of magnitude superiority of BFO over the other FE devices. In the revised manuscript, we\nhave replaced the previously used more optimized and extreme response time of the BFO device\n(0.36 ms) with a more conservative and directly comparable value of ~1.7 ms, measured under\nidentical conditions as the other ferroelectric devices; even on this uniform basis, BFO continues\nto outperform the other materials by approximately one to two order of magnitude (Figure R35).\nFigure R35 (Fig. 2i in the revised version) | Cross-material performance comparison of\nferroelectric photodetectors. i, Response time comparison for different ferroelectric materials.\nAll devices were fabricated using identical deposition methods, electrode configurations, and a\nfixed active area of 2 mm × 2 mm, enabling a fair comparison between conventional and thermally\nengineered architectures.\nWe believe these revisions address the reviewer’s concerns and provide a more accurate and\ntransparent interpretation of the performance differences observed among the ferroelectric devices.\nModification to the Manuscript:\nThe following paragraph has been revised in page 5, paragraph 2, in the revised manuscript.\n“All devices were fabricated and characterized in our laboratory under identical experimental\nconditions, using the same deposition method, electrode configuration, and a fixed active area of\n2 mm × 2 mm, ensuring a fair and direct comparison across different material systems.”\nAnswer 5: We thank the reviewer for this practical and relevant question. The devices were\nmechanically fixed onto the metal substrates using a small amount of silver paste applied only at\nthe peripheral edges of the conventional device, well outside the active channel region. Care was\ntaken to ensure that the silver paste does not contact any electrodes, does not electrically short the\ndevice, and does not obstruct the optical path during illumination. The central active area of the\ndevice remains in direct physical contact with the metal substrate, enabling efficient vertical heat\nextraction while preserving identical electrical and optical measurement conditions. To improve\nclarity and reproducibility, we have now included a detailed description of the mounting procedure\nin the Methods section.\nModification to the Manuscript:\nThe following paragraph has been revised in page 9, paragraph 4, in the revised manuscript.\n“Drift-free (drift-free) device configuration. Conventional devices were stacked onto different\nmetallic and thermoplastic plates using a small amount of silver paster applied exclusively at the\nperipheral edges, ensuring no contact with electrodes or obstruction of the central active optical\npath.”\ncopper substrate is once a minimum drift ratio is reached? It seems like the drift-ratio goes up\nagain for thicker substrates than 0.8mm, which to me is counterintuitive. How many substrates\nhave been included in that study per thickness?\nAnswer 6: We thank the reviewer for this insightful and technically important comment. We agree\nthat, from a thermal-management perspective, once the copper substrate exceeds a critical\nminimum thickness, further increases should not significantly enhance heat extraction. Our\nexperimental results are fully consistent with this expectation.\nIn our study, we first compared conventional devices with thermoplastic-backed and metal-\nbacked devices using substrates of identical lateral dimensions and thickness. Among these, the\nCu-backed configuration exhibited the most effective drift suppression and fastest response, with\nmarginally better performance than Al or stainless-steel substrates. Subsequently, to examine\nthickness dependence, we investigated 12 Cu-backed devices spanning eight different substrate\nthicknesses. As anticipated, the variation in drift ratio and response time beyond a certain thickness\nwas extremely small, typically within 0.2 - 0.4% (Figure R36), which is negligible compared to\nthe substantial improvement achieved when transitioning from conventional or thermoplastic\nsupports to a metallic heat sink.\nBecause an overly narrow range was chosen in the original manuscript, the drift ratio at 0.8 mm\nappeared visually unreasonable; however, in reality, the drift ratio at 0.8 mm is 0.58% which falls\nwithin the range of 0.58% to 0.62% observed over the measured thickness range. The highly\nzoomed scale used in previous version had visually amplified minor fluctuations that lie well\nwithin experimental uncertainty and do not represent a physically meaningful trend. We apologize\nfor any confusion caused by the original presentation, and in the revised manuscript, we have\nadopted a more appropriate y-axis scale in the Figure R36, clearly demonstrating that thickness-\ninduced variations are negligible once the optimal heat-extraction regime is reached. Overall, these\nresults confirm that the primary performance enhancement arises from the introduction of a high-\nthermal-conductivity metal heat sink.\nWe have also revised the explanation in the Supplementary Information to explicitly clarify this\npoint.\nof optimized device with 10 mm × 10 mm area.\nModification to the Supporting Information:\nThe following sentences have been revised in page 13, paragraph 1, in the revised\nSupplementary Information.\n“Supplementary Fig. 37b, presents devices with a fixed lateral size of 10 mm and varying copper\nthicknesses. The drift behavior remains nearly uniform, indicating that the optimal configuration\nfor thermal conduction and mechanical stability is not strongly dependent on substrate thickness.”\nrepeating the advantages of the drift-free concept. Almost no practical parameters nor limitations\nfor the use of the detector are discussed. The devices are quite big in terms of technological\nsignificance. The spatial imaging with “X” and “Z” masks is promising but relatively qualitative.\nQuantitative metrics like modulation transfer functon(MTF) or crosstalk ratio would be stronger.\nCovering the following questions would make this section more valuable for potential applications.\nHow small can the array be made until crosstalk even for the drift-free architecture happens? 2mm\npads are huge, what is the limit here? In that regard, from the methods section I understand, that\nto actually measure the device the ITO pads are directly contacted with silver paint (the device\nfigure in S28 is shown without those contacts). Having to place the contact on top would probably\nsignificantly influence device miniaturization as incoming light gets blocked. What other approach\nwould be necessary to deal with that issue?\nAnswer 7: We thank the reviewer for this constructive and forward-looking comment.\nThe spatial imaging experiment with “X” and “Z” masks were performed using the device\ngeometries with a 2 mm × 2 mm active pad, with a light source of 405 nm under identical\nillumination intensity for both freestanding and drift-free devices.\nIn the present work, our primary objective was to establish a clear, controlled comparison\nbetween conventional and drift-free architectures, focusing specifically on photocurrent drift\nsuppression and response-speed enhancement enabled by vertical heat extraction. We fully agree\nthat investigating the minimum achievable pixel size and the onset of crosstalk in drift-free\narchitectures is an important next step. The active pad size of our BFO device is determined by the\narea of top ITO electrode and can be readily reduced at least micro-meter scale by controlling the\nsputtering mask and electrode contact dimensions. It can be a future research direction.\nFollowing the reviewer’s suggestion, we have strengthened this section by adding quantitative\nspatial metrics using the most direct measurements accessible with our current instrumentation.\nSpecifically, we performed point-spread-function (PSF) derived line-spread function (LSF)\n(Response Figure R37) by illuminated a focused optical spot (diameter ≈ 230 µm) across a single\n2 mm × 2 mm channel at nine discrete lateral positions. For the conventional device, the\nnormalized signal drops to only ~48% at the edge of the channel, indicating a broad lateral\nresponse halo consistent with strong in-plane thermal diffusion. In contrast, the Cu-backed drift-\nfree device exhibits a rapid signal decay to < 7% at the same lateral position, demonstrating a\nsharply localized response and strongly suppressed lateral heat spreading.\nanalysis of lateral response. Normalized LSF extracted by illuminating a focused optical spot\n(~230 µm) across a single 2 mm × 2 mm channel for conventional and drift-free devices.\nSecond, we constructed quantitative crosstalk matrices by selectively illuminating channel 13\nand recording the time-resolved response of neighboring channels (Figure R38,39). In the\nconventional device, the nearest-neighbor channels (12 and 14) exhibit 10 - 11% of the primary-\nchannel signal, while next-nearest neighbors (11 and 15) show ~3 - 4% leakage. In striking contrast,\nthe drift-free device shows no measurable signal above the dark-noise floor in any adjacent channel,\ncorresponding to nearest-neighbor leakage below 1%. These results quantitatively confirm the\nsubstantial suppression of array-level crosstalk achieved through thermal-diffusion engineering.\nFigure R38 (Supplementary Fig. 33 in the revised version) | Crosstalk analysis in\nphotodetector arrays. Extracted from Figure R39. a, Crosstalk matrix obtained by selectively\nilluminating a single channel and recording responses from neighboring channels for conventional\nand drift-free devices. b, Schematic illustration of the 5 × 5 photodetector array used for crosstalk\nmeasurements.\na\nConventional\nb\nDrift-free\nFigure R39 (Supplementary Fig. 34 in the revised version) | Time-resolved crosstalk\nmeasurements. Time-resolved current responses recorded simultaneously from channels 11 - 15\nduring selective illumination of the central channel, comparing conventional a, and drift-free b,\ndevices. The drift-free device exhibits negligible transient signals in neighboring channels.\nSince the MTF is formally the Fourier transform of the LSF, the LSF data presented here already\ncaptures the essential spatial confinement characteristics in the spatial domain.\nthat silver paint was used for electrical contact, applied at a corner of the active area, covering\napproximately 5–10% of the total device area, while the majority of the channel remained\nunobstructed for optical illumination. For the crosstalk and PSF measurements, we additionally\nemployed microprobes with ~50 µm tip diameter, allowing localized electrical access while\noptically addressing different regions under a microscope. We agree that top-side contacts are not\nideal for aggressive scaling.\nOverall, while the present study deliberately focuses on mechanism validation and architectural\ncomparison at a fixed device size, the newly added quantitative PSF and crosstalk analyses\nsubstantively strengthen the spatial-imaging section and clarify both the capabilities and current\nlimitations of the drift-free photodetector platform.\nModification to the Manuscript:\nThe following lines have been revised in page 8, paragraph 1, in the revised manuscript.\n“This superior spatial fidelity is quantitatively validated by a comprehensive analysis of channel\ncrosstalk and the line-spread function (LSF), which show a seven-fold improvement in spatial\n74\nconfinement for the drift-free architecture (Supplementary Note. 7 and Supplementary Fig. 33-\n35).”\nThe following lines have been revised in page 8, paragraph 1, in the revised manuscript.\n“Further optimization, including systematic miniaturization to define the fundamental pixel-size-\nlimits of this thermal-management strategy, presents a clear pathway for future high resolution\nimaging applications.”\nModification to the Supporting Information:\nThe following paragraphs have been added in page 12, in the revised supplementary\ninformation.\n“Supplementary Note 7. Quantitaive anlaysis of high fidelity and imaging\nTo quantitatively assess pixel to pixel interference, a five-channel array (channels 11 - 15)was\nevaluated by illuminating only channel 13 with a 0.7 mm radius light spot (Supplementary Fig.\n33, 34). In the conventional standing device, adjacent channels (12 and 14) exhibited the ≈ 10%\nnormalized crosstalk, confirming lateral heat-induced signal leakage. In striking contrast, the drift-\nfree device shows no detectable crosstalk in adjacent channels, demonstrating complete\nsuppression of lateral heat flow and validating the channel isolation required for high precision\nimaging.\nTo further characterize spatial resolution, line-spread function (LSF) measurements were\nperformed by scanning a 220-µm radius laser spot across channel 13 in 200-µm steps\n(Supplementary Fig. 35). The conventional device shows a broad spatial response, retaining 48%\nof the peak signal at the channel boundary due to the lateral heat halo. By contrast, the drift-free\ndevice exhibits a sharply confined spatial profile, dropping to 7% at the boundary. This seven times\nimprovement in the spatial fall-off directly confirms that thermal-diffusion engineering enhances\npoint-spread localization, reduces heat-induced smearing, and enables high-resolution detection.”\n75"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript has been significantly improved with additional supporting data and it is now ready for publication consideration.", "reviewer_2": "The paper is well written and organized, and it makes a meaningful contribution to the literature. I have no major concerns and find no issues that would require revision. I therefore recommend this manuscript for acceptance in its current form.\nPoint-by-point assessment of revisions:\nComment 1: The authors have added a table to provide clear information and comparison. This adequately resolves the concern.\nComment 2: The authors have performed power density-dependent temporal response measurements over a broad range of illumination intensities for both the conventional and drift-free BFO devices. The newly added analysis strengthens the results and supports the conclusions. This comment has been fully addressed.\nComment 3 and 4: The authors have clearly explained the differences in the experimental results and modified the manuscript with detailed experimental information. This comment has been fully addressed.\nComment 5: The authors have clearly addressed this concern with the updated experimental results.\nComment 6: Response: This comment has been fully addressed with the inclusion of these references and the clarified discussion.", "reviewer_3": "The authors have considered all my comments and worked rigorously to improve a manuscript, that already from the start had high-quality results. With their extensive work on answering all reviewers comments, they have made a complete work, that will serve the community well. I am now in favor of publishing."}, "author_response": ""}]

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