Heterogeneously integrated lithium tantalate-on-silicon nitride modulators for high-speed communications

Domain: chemistry
Published: 2026-02-28
Editorial outcome: minor_revision

Abstract
Ultrabroadband integrated modulators involving materials beyond those available in silicon manufacturing increasingly rely on the Pockels effect. Among electro-optic materials, lithium tantalate offers comparable Pockels coefficients to lithium niobate but with significantly improved photostability, lower birefringence, higher optical damage threshold, and enhanced DC bias stability. Here we demonstrate wafer-scale heterogeneous integration of lithium tantalate films on low-loss silicon nitride photonic integrated circuits, achieving low optical losses (∼ 14.2 dB/m) while combining the mature processing of silicon nitride waveguides with the ultrafast electro-optic response of thin-film lithium tantalate. The resulting devices achieve a 6 V half-wave voltage, and support modulation bandwidths of up to 100 GHz. We use single intensity modulators and in-phase/quadrature (IQ) modulators to transmit PAM4 and 16-QAM signals reaching up to 333 and 581 Gbit/s net data rates, respectively. Our results establish lithium tantalate-on-silicon nitride as a viable platform for RF photonics, interconnects, and analog signal processing.

Introduction
Photonic integrated circuits (PICs) provide a unique approach to scaling standardized optoelectronic technologies. [1, 2]. Among PIC platforms, silicon nitride-based devices offer a range of features traditionally leveraged in optical fibers such as low propagation losses, large power handling [3], complementary metal-oxide-semiconductor (CMOS) compatibility, and a wide bandgap enabling transparency across a wide range of optical wavelengths [4–6]. Thicker nitride waveguides, e.g. manufactured with a photonic Damascene process, additionally provide strong optical confinement and easily-achievable anomalous group velocity dispersion [7–10]. These features become crucial while harnessing this material’s optical nonlinearities [11] and have thus lead to demonstrations pertaining to Kerr frequency combs [10–13], optical frequency conversion [14, 15], traveling-wave optical parametric amplifiers [16–18], and quantum information science [19–22]. Further deployment of this platform in applications such as optical communications and microwave photonics [23, 24] requires electro-optic modulation, which silicon nitride cannot directly provide due to its amorphous nature.

Ferroelectric-based photonic integrated circuits readily provide such features [25–27], and can thus introduce electro-optic modulation capabilities into silicon nitride PICs through the heterogeneous integration of materials such as lithium niobate [28, 29]. Recent advances that have demonstrated low loss lithium tantalate photonic integrated circuits [30], as well as its advantages such as lower birefrigence, lower bias drift, and higher power handling than in LiNbO3, motivate applying similar integration methods to alternative Pockels materials [31, 32]. Improved economies of scale for lithium tantalate substrates due to their role in 5G/6G RF filters [33] further encourages the use of this material in integrated modulators. At the current stage, heterogeneous integration of this material offers an immediately accessible route towards leveraging its features in high-volume manufacturing platforms such as silicon nitride.

Here, we develop a wafer-scale bonding process for lithium tantalate on silicon nitride PICs. With this platform, we demonstrate modulation defined by a VπL = 4.08 V ·cm and a 3-dB bandwidth close to 100 GHz in a push-pull Mach-Zehnder modulator (MZM). These results enable net data transmission rates of 333 Gbit/s when operating the modulators in pulse amplitude modulation (PAM) schemes. We further extend this data transmission demonstration to Si3N4-LiTaO3 IQ modulators, thereby achieving net data transmission rates of 581 Gbit/s using quadrature amplitude modulation (QAM) signals. With performance metrics that can already compete with those of monolithic lithium-tantalate-on-insulator (LTOI) PICs [31, 32], these results show that Si3N4-LiTaO3 devices can implement systems combining mature Si3N4 PICs with next-generation ferroelectric thin films, thereby enabling both low-loss strong optical confinement with GHz-rate modulation.

Results
Hybrid Si3N4-LiTaO3 PIC fabrication. Figure 1(a) outlines the process flow for the hybrid PICs, which starts with the fabrication of Si3N4 waveguide structures using the photonic Damascene process [9]. As detailed in Supplementary Note 1, the process flow begins with preform formation on a 100 mm-diameter silicon wafer with 4µm thick wet thermal oxide by means of deep ultraviolet (DUV) stepper lithography, dry etching and reflow under high temperatures. Low-pressure chemical vapor deposition (LPCVD) then deposits a layer of Si3N4, which subsequently undergoes chemical-mechanical polishing (CMP), oxide interlayer deposition, and annealing. The Si3N4 photonic Damascene process is free of crack formation in the highly tensile LPCVD Si3N4 film and provides high fabrication yields and low optical propagation losses of O(dB/m) [8]. The patterned sample supports strip waveguides with a 1µm width and a 0.5µm height. In addition, inverse nanotapers allow for efficient edge coupling between the chip’s waveguides and lensed fibers.

Our heterogeneous integration approach additionally involves 100 mm LTOI wafers fabricated by a hydrogen-based ion-slicing technique [30, 33]. The resulting wafer stack consists of a 300 nm x-cut lithium tantalate thin film, a 2µm buried oxide layer and a 525µm thick silicon substrate. Following surface preparation relying on cleaning and plasma activation, the Si3N4 and LTOI wafers undergo hydrophilic pre-bonding at room temperature, where van der Waals interactions establish a preliminary bond between the two wafers. A subsequent 300◦C thermal annealing process then enhances the bond strength by accelerating the polymerization of silanol (Si-OH) groups to covalent bonds (Si-O-Si) at the interface. After bonding, the process continues by thinning the LiTaO3-based donor substrate.

Backside wafer grinding followed by a tetramethylammonium hydroxide (TMAH) wet etch entirely removes the substrate’s silicon. A buffered hydrofluoric acid (BHF) solution then eliminates the thermal oxide layer, thereby exposing the LiTaO3 layer and forming a well-confined hybrid optical waveguide. This sequence entirely lacks plasma-based processing traditionally used in LiTaO3 nanofabrication, which for instance includes oxygen-based diamond-like carbon hard mask etching [34] and argon dry etching [35]. With such measures, we avoid introducing potential plasma-induced charges at the Si3N4 PIC wafer’s Si/SiO2 interface, which can lead to parasitic surface conduction RF losses [36].

To add high-speed coplanar waveguide (CPW) and heater electrodes to the sample, lithography based on a maskless aligner (Heidelberg Instruments MLA150) first defines the electrode layout. Thermal evaporation of 10 nm of titanium and 800 nm of gold followed by lift-off then completes the electrode’s fabrication. Argon-based ion beam etching subsequently patterns the LiTaO3 thin film to form adiabatic taper transitions between Si3N4 waveguides and the hybrid Si3N4-LiTaO3 waveguides. The process also fully removes the LiTaO3 near the chip facets to improve edge coupling efficiency to the hybrid Si3N4-LiTaO3 PIC. After cladding deposition, an additional exposure step followed by hydrofluoric wet etching selectively expose portions of the sample for landing probes on the manufactured electrodes. As discussed in Supplementary Note 2, these cladding openings also match the group velocities of the CPW RF field and optical waveguide mode.

Figure 1(b) shows a photograph of a finalized 100-mm hybrid Si3N4-LiTaO3 wafer, where a distinct boundary line delineates the contour of the bonded LiTaO3 film. The central nine stepper fields are fully-covered by the film, thereby highlighting the excellent fabrication consistency and large-scale yield of the wafer bonding technique. Supplementary Note 3 provides further details on wafer-scale yields, which include film thickness homogeneity and device performance analyses.

To characterize the optical properties of the wafer’s hybrid photonic structures, frequency-comb-assisted spectroscopy [37] relying on three external-cavity diode lasers probed optical waveguide loss in fabricated hybrid Si3N4-LiTaO3 microring resonators. Figures 1(c,d) show the derived external, κex, and intrinsic, κ0, loss rates of the ring’s TE polarization resonances near the C telecom band. Here, κ0 increases with optical frequency, which is consistent with the behavior of similar structures implemented in LiNbO3-on-Si3N4 heterogeneous PICs [28, 29]. In such structures, higher optical frequencies result in a smaller mode field defined by a stronger overlap with the higher-index ferroelectric slab, thus inducing additional bending losses in the rings. Over the frequency range considered in Fig. 1(d), finite element method simulations suggest the lithium tantalate film holds roughly 48% of the mode’s energy. Figure 1(e) shows a typical normalized resonance in the C-band, which indicates a fitted intrinsic linewidth of 72 MHz and a corresponding propagation optical loss of α ≈ 14.2 dB/m. This figure is larger than the 3-4 dB/m and 5.6 dB/m loss values achieved in monolithic Si3N4 [7, 8] and LiTaO3 [30] circuits. This comparison provides further evidence that the hybrid waveguide’s losses do not come from intrinsic material losses but rather radiative losses in the unpatterned and higher-index LiTaO3 film.

Electro-optic performance. To demonstrate electro-optic capabilities in our hybrid Si3N4-LiTaO3 platform, we rely on electro-optic modulators consisting of 6.8 mm-long MZM pairs operating in a push-pull configuration with high-speed CPW electrodes. Figure 2(a) shows a micrograph of such a modulator. Supplementary Note 4 provides simulated data regarding the transmission of some of its underlying components. The CPW design features a signal-to-ground spacing that accounts for tradeoffs between metal-induced optical losses and modulation. From the cross-sectional SEM images in Fig. 2(b) and the inset diagram in Fig. 2(c), our CPW design balances out these two factors with a 23µm signal electrode width and a 6µm gap separating the ground and signal electrodes. Simulations presented in Supplementary Note 2 suggest this configuration yields metal spacing-induced propagation losses of 0.1 dB/cm and a voltage length product of VπL ∼ 4.5 V ·cm. As further discussed in Supplementary Note 2, the chosen signal width also affects the modulator’s underlying bandwidth by altering RF propagation losses and velocity mismatching between the modulator’s co-propagating RF and optical fields.

We first characterize the modulator’s electro-optic performance under quasi-DC operation. Coupling 1550 nm continuous-wave light into the Si3N4-LiTaO3 MZM exhibited a fiber-to-chip coupling loss of approximately 5dB. Figure 2(c) shows the measured output power while applying a 100 Hz triangle voltage wave via an RF probe. These results indicate a Vπ = 6 V half-wave voltage for a 6.8 mm long push-pull configuration, which corresponds to a VπL = 4.08 V ·cm voltage-length product. Additional data provided in Supplementary Note 5 suggest this response is consistent down to modulation frequencies of at least 1Hz and over optical wavelengths ranging from 1500nm to 1630nm.

As highlighted in Fig. 2(d), probing the modulator’s response with a vector network analyzer (VNA) provided the modulator’s bandwidth. First, a two-port calibration using short-open-load-through standards on a commercial calibration substrate (AC2-2 MPI Corporation) is performed, which sets the reference plane at the tips of the two high-speed electrical probes. The electro-optic-electric (EOE) measurement then relies on a modification in the setup where one of the VNA’s output connects to the high-speed photodiode detecting the optical output rather than to the electrical probe. De-embedding independently characterized frequency responses from the photodiode and one electrical probe finally yields the intrinsic electro-optic (EO) response of the modulator.

The resulting S21 EO response exhibits excellent flatness within 3 dB across frequencies ranging from 25 MHz to 110 GHz while keeping S11 microwave reflections below −15 dB. These metrics reach levels comparable to state-of-the-art figures achieved in other electro-optic PIC platforms [31, 38, 39], which underscores the significant potential of Si3N4-LiTaO3 for applications requiring high-speed electro-optic modulation.

Monitoring the modulator’s optical output power while set to its quadrature point quantified its stability under a DC bias. As outlined in Supplementary Note 6, a dedicated photonic package mitigated the influence of drifting chip-to-fiber coupling efficiency in this measurement. As shown in Fig. 2(e), the Si3N4-LiTaO3 MZM exhibits low power shifts of less than 0.5 dB over a one-hour period, thus outperforming results reported in prior work on monolithic lithium niobate and lithium tantalate PIC platforms [38, 40, 41]. Minimal degradation of the unetched LiTaO3 film, where defect-driven photorefractive effects typically give rise to drift [42], likely contributes to the modulator’s enhanced DC bias stability.

Data transmission experiments. To demonstrate the viability and performance of our Si3N4-LiTaO3 platform, we use our devices in high-speed optical data communication experiments, covering both intensity-modulation and direct-detection (IMDD) and coherent modulation schemes. Figure 3(a) shows the setup for the IMDD experiment. Herein, a tunable external-cavity laser (ECL) operating in the C-band with a power of 17.8dBm provides the optical carrier. A fiber-based polarization controller (FPC) then adjusts the output polarization before coupling to the quasi-TE mode, having a dominant electric field parallel to the substrate plane, of the Si3N4-LiTaO3 MZM via a pair of lensed fibers. We drive the MZM with electrical PAM4 signals, that are synthesized by offline digital signal processing (Tx-DSP) based on pseudo-random bit sequences (PRBS) and root-raised cosine (RRC) pulse-shaping filters [31], and that are converted to the analog domain using a high-speed arbitrary waveform generator (AWG, M8199B, Keysight Technologies Inc.). A 20 cm-long RF cable followed by a broadband RF amplifier (SHF T850 B, SHF Communication Technologies AG) sends the drive signals to the coplanar transmission line of the MZM via a first impedance-matched probe in a ground-signal-ground configuration. A second probe terminates the transmission line with a 50Ω coaxial termination. Tx-DSP compensates for frequency-dependent RF loss up to the input of the feeding probe by implementing a linear minimum-mean-square-error predistortion. An erbium-doped fiber amplifier (EDFA) brings the rather weak output signal of -10 dBm from the MZM to a power level near 10 dBm compatible with the receiver. This receiver comprises a high-speed photodiode (PD, Finisar Corp.), which is directly connected to a high-speed real-time oscilloscope (RTO, UXR 1004A, Keysight Technologies Inc.) with a sampling rate of 256 GSa/s and an analogue bandwidth of 105 GHz. The EDFA is followed by an optical bandpass filter (BPF) to suppress out-of-band amplified spontaneous-emission (ASE) noise and by a variable optical attenuator (VOA, LTB-1, EXFO Inc.) that adjusts the optical power to 10dBm, the maximum input power accepted by the photodiode. At the receiver, we use offline digital signal processing (Rx-DSP) to extract and demodulate the PAM4 data. As further elaborated in Supplementary Note 7, the Rx-DSP suite comprises standard algorithms such as timing recovery, linear Sato equalization, and an additional decision-directed least-mean-square equalizer.

In our experiment, we used this setup to operate the device at different PAM4 symbol rates between 144 GBd and 200 GBd. Figure 3(b) provides the resulting pre-forward error correction (pre-FEC) bit error ratios (BER) along with the BER thresholds for different forward error correction (FEC) schemes indicated by dashed horizontal lines. For symbol rates of 160 GBd (line rate 320 Gbit/s) and lower, the BER is below the threshold for hard-decision FEC with 7% coding overhead (7% HD-FEC) [43], and it stays below the threshold for soft-decision FEC (SD-FEC) with 15% coding overhead [44, Table 7.5] up to symbol rates of 192 GBd (line rate 384 Gbit/s). For 200 GBd (line rate 400 Gbit/s), we find a BER value that is still compatible with soft-decision forward-error correction (SD-FEC) with 25% coding overhead (25% SD-FEC) [44, Table 7.5]. Figure 3(d) and Figure 3(e) depict the reconstructed eye diagrams of the 160 GBd and the 200 GBd signals, respectively, along with the histograms taken at the center of the symbol slot. The data points corresponding to these eye diagrams are marked ○A and ○B in Figure 3(b). While the signal quality clearly leaves room for improvement, the demonstrated symbol rates and line rates can already compete with those achieved by standalone lithium-tantalate-on-insulator (LTOI) MZM [31].

To quantify the information transfer efficiency of our transmission system, we derive the generalized mutual information (GMI) of the transmitted signals from the log-likelihood ratios (LLRs) of the received symbols based on an additive white Gaussian noise (AWGN) channel model [46]. Figure 3(c) shows the resulting achievable information rates (AIR), which correspond to the product of the symbol rate and the GMI of each symbol and provide an upper bound for the transmission capacity of the system.

To estimate practically achievable net data rates (NDR), we additionally have to include penalties introduced by typical FEC codes. By comparing the normalized GMI (NGMI) estimated in our experiments (see Supplementary Note 8) to the NGMI thresholds provided in [47], we select suitable FEC codes and evaluate the NDR by multiplying the line rates and the associated FEC code rates. The highest NDR of 333 Gbit/s is obtained for a symbol rate of 192 GBd (line rate 384 Gbit/s) in combination with 15% SD-FEC, while the AIR at this symbol rate amounts to 349 Gbit/s.

Besides MZM and IMDD signaling, our Si3N4-LiTaO3 platform also supports more advanced systems, such as IQ modulators (IQM) for coherent communications. Figure 4(a) shows a micrograph of such an IQM. The device consists of an input waveguide followed by a Y-splitter leading to two 13.5 mm-long MZMs, such as the one shown in Figure 2(a). The individual MZMs are DC-biased at their minimum transmission point using two 110 GHz bias-Tees (BT110R-C, SHF Communication Technologies AG). A heater-based phase shifter, which can produce a drift-free thermo-optic phase shift [48], then sets the relative phase between the signals generated in the in-phase (I) and the quadrature (Q) arms of the IQM to 90◦. Thereafter, a power combiner, like the one in Figure 2(a), synthesizes the output signal with two modulated quadratures. To demonstrate the viability of the device, we use the setup shown in Figure 4(b) to transmit quadrature phase-shift keying (QPSK) and 16-state quadrature-amplitude modulation (16QAM) signals at symbol rates between 144 GBd and 204 GBd. The setup shares several similarities with the one used for IMDD transmission shown Figure 3(a). Here, the two AWG channels however individually drive the I and Q arms of the IQM. Supplementary Note 8 gives further details on the drive voltages used in this demonstration. After modulation, an EDFA amplifies the optical signal from −13dBm to 16dBm and subsequent bandpass filting reduces out-of-band spontaneous emission noise. At the receiver, the modulated signal enters a 90◦ optical hybrid module, where it interferes with a continuous-wave local oscillator provided by a second ECL to down-convert the optical signal to four electrical baseband signals. Subsequent balanced photodiodes (BPD, Fraunhofer HHI) then detect the resulting waveforms. Finally, as clarified in Supplementary Note 7, offline receiver DSP recovers and evaluates the signals.

Figure 4(c) shows the measured pre-FEC BER along with the BER thresholds for different FEC schemes indicated by dashed horizontal lines. Figure 4(d) provides the corresponding AIR and NDR based on the same calculation used for the IMDD results. Additionally, Supplementary Figures 9(b,c) provides the NGMI and error vector magnitude (EVM), respectively. For QPSK signals, we obtained reliable BER values only for our measurements at symbol rates of 176 GBd and higher, whereas the measurements at 144 GBd and 160 GBd led to BER values below 1.24×10−6, which corresponds to the detection limit for the length of the recorded waveforms. As shown in Figure 4(e), the constellation diagram at 160 GBd features a constellation signal-to-noise ratio (CSNR) of 15.8 dB, which would correspond to an estimated BER of 3.5 × 10−10 [49, Eq. 2.18]. At the highest QPSK symbol rate of 204 GBd, we measure a BER of 7×10−3, which is still below the BER threshold for SD-FEC schemes with 25 % overhead. In this case, the CSNR amounts to 8.5 dB as displayed in the corresponding constellation diagram from Figure 4(f). Here, the 75 GHz electrical 3 dB-bandwidth of the AWG [50] mainly limits the performance of this experiment. As indicated in Figure 4(c), the highest symbol rate achieved for 16-QAM signaling amounts to 176 GBd and leads to a measured BER of 3.08×10−2 – still below the BER threshold for SD-FEC codes with 25 % overhead. This corresponds to a line rate of 704 Gbit/s. The corresponding NDR of up to 581 Gbit/s validates the feasibility of telecommunication systems based on hybrid Si3N4-LiTaO3 circuits. Figure 4(g) shows the corresponding constellation diagram, from which we measure a CNSR of 12.0dB. For our IMDD demonstration, achieving greater data rates will likely require specialized driver electronics given the Si3N4-LiTaO3 modulator’s >100GHz bandwidth.

To the best of our knowledge, these experiments demonstrate high symbol-rate communication capabilities that surpass the performance metrics of other Si3N4-based heterogeneous photonic platforms reported to date [51–53]. Meanwhile, our results exceed the previously reported sub 100 GBd symbol rates achieved in hybrid silicon lithium niobate coherent modulators [54].

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors showed a heterogeneously integrated LiTaO3 on silicon nitride modulators. The fabrication process is quite\nsimilar as previous works of LiNbO3 on silicon nitride. However, due to improved photostability, low biregringence and\nenhanced DC bias stability, this work showed enhanced performance of heterogeneous integrated modulators.\nNevertheless, some measurements, which could highlight the enhanced performance of heterogeneous integrated\nmodulators, is missing. Wafer level analysis is also missing for this wafer level heterogeneous integration. The authors\nshould add this analysis before this paper could be accepted in Nature Communication. Please see me comments as\nfollowing:\nMain text:\n1. In line 48, the authors highlight travelling-wave optical parametric amplifier in 2022 and 2024, but didn’t cite the first work\nfrom Chalmers in Science advances 2021.\n2. Could authors add wafer mapping of the thickness of LiTaO3 thin film and estimated silicon nitride waveguide height due\nto the nonuniformity of etching and CMP process? Thickness variation of SiO2 interlayer? Its impact on modulator?\n3. The authors highlight LiTaO3 superior feature of enhanced DC bias stability compared to LiNbO3. Could authors add DC\nbias stability measurement of this heterogeneously integrated LiTaO3 on Si3N4 modulator? Does heterogeneous\nintegration maintain superior feature of enhanced DC bias stability?\n4. Heterogeneous integration in Wafer level is crucial for massive production. The authors showed high yield wafer bonding\nof LiTaO3 thin film on processed Si3N4 wafer through a camera image. However, the authors didn’t provide any wafer level\nanalysis of the performance of heterogeneously integrated LiTaO3 on Si3N4 modulator. Could authors provide more\ndetailed analysis? Probably provide 5 measurements regarding a fixed design from 5 different shots.\nSupplementary Information\n1. Figure S1 has an error, in the process step of DUV lithography, the mask should be resist instead of silicon nitride.\n2. The authors’ previous work typically use LPCVD TEOS, HTO, LTO. Could authors explain why ICPCVD SiO2 is used as\ninterlayer in this work?\n3. The authors use a inversed LiTaO3 taper with tip width of 500nm to form a low-loss transition from Si3N4 waveguide to\nSi3N4/LiTaO3 hybrid waveguide. However, the large LiTaO3 tip introduce insertion loss of near 0.28 dB. Why not use a\neven narrower tip to further reduce the insertion loss? Could authors comment the introduced return loss, and its potential\nimpact on optical communication?", "reviewer_2": "The manuscript positions itself as the first wafer‑scale LT‑on‑Si₃N₄ platform achieving ~100 GHz electro‑optic bandwidth\nand high‑symbol‑rate IM/DD and coherent communications on‑chip. This novelty claim is broadly convincing, as earlier LT\nefforts emphasized either monolithic LTOI photonics or discrete thin‑film LT modulators without co‑integration on Si₃N₄,\nwhile prior heterogeneous integration on Si₃N₄ used LiNbO₃ (LN). By transposing the proven LN‑on‑Si₃N₄ heterogeneous\ntemplate to LT and then validating with both IM/DD and coherent formats the manuscript advances the material–platform\ncombination in a way that is more than cosmetic. That said, the narrative would benefit from a concise related‑work\ncomparison in a paragraph or table format (bandwidth, symbol rate, efficiency, routing loss) to precisely delimit what is new\nand what is inherited.\nThe bonding and thinning flow is described in a level of detail suitable for reproduction in a capable fab, including\nhydrophilic pre‑bonding and a moderate‑temperature anneal. Avoiding aggressive plasma exposure of LT during transfer is\nmethodologically sound. The wafer‑level photograph supports the ‘wafer‑scale’ claim qualitatively, and the text asserts\ncomplete bonding across nine central fields. However, for genuine manufacturing credibility the paper should substantiate\nthis with wafer maps and statistics, showing device‑to‑device distributions (Vπ, VπL, bandwidth, insertion loss) and a\nquantitative definition of ‘bonding yield’ in terms of devices meeting specific specs. This addition would convincingly elevate\nthe scalability argument.\nLoss extraction via ring‑resonator linewidths is appropriate for a hybrid stack. The reported ~14.2 dB/m is reasonable for a\nfirst‑generation LT‑on‑Si₃N₄ hybrid mode but, in absolute terms, is higher than state‑of‑the‑art bare Si₃N₄ routing and also\nsomewhat above the best published heterogeneous LN‑on‑Si₃N₄ values. Because the abstract uses the phrase ‘ultralow\nloss,’ the manuscript should reframe the language and present the number contextually, explicitly distinguishing\nhybrid‑mode loss from bare‑core Si₃N₄, summarizing the measurement method and uncertainty, and clarifying whether the\nfigure reflects the hybrid optical mode or the Si₃N₄ core. Providing measured mode‑converter insertion loss (rather than\nsimulation only) would further clarify where loss accrues in large‑scale circuits. In line “79” the statement – “ultralow optical\npropagation losses of O(dB/m)” is also misleading as the material is not lossless, please find a way to mitigate the statement.\nThe inclusion of a measured VπL ≈ 4.08 V·cm is a strength, as it enables cross‑platform comparisons without ambiguity. The\ntext also references simulations that predict a somewhat larger VπL for nominal geometry, which is encouraging but calls for\nreconciliation. A discussion aligning measured and simulated results, stating the electro‑optic coefficients used, optical and\nmicrowave mode overlaps, exact electrode geometry, and layer thicknesses, would resolve possible confusion and reinforce\nconfidence in the reported efficiency.\nBroadband S‑parameter data up to ~110 GHz, including |S₁₁| below −15 dB, indicate careful impedance control and low\nreflection at the device ports. The small‑signal |S₂₁| roll‑off appears consistent with ohmic attenuation (∝√f) and modest\nvelocity mismatch. For completeness and reproducibility, the paper should aggregate RF attenuation in dB/cm versus\nfrequency with line length, calibration plane, and termination explicitly stated, and add S₂₂ if available. Because recent LT\nwork has highlighted the conductivity advantages of silver electrodes above 100 GHz, a short discussion of metal choice (Au\nvs Ag) and anticipated performance–reliability trade‑offs would be valuable for readers planning to push to even higher\nbandwidths.\nThe communications results are a major strength of the manuscript. Demonstrations of PAM‑4 in the ~192 GBd range\nyielding ~333 Gb/s net after FEC, and coherent 16‑QAM around 176 GBd yielding ~581 Gb/s net, establish that the devices\nfunction beyond small‑signal curiosity. The setups are described clearly and the constraints of the electrical instrumentation\nare acknowledged (e.g., AWG bandwidth limiting the 204‑GBd QPSK case). To fully enable external benchmarking, the\nmanuscript should report per‑format pre‑FEC BER and EVM, the precise FEC overheads and thresholds, estimated or\nmeasured energy‑per‑bit based on the drive swing at the device pads, and a compact link budget that separates fiber‑to‑chip\ncoupling, on‑chip routing, modulator section, and any amplification used.\nThe introduction fairly motivates LT over LN with lower birefringence and improved photostability and DC‑bias stability.\nHowever, these advantages are not yet quantified on the presented platform. Including time‑resolved bias drift under realistic\noptical powers and over a relevant temperature range would turn a qualitative motivation into a platform‑level evidence\npoint. Even a modest A/B comparison versus a representative LN‑on‑Si₃N₄ control device, if available, would strengthen the\ncase materially.\nThe text describes fiber‑to‑chip coupling on the order of several dB per facet and indicates the need for amplification to meet\nreceiver power targets in some experiments, which is entirely plausible for early‑stage devices. Nevertheless, a clear\nseparation of external‑coupling loss from intrinsic on‑chip insertion loss of the MZM and IQM would meaningfully assist\npractitioners. Reporting measured mode‑converter loss, on‑chip device IL, and facet coupling with uncertainties would also\nhelp reconcile the communications‑experiment power budgets with the passive‑loss measurements.\nOverall methods transparency is good and likely, nevertheless the paper would still benefit from explicit uncertainty\nestimates for VπL, loss extractions, and bandwidth fitting; precise VNA calibration details and de‑embedding steps; and,\nwhere applicable, references to public PDK‑like parameters (layer thicknesses, refractive indices at test wavelengths).\nAdding micrographs or cross‑sections of critical transitions (e.g., vertical adiabatic converters) would also round out the\nreproducibility story.\nMonolithic LTOI platforms have reported low propagation loss and strong efficiency at ~40‑GHz‑class bandwidths; discrete\nthin‑film LT modulators have reached beyond 100 GHz; heterogeneous LN‑on‑Si₃N₄ has already proven the architectural\nvalue of marrying a strong Pockels material to ultra‑low‑loss routing. This manuscript’s unique contribution is to combine the\nLT material with Si₃N₄ at wafer scale and then validate with both IM/DD and coherent communications at symbol rates\ncompetitive with the best contemporary demonstrations. Given the sustained interest in coherent transceivers and\nRF‑photonics, the practical impact is credible, particularly if the authors add the benchmarking details outlined above and as\nsuch the manuscript could be accepted for publications after the requested revisions have been added.", "reviewer_3": "This is a very good paper which addresses one of the current largest bottlenecks associated with high speed data\ntransmission. Silicon and lithium niobate based optical modulators are areas of active research and the competing\ntechnologies; Single lane speeds are limited by modulator speeds and therefore, companies and researchers are intensely\nstudying ways to achieve better on-chip modulators. In this paper authored by Cai et. al, they introduce lithium tantalate\nbased modulators which are integrated with ultra-low loss silicon nitride waveguide devices. I recommend publishing this\npaper and have some suggestions for the authors:\n1) The reported Vpi is 6V. Can the authors comment on how this compares with other state of the art modulators? A\ncommonly used figure of merit is Vpi.L. A table which compares this figure of merit, bandwidth and other commonly used\nperformance metrics with other modulators would be great for readers.\n2) The authors demonstrate the modulator platform with both IMDD and coherent data transmission. Can they comment on\nwhether the highest rates demonstrated were limited by the equipment or the modulator platform itself?\n3) Because of the lack of an easily available direct bandgap material in CMOS, III-V continues to be relevant. In the\ndiscussion section, the authors discuss III-V active materials and their potential integration with the lithium tantalate-Si3N4\nplatform in the future through bonding. Can the authors comment on the difficulties that may be associated with the adoption\nof this in CMOS foundries.\n4) In a similar vein, lithium tantalate is not currently widely used in CMOS foundries. Some discussion about what could\naccelerate adoption in foundries would be useful.", "reviewer_4": "The authors achieved a wafer-scale heterogeneous bonding of LiTaO3 onto Si3N4 PIC platform. Compared to monolithic\nelectro-optic photonics platforms, this approach leverages the low-loss Si3N4 waveguides while simultaneously exploiting\nthe electro-optic properties of LiTaO3. The work demonstrates EO modulation with a Vπ of 6 V and bandwidths up to 100\nGHz, as well as both IMDD and coherent signal transmission, showing performance comparable to EO modulators realized\non other PIC platforms. The manuscript is suitable for publication, but the authors should address the following points before\nacceptance:\n1. Compared to state-of-the-art EO modulators, this work exhibits relatively low modulation efficiency, with a reported Vπ·L of\n4.08 V·cm. For the MZM used in coherent data transmission, the electrode length is up to 13.5 mm. What is the underlying\nreason for this low efficiency? Is it due to limited optical mode overlap or insufficient electric-field overlap with the active\nLiTaO₃ film?\n2. Was the EO MZM measured under a DC bias? If so, was any bias drift observed, and how severe or minor was it\ncompared to that in lithium niobate devices?\n3. In line 251, some works in other heterogeneous platforms are cited (mainly LiNbO3 or LiTaO3). However, there have also\nbeen similar demonstrations using other EO materials, such as barium titanate or SiN PIC platforms, which provide\ncomparable integration advantages but with significantly larger Pockels coefficients. These works should also be discussed\nin comparison. In addition, including other types of EO modulators and summarizing performance metrics (both monolithic\nand heterogeneous) in a comparison table would make the advantages of this heterogeneous Si3N4-LiTaO3 platform over\nother photonic platforms clearer.\n4. The advantages of LiTaO3 over lithium niobate in this context of modulators were not really very clear to me – lower\nbirefringence and reduced photorefractive effects were mentioned, but how significant is the advantage (especially if we\nwere to compare LN-with-nitride and LT-with-nitride side by side)? I suppose the “fast tunable LIDAR” application makes\nsense, but a more quantitative discussion would strengthen the motivation.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "A point-by-point response to the reviews’ comments of the manuscript\n“Heterogeneously integrated lithium tantalate-on-silicon nitride\nmodulators for high-speed communications”\nWe would like to express our sincere gratitude to all the reviewers for their detailed review and\ninsightful suggestions regarding the manuscript. Here, we provide point-by-point answers to\nthe reviewers’ comments. For clarity, the original text of the comments is shown in black, the\nreplies are in blue, and the actions taken are highlighted in red and corresponding modifications\nin the manuscript are underlined.\nReviewer #1 (Remarks to the Author):\nThe authors showed a heterogeneously integrated LiTaO3 on silicon nitride modulators. The\nfabrication process is quite similar as previous works of LiNbO3 on silicon nitride. However, due\nto improved photostability, low birefringence and enhanced DC bias stability, this work showed\nenhanced performance of heterogeneous integrated modulators. Nevertheless, some\nmeasurements, which could highlight the enhanced performance of heterogeneous integrated\nmodulators, is missing. Wafer level analysis is also missing for this wafer level heterogeneous\nintegration. The authors should add this analysis before this paper could be accepted in Nature\nCommunication. Please see me comments as following:\nReply: We are grateful for the reviewer’s recognition of the enhanced performance our work\ndemonstrates—notably the advancements on broadband electro-optic modulation and record-\nhigh data transmission rate. We also thank the reviewer for the constructive comments on the\nwafer-scale measurements to further highlight the capabilities of our platform and\ncorresponding heterogeneous integration technique. We have addressed these points carefully\nin our response below.\nMain text:\n1. In line 48, the authors highlight travelling-wave optical parametric amplifier in 2022 and 2024,\nbut didn’t cite the first work from Chalmers in Science advances 2021.\nReply: We thank the reviewer for the careful literature review and mentioning the missing\ncitation on integrated optical parametric amplifier (OPA). In our revised version, we have now\nadded the citations of the first integrated OPA work from Ye, Z. et al. (Science advances 7.38\n(2021): eabi8150.).\nAction taken: To highlight the capacity of silicon nitride waveguide on on-chip OPA, we have\nupdated our reference as below:\nNew citations in main text (Page 1):\n[16] Z. Ye, P. Zhao, K. Twayana, M. Karlsson, V. Torres Company, and P. A. Andrekson,\nOvercoming the quantum limit of optical amplification in monolithic waveguides, Science\nAdvances 7, eabi8150 (2021).\n2. Could authors add wafer mapping of the thickness of LiTaO3 thin film and estimated silicon\nnitride waveguide height due to the nonuniformity of etching and CMP process? Thickness\nvariation of SiO2 interlayer? Its impact on modulator?\nReply: We thank the reviewer for raising this concern on wafer-scale uniformity. For the\nLiTaO3 thin film, the LiTaO3-on-insulator (LTOI) wafers are first produced via ion slicing (Smart-\nCut) followed by bonding to a silicon wafer covered with wet oxide. We then use this wafer to\nbond the LiTaO3 layer onto the Damascene photonic Si3N4 wafer. This technique preserves the\noriginal thickness variations of the bonded thin film, which has a nonuniformity of less than 5%\n(Supplementary figure 4(b)). Instead of using a LiTaO3 film from a LTOI wafer, we could directly\nbond an ion-sliced LiTaO3 film from a bulk wafer to our patterned silicon nitride wafer as\nperformed for lithium niobate in [Laser Photonics Rev. e00138 (2025)]. Such an approach would\ncircumvent any film degradation experienced by the LiTaO3 film in the preparation of the LTOI\nwafer used for this work.\nFor the Si3N4 layer, as seen in what is now Supplementary figure 4(a) of our revised SI, its\nvariation typically lies on the scale of several tens of nanometers and arises primarily from the\nCMP process. Specifically, the process is sensitive to the polishing stress distribution and\nsurface relief induced by small filler patterns. As emphasized in Supplementary Figure 4(c),\nthese thickness variations can affect the modulator’s half-voltage (Vpi) and optical loss, albeit\nto a negligible extent.\nFor the interlayer oxide, its thickness lies around 50 nm and realistically only varies by a few\nnanometers. Such small variations have a negligible impact on both the optical effective index\nand the electro-optic mode overlap and thus does not affect the modulator’s performance.\nAction taken: In the Supplementary Information (SI), we have put our measured wafer mapping\ndata of the LiTaO3 thin film and Si3N4 waveguide height using an optical profilometer (FilMetrics\nF54-XY-200). We have also simulated the modulator’s optical loss and Vpi for different Si3N4\nwaveguide heights. We then discuss the non-uniformity sources for each layer and analyze\ntheir impacts on the modulator’s performance.\nAddition to the main text (Page 3):\nSupplementary Note 3 provides further details on wafer-scale yields, which include film\nthickness homogeneity and device performance analyses.\nAddition to the SI (Supplementary Section 3):\nSupplementary Figure 4. Analysis of thickness variations and their impact on device\nperformance. Thickness maps of the (a) silicon nitride layer and (b) lithium tantalate thin\nfilm. The maps are interpolated from measurements at the nine central stepper fields\n(marked as black dots). Simulated modulation efficiency and optical loss as a function of\n(c) the silicon nitride waveguide height and (d) the lithium tantalate thin film thickness. The\nshaded regions indicate the measured range of film thickness on the wafers.\nTo quantify the wafer-scale process yield of the Si N -LiTaO platform, Supplementary\n3 4 3\nFigures 4(a,b) show the measured thickness maps of the Si N waveguide (post-\n3 4\nDamascene process) and thin film LiTaO (post-bonding) layers, respectively. The raw\n3\ndata, measured at the center of the nine stepper fields defined in Supplementary Figure\n5(a) are (532 nm, 539 nm, 511 nm; 559 nm, 571 nm, 531 nm; 535 nm, 543 nm, 505 nm)\nfor the Si N layer and (306 nm, 300 nm, 298 nm; 293 nm, 295 nm, 289 nm; 294 nm, 298\n3 4\nnm, 299 nm) for the LiTaO3 layer. The Si N thickness features height variations\n3 4\nintroduced by the surface polishing step of the Damascene process. Additional CMP on\nthe interlayer used for bonding can further modify the absolute Si N thickness across the\n3 4\nwafer. In contrast, the bonded LiTaO thin film exhibits much higher uniformity, with total\n3\nvariations near 17 nm.\nBased on the simulated results in Supplementary Figure 4(c,d), increasing the waveguide\nheight within the measured range of 500 to 560 nm only slightly decreases optical loss due\nto stronger mode confinement in the Si N core. This reduction comes at the cost of an\n3 4\nincreased Vπ from 4.55 to 4.85 V. As for the LiTaO thickness, a tens-of-nanometer\n3\nvariation significantly alters Vπ, while a thicker LiTaO thin film increases metal-induced\n3\nabsorption loss. Therefore, we identify the uniformity of the LiTaO layer as the most critical\n3\nparameter for achieving consistent device performance. The excellent uniformity provided\nby the ion-slicing and wafer-bonding techniques is therefore crucial for this platform. To\nfurther improve wafer-level device consistency, using a subtractive process to fabricate the\nSi N waveguide [7] can mitigate dimensional changes that arise from the polishing and\n3 4\nthermal reflow fabrication steps. However, achieving a flat surface required for high-yield\nbonding is considerably more complex with this subtractive method compared to the\nDamascene process.\n3. The authors highlight LiTaO3 superior feature of enhanced DC bias stability compared to\nLiNbO3. Could authors add DC bias stability measurement of this heterogeneously integrated\nLiTaO3 on Si3N4 modulator? Does heterogeneous integration maintain superior feature of\nenhanced DC bias stability?\nReply: Thanks for the reviewer’s suggestion. Although prior work (C. Wang, Nature 629, 784\n(2024)) had shown the superior modulation stability inherited from LiTaO3, adding a DC bias\ndrift measurement is still reasonable to demonstrate our platform’s advantage in preserving this\nintrinsic material property. Here, the measurements are carried out through a homemade\nphotonic packaging process, thereby removing the effects of chip-to-fiber alignment drift while\ncoupling the modulator’s output to a fiber mounted on a flexure stage. By setting the packaged\nmodulator at its working point (half Vpi for the balanced MZM), the measured DC bias drift\nexhibits a low power shift of less than 0.5 dB over one hour, thus highlighting its superior\nperformance compared to the current LiTaO3-based performance benchmark [Optica 11, 1614-\n1620 (2024), Optics Express 32, 44115 (2024).]. We believe that this progress is largely due to\nthe stable fiber-to-chip coupling and lower defect density of the unetched LiTaO3 layer, which\nprovides a baseline for future applications.\nAction taken: We have added a new paragraph to the main text highlighting the DC bias drift\nmeasurement and updated Figure 2 to highlight this aspect of our platform. We have also added\na new section to the SI, including the image of the packaged LiTaO3 -Si3N4 modulator and\ndetails about our packaging method.\nModification to the main text (Figure 2 and Page 5):\nFigure 2. Hybrid Si N -LiTaO electro-optic modulators. (a) Optical micrograph of a\n3 4 3\nfabricated 6.8 mm-long hybrid modulator. Insets: optical micrographs of the modulator’s\nunderlying components, which include Y-splitters, a coplanar waveguide electrode, and\ntapered transitions between Si N waveguides and Si N -LiTaO waveguides with a\n3 4 3 4 3\nsimulated 0.28 dB insertion loss. (b) False-colored scanning electron microscopy (SEM)\nimage of the cross-section of a manufactured Si N -LiTaO modulator. (c) Normalized\n3 4 3\ntransmission of a 6.8 mm-long push-pull MZM versus applied voltage. The inset diagram\nshows the high-speed electrode geometry with a 23 μm signal width and a 6 μm electrode\ngap. (d) Measured electro-optic response (electro-optic S21) and microwave return loss\n(electrical reflection S11), revealing a high 3-dB bandwidth near 100 GHz. (e) Output\noptical intensity of the modulator while biased at its working voltage, i.e. its quadrature\npoint, over one hour.\nMonitoring the modulator’s optical output power while set to its quadrature point quantified\nits stability under a DC bias. As outlined in Supplementary Note 6, a dedicated photonic\npackage mitigated the influence of drifting chip-to-fiber coupling efficiency in this\nmeasurement. As shown in Fig. 2(e), the Si N - LiTaO MZM exhibits low power shifts of\n3 4 3\nless than 0.5 dB over a one-hour period, thus outperforming results reported in prior work\non monolithic lithium niobate and lithium tantalate PIC platforms [38, 40, 41]. Minimal\ndegradation of the unetched LiTaO film, where defect-driven photorefractive effects\n3\ntypically give rise to drift [42], likely contributes to the modulator’s enhanced DC bias\nstability.\nAddition to the SI (Supplementary Section 6):\n6. Photonic package for DC-bias drift measurements\nSupplementary Figure 8. Photonic package for DC electro-optic stability measurements.\nImage of the packaged Si N - LiTaO modulator.\n3 4 3\nThe DC bias drift of a modulator consists of the temporal variations in its optical output\npower while biased at a constant voltage (e.g., the quadrature point). These fluctuations\ntypically arise from mechanisms such as charge relaxation and the photorefractive effect\n[14, 15]. Though the DC-stability of monolithic LiTaO modulators was demonstrated in\n3\nprevious work [16], a similar study is still essential to verify whether this feature also applies\nto hybrid Si N - LiTaO MZMs. To remove the influence of varying chip-to-fiber coupling\n3 4 3\nefficiency, a robust and custom photonic package was engineered for this experiment. As\nshown in Supplementary Figure 8, the chip was first manually mounted onto a copper sub-\nmount with a UV-cure adhesive and then electrically connected with\nwire bonding (F&S Bondtec 56i). This step was followed by active alignment of the\ninput/output fibers that were precisely maneuvered using multi-axis fixture, while the\noptical power was monitored with a photodiode. Once the coupling efficiency was\noptimized, the fibers were permanently fixed in place at the chip facets using high-precision\nepoxies.\n4. Heterogeneous integration in Wafer level is crucial for massive production. The authors\nshowed high yield wafer bonding of LiTaO3 thin film on processed Si3N4 wafer through a camera\nimage. However, the authors didn’t provide any wafer level analysis of the performance of\nheterogeneously integrated LiTaO3 on Si3N4 modulator. Could authors provide more detailed\nanalysis? Probably provide 5 measurements regarding a fixed design from 5 different shots.\nReply: We thank the reviewer for this constructive comment. Demonstrating wafer-level\nperformance is essential to validate the potential of our platform for volume manufacturing. We\nhave selected five fields across the main area of the wafer and conducted a series of\nmeasurements on nominally identical devices including ring resonators, long, and short\nmodulators. Besides higher levels of optical losses in one field (F7), these properties are\nconsistent across the wafer.\nAction taken: We have added a new section to the SI, which presents the detailed wafer-level\nanalysis of key performance metrics (high-frequency EE response, frequency microwave loss,\noptical loss and half-wave voltage) from five distinct fields. The attached illustration summarizes\nthese important uniformity results.\nAddition to the SI (Supplementary Section 3):\nSupplementary Figure 5. Wafer-scale characterization of EO modulators and\nmicroring resonators. (a) Schematic showing the five fields selected for wafer-level\ntesting (left panel) and the selected chips for inter-field device comparison (right\npanel). (b) Measured microwave electrical-to-electrical S21 response for 6.8 mm long\nand 16 mm long CPW electrodes, showing high uniformity across the five fields. (c)\nIntrinsic linewidth, κ0/2π, histogram for microrings from the selected fields. All rings\nshare the same design featuring a 112 GHz free spectral range. (d,e) Measured\nmodulation efficiency and RF loss slope for three 6.8 mm MZMs within C4. The\nmodulators share the same signal-ground gap yet have different signal electrode\nwidths. Missing data points are due to device damage during handling or packaging.\nAs depicted in Supplementary Figure 5(a), device performance uniformity across the\nwafer is characterized by choosing five representative fields for testing as well as\nselecting three chips within a given field for evaluating inter-field device variations.\nFrom the obtained microwave transmission displayed in Supplementary Figure 5(b),\nthe 3 dB EO bandwidth can be approximately calculated from the 6 dB electrical\nbandwidth when only considering the effects of microwave loss and assuming\nimpedance matching and velocity matching [8, 9]. The electrical S21 curves for both\nthe 6.8 mm and 16 mm long devices are tightly grouped, indicating excellent\nconsistency. Correspondingly, the estimated 3 dB EO bandwidth exceeds 67 GHz for\nthe 6.8 mm modulators and is approximately 20 GHz for the 16 mm modulators across\nall tested fields. Optical loss uniformity was evaluated by measuring the intrinsic\nlinewidth κ /2π of microring resonators across the same five fields. Supplementary\nconsidered frequency range of 184-200 THz. The microrings are located on C5 and\nshare the same design featuring a 112 GHz free-spectral range. Relevant statistics\npresented as (Field number, Mean value, Median value, Max value) are listed as\nfollows: (F1, 66.8 MHz, 58.6 MHz, 45 MHz), (F3, 69.1 MHz, 57.7 MHz, 55 MHz), (F5,\n77.8 MHz, 71.7 MHz, 55 MHz), (F7, 164 MHz, 187 MHz, 50 MHz), (F9, 85.6 MHz,\n57.1 MHz, 55 MHz). Inter- and intra-field uniformity is highlighted in Supplementary\nFigure 7(d,e). The modulation efficiency and RF loss for three 6.8 mm long MZMs,\nwith the same electrode gap and different signal widths (marked as Devices 101-103)\nin C4, demonstrate comparable performance to the reported device in the main text,\nwith minor variations attributed to fabrication tolerances.\nSupplementary Information\n1. Figure S1 has an error, in the process step of DUV lithography, the mask should be resist\ninstead of silicon nitride.\nReply: We thank the reviewer for catching this mistake in the SI. The layer should be marked\nas red to represent the ‘Resist’ for patterning the silicon nitride waveguide.\nAction taken: We have modified Supplementary Figure 1 as shown below:\n2. The authors’ previous work typically use LPCVD TEOS, HTO, LTO. Could authors explain\nwhy ICPCVD SiO2 is used as interlayer in this work?\nReply: We sincerely thank the reviewer for raising this question, which allows us to correct a\nsignificant typo in the manuscript. The typo is that the interlayer is indeed deposited using\nLPCVD rather than ICPCVD. The mistake is because some of our more recent works have\nutilized an ICPCVD process with a SiCl4 precursor to deposit a high-density and hydrogen-free\ncladding, which is an efficient deposition method as detailed in [arXiv preprint arXiv:2312.07203\n(2023)]. However, for the specific requirements of the high-speed modulator, ICPCVD is not a\nsuitable choice owing to the involved high-power oxygen plasma (normally exceeding 2000 W).\nThis kind of plasma could introduce defects at the Si/SiO2 interface, thereby leading to\nsignificant parasitic conduction losses at microwave frequencies. Therefore, we chose the\nestablished method of LPCVD followed by annealing to ensure the lowest RF loss.\nAction taken: We have corrected the description of the interlayer deposition method in the SI.\nRevision to the SI (Supplementary Section 1):\nA 1 μm thick oxide interlayer is deposited by LPCVD, after which an additional 1200\nannealing is conducted to eliminate absorption losses associated with hydrogen impurities.\nFinally, a second CMP step is adopted to polish the interlayer to a thickness of 50 nm℃ so\nas to remove the residual topography due to the Damascene process, as well as ensuring\nsub-nanometer surface roughness for subsequent wafer bonding of lithium tantalate on\nDamascene silicon nitride wafers.\n3. The authors use a inversed LiTaO3 taper with tip width of 500nm to form a low-loss transition\nfrom Si3N4 waveguide to Si3N4/LiTaO3 hybrid waveguide. However, the large LiTaO3 tip\nintroduce insertion loss of near 0.28 dB. Why not use a even narrower tip to further reduce the\ninsertion loss? Could authors comment the introduced return loss, and its potential impact on\noptical communication?\nReply: We would like to thank the reviewer for these insightful questions. Regrettably, our\ncurrent design does not include test structures (e.g., cascaded tapers) required to accurately\nde-embed the loss profile of a single taper transition, but we would like to offer a detailed\ndiscussion on this topic.\nRegarding the taper tip width and its effect on insertion loss, we fully agree that a narrower tip\nwidth would further reduce the mode transition loss, which is proven by our new simulation\nusing different tip widths (Supplementary Note 4). The 500 nm tip width in this work is a direct\nconsequence of the resolution limit of our maskless lithography tool (Heidelberg Instruments\nMLA150). This could be improved by migrating this lithography step to a higher-resolution\ntechnique, such as DUV and electron-beam lithography, whose resolution can readily achieve\n150 nm or less.\nFurthermore, we acknowledge that any non-ideal mode transition can introduce a return loss.\nHowever, the back reflection from a single taper is sufficiently low so as not to create a Fabry-\nPérot cavity effect between input and output mode transitions, thus we are confident that this\nwas not a dominant factor impairing our high-speed EO modulation. The quality of our\nexperimental communications results presented in the manuscript provides strong evidence for\nthe above conclusion.\nAction taken: We have updated the main text to clarify that the current taper geometry is limited\nby our lithography resolution and have added a discussion on the potential for performance\nimprovement using DUV lithography. In the SI, we have updated the simulation to consider tip\nwidths from 200 nm to 500 nm. The results show the potential improvements of using a\nnarrower transition taper.\nRevision to the SI (Supplementary Section 4):\nAs shown in Supplementary Figure 6(c), the 500 nm minimal width of the taper primarily\nlimits transmission. The patterning of the LiTaO layer relied on a mask-less lithography\n3\nmethod. As shown in the scanning electron micrograph of Supplementary Figure 6(d), this\ntype of lithography produces coarser features resulting in greater insertion losses. The\nsimulated data provided in Supplementary Figure 6(b) suggest that decreasing this width\ndown to 200 nm can decrease the taper’s insertion loss down to 0.08 dB. As previously\ndemonstrated for heterogeneously integrated LiNbO -Si N circuits, DUV optical\n3 3 4\nlithography can reach these widths and insertion loss figures.\nSupplementary Figure 6 Simulated transmission of silicon nitride-lithium tantalate\nwaveguide transitions. (a) Schematic diagram and corresponding FDTD simulations of the\nadiabatic coupling from the Si N waveguide to the hybrid Si N -LiTaO waveguide. (b)\n3 4 3 4 3\nSimulated transmission spectrum of the waveguide transition for various minimum LiTaO\n3\ntaper widths. (c) Simulated electric field distribution of the adiabatic taper for a 500 nm\nminimum LiTaO taper width. (d) Top-view scanning electron micrograph of the waveguide\n3\ntransition where the patterned LiTaO layer features blue false colors.\nThe manuscript positions itself as the first waferscale -LT--on--Si N platform achieving ~100\nGHz electro-optic bandwidth and high -symbol rate IM/DD and coherent\ncommunications -onchip. This novelty claim is broadly convincing₃, a₄s earlier LT efforts\nemphasized either monolithic LTOI photonics or discrete -thin-film- LT modulators without\nco-integration on Si N , while prior heterogeneous integration on Si N used LiNbO (LN). By\ntransposing the proven LN--on--Si N heterogeneous template to LT and then validating with\nboth IM/DD and coh₃er₄ent formats the manuscript advances the mat₃eri₄al–platform co₃mbination\nin a way that is more than cosmeti₃c. T₄hat said, the narrative would benefit from a concise related w-ork comparison in a paragraph or table format (bandwidth, symbol rate, efficiency, routing loss) to precisely delimit what is new\nand what is inherited.\nReply: We sincerely appreciate the reviewer’s acknowledgements of our efforts in advancing\nheterogenous photonic platforms exploiting multi-material capabilities and pushing towards\npractical telecommunication applications. Following the reviewer’s constructive suggestion, we\ncompletely agree that a comprehensive comparison to related works helps readers precisely\nunderstand the advances made by our platform . As such, we have added a table in the SI to\ncompare our work with other state-of-the-art platforms.\nAction taken: In accordance with the reviewer’s suggestion, we have added a comparison table\nin the SI (Supplementary Note 9). This table lists the key performance metrics (e.g. Bandwidth,\nVpi L, Data rate, BER) of current integrated EO modulator technologies, which includes\nmonolithic thin film (LNOI, LTOI, BTOOI) and recent heterogeneous works (BTO-on-Si3N4,\nLiNbO3-on-Si3N4 and LiTaO3-Si3N4).\nAddition to SI (Supplementary Section 9):\n8. Comparison to the state-of-the-art\nTo benchmark our device against the state-of-the-art, we summarize the performance\nmetrics of leading EO modulator technologies in Supplementary Table 3. These\ntechnologies include monolithic platforms such as LiNbO -on-insulator (LNOI), LiTaO -on-\n3 3\ninsulator (LTOI) and BaTiO -on-insulator (BTOOI) as well as heterogeneous photonic\n3\nplatforms based on BaTiO -Si N , LiNbO3-Si N and LiTaO -Si N . Our devices exhibit a\n3 3 4 3 4 3 3 4\ncompelling performance profile for both single MZM and IQ modulator architectures by\nachieving superior data throughput while maintaining competitive modulation efficiency.\nSupplementary Table 3. Performance metrics of high-speed EO modulators.\nLine\nBandwidth Vπ·L\nRef. Platform Type Format rate BER\n[GHz] [V·cm]\n[Gbit/s]\nsingle\nMZM\nsingle ⁻\nLNOI > 45 2.8 PAM 8 210 1.50 × 10 ²\nMZM\nsingle ⁻\nLNOI ~ 100 2.2 NA NA NA\nMZM\nsingle\nMZM\nsingle ⁻\nLNOI ~ 170 3.3 PAM 8 240 1.10 × 10 ²\nMZM\nLNOI IQ 48 2.47 16 QAM 320 8.41 × 10⁻³\nDP 16\nQAM\nDP 64\nLNOI IQ 110 2.3 2220 NA\nQAM\nsingle\nMZM\nsingle ⁻\nMZM\nsingle\nMZM\n₃ ₃ ₄ single ⁻\nMZM\n₃ ₃ ₄ single ⁻\nMZM\nThis ₃ ₃ ₄ single\nLiTaO -Si N 100 4.08 PAM 4 400 3.00 × 10 ²\nwork MZM\nThis ₃ ₃ ₄ ⁻\nLiTaO -Si N IQ 34 4.05 QPSK 408 7.00 × 10 ³\nwork\nThis ₃ ₃ ₄ ⁻\nLiTaO -Si N IQ 34 4.05 16 QAM 704 3.08 × 10 ²\nwork\n₃ ₃ ₄ ⁻\nThe bonding and thinning flow is described in a level of detail suitable for reproduction in a\ncapable fab, including hydrophilic prebonding and a -moderate temperature anneal. Avoiding\naggressive plasma exposure of LT during transfer is methodologically sound. The -wafer-level\nphotograph supports the ‘-wafer-scale’ claim qualitatively, and the text asserts complete\nbonding across nine central fields. However, for genuine manufacturing credibility the paper\nshould substantiate this with wafer maps and statistics, showing -device--to-\ndevice- distributions (Vπ, VπL, bandwidth, insertion loss) and a quantitative definition of\n‘bonding yield’ in terms of devices meeting specific specs. This addition would convincingly\nelevate the scalability argument.\nReply: We would like to thank the reviewer for the positive feedback on our fabrication\ntechnique and highlighting the necessity of wafer-level statistics. Concerning the device\nperformance across the whole wafer, we ideally would present measured data from all nine\ncentral fields. However, several devices with available designs were already committed to\ncleaving, packaging and high-speed telecommunication testing in collaborating labs. To provide\na representative performance map, we have characterized identical devices from five\ndistributed fields spanning the full bonded area. This analysis, now presented in the SI, provides\nthe device-to-device distributions for key performance metrics (e.g. modulation efficiency,\nbandwidth, optical loss).\nFor the ‘bonding yield’ mentioned by the reviewer, we define it as the percentage of devices\nthat meet a specific set of performance specifications. To quantify this, yielding a complete\nanalysis would require testing a very large population of different devices and even multiple\nfabrication runs, which is essential for commercial manufacturing but beyond the scope of this\ninitial demonstration. However, to provide more quantitative figures involving the bonded wafers,\nwe now provide the thickness mapping for nine field to further emphasize our wafer-scale\nintegration capabilities. Furthermore, the wafer-scale device measurements mentioned above\ndisplays a level of uniformity serving as a strong proxy for the commercial potential and high\nscalability of our platform.\nAction taken: In the SI, we have added a new section titled “Wafer-level characterization” with\nSupplementary Figure 4 and Supplementary Figure 5, which presents the detailed wafer-level\nanalysis of thickness mapping and key performance metrics for MZMs (high-frequency electric-\nto-electric (EE) response, frequency microwave loss, optical loss, and half-wave voltage) from\nfive distinct fields.\nTo avoid repetition, the modification details can be seen in our above response to Reviewer #1,\nComment #2 and Comment #4.\nLoss extraction via ring-resonator linewidths is appropriate for- a hybrid stack. The reported\n~14.2 dB/m is reasonable for a first--generation LT--on--Si N hybrid mode but, in absolute\nterms, is higher than state--of--the-art- bare Si N routing and also somewhat above the best\npublished heterogeneous LN--on--Si N values. Because the₃ ab₄stract uses the phrase ‘ultralow\nloss,’ the manuscript should reframe the lan₃gu₄age and present the number contextually,\nexplicitly distinguishing hybrid--mo₃de₄ loss from bare--core Si N , summarizing the\nmeasurement method and uncertainty, and clarifying whether the figure reflects the hybrid\noptical mode or the Si N core. Providing measured mode-converter in₃se₄rtion loss (rather than\nsimulation only) would further clarify where loss accrues- in largescale circuits. In line “79” the statement – “ultralow o₃pt₄ical propagation losses of O(dB/m)-” is also misleading as the material\nis not lossless, please find a way to mitigate the statement.\nReply: We thank the reviewer for pointing out the inaccurate statement on optical loss in this\nmanuscript. We would like to clarify that the reported loss value of 14.2 dB/m was extracted\nsolely from the linewidth of the LiTaO3-on-Si3N4 ring resonators, which are specifically designed\nto probe their losses. Compared to other state-of-the-art photonic integrated circuits, the\npropagation loss for this hybrid mode is not considered as ultralow, especially when compared\nto other monolithic platforms such as Si3N4 and lithium tantalate. Thus, we agree that our\nlanguage regarding ‘ultralow’ needs to be rephrased as ‘low optical loss’. On their own, silicon\nnitride waveguides fabricated using the photonic Damascene process leveraged in our work do\nfeature losses on the scale of 3-4 dB/m [Optica 5.10 (2018): 1347-1353, Nature\ncommunications 12.1 (2021): 2236]. As for low-loss LTOI PICs [Nature 629, 784–790 (2024)],\nthey can feature losses down to 5.6 dB/m. These very low figures suggest that our reported\n14.2 dB/m figure is not limited by material absorption but rather increased bending loss radiating\nin the unpatterned LiTaO3 layer.\nMoreover, we fully agree with the reviewer that presenting the measured insertion loss of the\nmode converters would offer a more complete picture of the total on-chip loss budget.\nUnfortunately, our current reticle design does not include dedicated test structures (e.g.,\ncascaded tapers) that are necessary to accurately de-embed the insertion loss of a single mode\nconverter. Without these structures, any measurement would be convoluted with other sources\nof loss, such as fiber-to-chip coupling. However, we believe that its insertion loss has a\nnegligible impact on our modulation performance, as supported by our high-speed transmission\nexperiments in the main text. To compensate for this unavailable experimental data, we\nprovided additional simulated mode converter insertion loss data for various minimum taper\nwidth values. The results show how reducing this width with sharper lithography methods can\nreduce insertion losses. Such claims are also supported by our group’s prior work exploiting\nsuch methods to implement mode converters in a heterogenous LiNbO3-Si3N4 photonic\nintegrated circuits [Nat. Commun. 14, 3499 (2023)].\nAction taken: We still label losses as “ultra-low” when referring to prior work on silicon nitride\nwaveguides fabricated with the photonic Damascene process. However, while referring to our\nheterogeneous platform, we now refer to these losses as “low” in the main text:\n... we implement modulators that achieve low optical losses (~ 14.2 dB/m) while\ncombining ...\n... Built-in integration with thick Si N waveguides also enables a new generation of on-\n3 4\nchip systems leveraging both electro-optic and low loss waveguides ranging from\nmicrowave oscillators ...\nIn the main text (Page 3), we now also reinforce that our 14.2 dB/m likely arises from bending\nlosses due to the smaller achieved losses in monolithic silicon nitride and lithium tantalate\nplatforms:\nThis figure is larger than the 3-4 dB/m and 5.6 dB/m loss values achieved in monolithic\nSi N [7, 8] and LiTaO [30] circuits. This comparison provides further evidence that the\n3 4 3\nhybrid waveguide’s losses do not come from intrinsic material losses but rather radiative\nlosses in the unpatterned and higher-index LiTaO film.\nconverter in the SI (Please see our above response to Reviewer #1, Comment #3 in\n“Supplementary information” part).\nThe inclusion of a measured VπL ≈ 4.08 V·cm is a strength, as it enables cross-platform\ncomparisons without ambiguity. The text also references simulations that predict a somewhat\nlarger VπL for nominal geometry, which is encouraging but calls for reconciliation. A discussion\naligning measured and simulated results, stating the electrooptic coefficients used, optical and\nmicrowave mode overlaps, exact electrode geometry, and layer thicknesses, would\nresolve -possible confusion- and reinforce confidence in the reported efficiency.\nReply: We thank the reviewer for this comment. Our original model was based on a simplified\nmodel used for bulk LNOI modulators, where the optical mode is entirely confined within the\nferroelectric material. However, for our proposed hybrid LiTaO3-on-Si3N4 platform, this\napproximation is less accurate due to the spatially varying refractive index and the optical mode\nacross different materials. Given this limitation, we have updated our model based on\nperturbation theory, thereby allowing it to universally model any integrated waveguide structure\n(see Eqs. 2-5 in the revised SI). The simulated results using this model yields a Vpi·L that is in\nmuch closer agreement with our measured one, where a small deviation might be due to the\nfabrication tolerance. For the detailed parameters used in our model, we now list them in the\nTable S1 of the SI to enable readers to easily reproduce our results.\nAction taken: We have revised Section 2. A in the SI. The previous simplified model for\ncalculating the electro-optic efficiency has been replaced with the full derivation based on a\nperturbation theory model. We have also updated the text to discuss the agreement between\nthe new simulation and our experimental data, and to detail the fabrication tolerances that\naccount for the remaining deviation.\nRevision to the SI (Supplementary Section 2.A. and Supplementary Section 4.A.):\n• Eqs. (2-5): Derivation of a modified VπL expression starting from perturbation theory.\n• Table S1: List of model parameters used to calculate the reported efficiency.\n• Passage in Section 4 of the SI attributing efficiency discrepancies to an uneven silicon\nnitride layer and a slightly varied LiTaO3 thin film:\nBased on the simulated results in Supplementary Figures 4(c,d), increasing the\nwaveguide height within the measured range of 500 to 560 nm only slightly decreases\noptical loss due to stronger mode confinement in the Si N core. This reduction comes\n3 4\nat the cost of an increased Vπ from 4.55 to 4.85 V. As for the LiTaO thickness, a tens-\n3\nof-nanometer variation significantly alters Vπ, while a thicker LiTaO thin film increases\n3\nmetal-induced absorption loss. Therefore, we identify the uniformity of the LiTaO layer\n3\nas the most critical parameter for achieving consistent device performance.\nBroadband S--parameter data up to ~110 GHz, including |S | below −15 dB, indicate careful\nimpedance control and low reflection at the device ports. The small--signal |S | roll-off- appears\nconsistent with ohmic attenuation ( √f) and modest velocity ₁m₁ismatch. For completeness and\nreproducibility, the paper should aggregate RF attenuation in dB/cm versus f₂r₁equency with line\nlength, calibration plane, and term∝ination explicitly stated, and add S if available. Because\nrecent LT work has highlighted the conductivity advantages of silver electrodes above 100 GHz,\na short discussion of metal choice (Au vs Ag) and anticipated performa₂n₂ce–reliability trade-offs\nwould be valuable for readers planning to push to even higher bandwidths.\nReply: We agree with the reviewer’s suggestion that an explanation on electro-optic S21\nmeasurements is essential for completeness and reproducibility.\nRegarding the RF attenuation, we have updated Supplementary Figure 7(a) in the SI to show\nthe modulator RF loss for different device lengths (6.8 mm and 16 mm), which feature\ncomparable values. For this measurement, the VNA was calibrated and terminated using high\nspeed GSG probes. In the electro-optic (EO) S-parameter measurements, the CPW was\nterminated by a GSG probe connected to a 50 Ω coaxial load. The frequency response of a\nhigh-speed photodiode and one GSG probe were then de-embedded from the raw EO S21\nsignals.\nRegarding the requested S22 data, we infer the reviewer is referring to the EO S22 involved in\nour EO measurements. In these measurements, the microwave input is port 1, and the\nmodulated light received at the output is in port 2. Owing to the non-reciprocal features of\ntravelling wave EO modulators, the dominant contribution to the measured EO S22 comes from\nco-propagating microwave reflections while sending the modulating microwave from port 2.\nHowever, this information can be deduced from the EO S21 (modulation bandwidth) and the\nEE S11 (impedance matching affecting microwave reflections) parameters, which universally\ndescribe the performance of travelling-wave EO modulators. Both values are already provided\nand analyzed in our manuscript. Adding the S22 parameter would add a level of redundancy\nwhich is not necessarily required nor typically investigated in other works.\nThe reviewer also raises a very important point on the metal selection for high-speed electrodes.\nTo provide a comprehensive discussion, we have added a new analysis to Supplementary\nFigure 7(a), which directly benchmarks the RF loss of our Au electrodes against both Au and\nAg electrodes on the monolithic LTOI platform from our recent work [Optica 11, 1614-1620\n(2024)]. The updated data confirms that silver offers a slight advantage due to its lower\nconductivity at higher frequencies. However, our choice of gold was a strategic decision based\non the trade-off between peak performance and device reliability. The chemical stability of gold\nelectrodes ensures fabrication and device robustness, whereas silver's susceptibility to\noxidation poses a significant challenge for wafer-scale manufacturing and long-term device\nstability.\nAction taken: We have modified Supplementary Figure 7 (a) in the SI with additional RF loss\ndata, and have updated the description in Supplementary Section 5.\nRevision to the SI:\nSupplementary Figure 7. Modulator microwave transmission and modulation efficiency. (a)\nMeasured co-planar waveguide microwave losses on a square-root frequency axis,\nincluding the performance comparison with monolithic thin film LiTaO modulators using 6\n3\nmm long gold electrodes and silver electrodes from Ref.[11]. (b) Measured transmission\nof the Si N -LiTaO modulators driven by modulation frequencies from 1 Hz to 100 kHz.\n3 4 3\nExtracted modulator Vπ value at (c) different modulation frequencies at a fixed optical 1550\nnm optical wavelength and (d) different optical wavelengths at a fixed 100 Hz modulation\nfrequency.\nWe first characterized the RF attenuation properties of the fabricated CPW-type\nmodulators. Supplementary Figure 7(a) presents the extracted microwave loss of the 6.8\nmm long gold electrode on the Si N -LiTaO platform, using a 67 GHz vector network\n3 4 3\nanalyzer (VNA). The square-root frequency dependence of RF loss (the black dashed line\nshown in Supplementary Figure 7(a)) indicates the dominant ohmic loss mechanism (α\n√fM W ), confirming that our fabrication techniques can effectively prevent parasitic-\ncapacitance-induced loss [12, 13]. The measured RF attenuation is comparable to sta∝te-\nof-the-art ultrabroadband LTOI modulators with gold CPW electrodes [11]. Employing\nsilver electrodes could further reduce ohmic losses, thus potentially enabling a higher EO\nbandwidth. However, this choice risks degrading the stability of the electrodes owing to\nsilver’s vulnerability to oxidation. Therefore, incorporating silver-based electrodes in Si N -\n3 4\nLiTaO modulators would require adapting them to wafer-scale fabrication and\n3\nguaranteeing long-term device reliability.\nThe communications results are a major strength of the manuscript. Demonstrations of PAM-4\nin the ~192 GBd range yielding ~333 Gb/s net after FEC, and coherent 16-QAM around 176\nGBd yielding ~581 Gb/s net, establish that the devices function beyond small-signal curiosity.\nThe setups are described clearly and the constraints of the electrical instrumentation are\nacknowledged (e.g., AWG bandwidth limiting the 204-GBd QPSK case). To fully enable\nexternal benchmarking, the manuscript should report per-format pre-FEC BER and EVM, the\nprecise FEC overheads and thresholds, estimated or measured energy-per-bit based on the\ndrive swing at the device pads, and a compact link budget that separates fiber-to-chip coupling,\non-chip routing, modulator section, and any amplification used.\nReply: We thank the reviewer for the positive feedback regarding the communication results\nand for the constructive suggestions to add further data of our measurement results for better\nexternal benchmarking. As outlined below, we have added this information in various parts of\nthe revised manuscript and the SI.\nActions taken: Figures 3(b) and 4(c) already provide the pre-FEC BER in the original manuscript.\nWe now explicitly mention “pre-FEC BER” while referring to the figures and in the figure\ncaptions themselves.\nFigure 3(b) provides the resulting pre-forward error correction (pre-FEC) bit error ratios\n(BER)…\n(b) Measured pre-forward error correction bit error ratio (BER)…\nFigure 4(c) shows the measured pre-FEC BER along…\n(c) Measured pre-forward error correction bit error ratio (BER)\nWhat is now Supplementary Note 8 provides the corresponding EVM for the coherent\ncommunications measurements:\nAddition in the main manuscript:\nIn addition, the error vector magnitude (EVM) for QPSK and 16QAM signals are shown in\nSupplementary Figure 9(c)….\nAddition in the Supplementary information:\nThe EVM is an alternative metric of the quality of the received signal in coherent\ntransmission systems and quantifies the root-mean-square (RMS) distance between the\nreceived complex modulation symbols and their ideal positions in the constellation\ndiagram [27]. We calculate the EVM normalized to the average power as Eq. (7), where\nis the number of symbols over which the EVM is evaluated. Supplementary Figure 9(c)\nshows the resulting EVM for QPSK and 16QAM symbols after equalization as a function\nof the symbol rate. As expected, the EVM increases as the signal quality decreases for\nlarger symbol rates.\nSupplementary Figure 9. Extended signal quality analysis for high-speed data\ntransmission experiments. (a) Normalized generalized mutual information (NGMI) for four-\nlevel pulse-amplitude modulation (PAM4) signals received in our intensity-modulation and\ndirect-detection (IMDD) experiment. (b) NGMI for quadrature phase-shift keying (QPSK,\nbrown) and 16-state quadrature-amplitude modulation (16QAM, blue) signals received in\nour coherent transmission experiment. All signals were transmitted using Si3N4-LiTaO3\nmodulators across various symbol rates. (c) Error vector magnitude (EVM) of the QPSK\n(brown) and 16QAM (blue) symbols extracted at the receiver for different symbol rates.\nAs detailed in the IMDD section, we estimate practically achievable net data rates (NDR) by\ncomparing the normalized generalized mutual information (NGMI) estimated in our experiments\nto the NGMI thresholds provided in reference [46] of the main manuscript. For clarity, we added\na sentence to the coherent communication section that refers to the description in IMDD section.\nIn addition, we added a plot of the NGMI to the supplementary (now: Supplementary Note 8)\nfor better comparison with the thresholds given in the cited reference.\nAddition in the main manuscript:\nFigure 4(d) provides the corresponding AIR and NDR based on the same calculation used\nfor the IMDD results. Additionally, Supplementary Figures 9(b,c) provides the NGMI and\nerror vector magnitude (EVM), respectively.\nAddition in the Supplementary information:\nThe generalized mutual information (GMI) is directly related to the achievable information\nrate (AIR) for bit-interleaved coded modulation and is commonly used to define\nperformance prediction thresholds, especially for soft-decision forward error correction\n(SD-FEC) schemes [24]. We derive the GMI of the transmitted signals from the log-\nlikelihood ratios (LLRs) of the received symbols based on a linear channel model with\nadditive white Gaussian noise (AWGN) as the only impairment [25]. To compare different\nmodulation formats, we calculate the NGMI by normalizing the GMI by the number of bits\nthat can be encoded into a single symbol. Supplementary Figure 9(a) shows the NGMI for\nfour-level pulse-amplitude modulation (PAM4) signals transmitted at different symbol rates\nin our IMDD experiment. The NGMI for QPSK and 16QAM signals transmitted in our\ncoherent transmission experiment are depicted in Supplementary Figure 9(b). Based on\nthese values, we select suitable forward error correction (FEC) codes with the nearest\nlower NGMI threshold as given in [26, Table 2] and evaluate the net data rate (NDR) by\nmultiplying the line rates and the associated FEC code rates, see Figure 3(c) and Figure\n4(d) of the main manuscript. As the symbol rate increases, the signal-to-noise-and-\ndistortion ratio (SNDR) of the received signal decreases, and FEC implementations with\nlarger overheads are required to enable error-free decoding. As a consequence, we\nobserve a reduction of the NDR beyond 192 GBd for PAM4 signals, see Figure 3(c) of the\nmain manuscript.\nSupplementary Note 8 B now provides the estimated electrical energy dissipation per payload\nbit based on the drive voltages at the input RF-probe. We also added a more detailed\ndescription of the electrical transmitter setup used in the IMDD experiment and added a\npreviously missing RF amplifier in the setup in Figure 3(a):\nAddition in the main manuscript:\n… We drive the MZM with electrical PAM4 signals, that are synthesized by offline digital\nsignal processing (Tx-DSP) based on pseudo-random bit sequences (PRBS) and root-\nraised cosine (RRC) pulse-shaping filters [30], and that are converted to the analogue\ndomain using a high-speed arbitrary waveform generator (AWG, M8199B, Keysight\nTechnologies Inc.). A 20 cm-long RF cable followed by a broadband RF amplifier (SHF\nT850 B, SHF Communication Technologies AG) sends the drive signals to the coplanar\ntransmission line of the MZM via a first impedance-matched probe in a ground-signal-\nground configuration. A second probe terminates the transmission line with a 50 Ω coaxial\ntermination. …\nFigure 3. Transmission experiments using intensity-modulation and direct detection (IMDD)\n(a) Experimental setup. ECL: External cavity laser; FPC: Fiber polarization controller; AWG:\nArbitrary-waveform generator; Tx-DSP: Transmitter-digital signal processing (offline);\nAmp.: RF amplifier; EDFA: Erbium-doped fiber amplifier; BPF: Bandpass filter; VOA:\nVariable optical attenuator; PD: Photodiode; RTO: High-speed real-time oscilloscope (256\nGSa/s, 105 GHz); Rx-DSP: Receiver-digital signal processing (offline). (b) Measured pre-\nforward error correction bit error ratio (BER) versus symbol rate for PAM4 transmission,\nwith horizontal dashed lines indicating the thresholds for soft-decision forward error\ncorrection with 15 % and 25 % coding overhead (15 % SD-FEC, 25 % SD-FEC) and for\nhard-decision FEC with 7 % coding overhead (7 % HD-FEC). The eye diagrams of two\nselected data points A and B are shown in panels (d) and (e). (c) Achievable information\nrates (AIR, orange dashed line) and corresponding net data rates (NDR, solid brown line)\nof the IMDD measurements, showing that the highest achieved NDR is 333 Gbit/s using a\nPAM4 signals at a symbol rate of 192 GBd. (d), (e) Eye diagrams (left) at selected symbol\nrate of 160 GBd PAM4 (d) and 200 GBd PAM4 (e), labeled by A and B in panel (b), along\nwith the associated histograms (right) of the reconstructed signal amplitudes in the center\nof the corresponding symbol slot.\nAddition in the Supplementary information:\nTo estimate the electrical energy dissipation per transmitted bit, we first need to extract the\nvoltage levels that were fed to the modulator during the transmission experiment. To this\nend, the electrical transmitter was directly connected to the real-time oscilloscope (RTO,\nUXR 1004A, Keysight Technologies Inc.) and operated with the same parameters and\nsignals as in the optical transmission experiment.\nIn case of the IMDD experiment, the transmitter consisted of a high-speed arbitrary-\nwaveform generator (AWG, M8199B, Keysight Technologies Inc.) and a subsequent RF\namplifier (SHF T850 B, SHF Communication Technologies AG), which was directly\nconnected the RF probe during the transmission experiment. The AWG and the RF\namplifier are connected by a 20 cm-long RF cable. For measuring the voltage levels that\nwere fed to the modulator during the transmission experiment, we connect the output of\nthe RF amplifier directly to the RTO. To estimate the electrical energy dissipation during\nthe data transmission experiment, we calculated the average electrical power fed to the\nmodulator from the voltage samples recorded by the RTO. Assuming impedance matching\nbetween the terminated modulator and the internal resistance of the transmitter (R = 50\nΩ), the average electrical power is calculated from the instantaneous voltage u(t) by Eq.\n(8), where T is an observation time that covers many symbols (T = 32.8 μs in our\nexperiments). The electrical energy dissipation per transmitted payload bit is given by\ndividing the average electric power by the NDR Eq. (9). Note that the transmission\nexperiments at the various symbol rates were conducted with a constant drive-voltage\nswing applied to the MZM, which was ensured by re-adjusting the output voltage settings\nof the AWG for each symbol rate as to maintain the voltage difference between the highest\nand the lowest PAM4 signal level in the range (1.25 ± 0.05) V. Supplementary Figure 10(a)\nshows the net energy per bit for transmitted PAM4 signals at various symbol rates. Since\nthe drive voltages were kept constant, the energy dissipation follows the trend of the\ninverse NDR, see Figure 3(c) of the main manuscript. The lowest energy dissipation per\npayload bit amounts to 12.9 fJ bit−1, reached at a symbol rate of 176 GBd. Non-ideal\nvoltage settings lead to smaller deviations from of the measured values from a continuous\ntrend, as indicated by a dashed line in Supplementary Figure 10(a).\nFor the coherent data transmission experiments, the drive voltages applied to both MZMs\nmust be considered individually, since the MZM in our sample device turned out to have\ndifferent half-wave voltages. Due to the limited physical space between the two input\nconnectors at the feeding RF-probe, the electrical transmitter configuration differs from\nthat one used in the IMDD setup. Specifically, two outputs of the AWG were each directly\nconnected to separate broadband RF amplifiers, followed by two broadband bias-tees and\ntwo 20 cm-long RF cables. The components in both signal paths are nominally identical.\nIn this experiment, we kept the voltage settings of the AWG constant, leading to a decrease\nof the voltage swings coupled to the MZM with increasing symbol rates. As in the IMDD\ncase, the voltage levels for the transmitted QPSK and 16QAM symbols are extracted by\nconnecting the RF cables to the input of the RTO, and the average electrical power and\nthe electrical energy dissipation per transmitted payload bit were estimated in analogy to\nEquations 8 and 9. For the MZM transmitting the in-phase signal, the drive voltage\ndifference between the lowest and the highest signal level decreases from 0.8 V at 144\nGBd to 0.4 V at 204 GBd. For the quadrature MZM, the voltage swing reduced from 1.6 V\nto 0.8 V over the same symbol rate range. The electrical energy per transmitted payload\nbit is depicted in Supplementary Figure 10(b). The minimum net electrical energy\ndissipation per bit amounts to 9.4 fJ bit−1 and was achieved for 16QAM signals with a\nsymbol rate of 176 GBd.\nSupplementary Figure 10. Electrical energy efficiency analysis for high-speed data\ntransmission experiments. (a) Estimated electrical energy dissipation per payload bit\ncalculated from the drive voltages measured for four-level pulse-amplitude modulation\n(PAM4) signals using intensity-modulation and direct-detection (IMDD) and (b) for\nquadrature phase-shift keying (QPSK, brown) and 16-state quadrature-amplitude\nmodulation (16QAM, blue) signals. A third-order polynomial fit (dashed line) provides a\nguide to the eye. All quantities are plotted for different symbol rates. Please note the\ndifferent scales of the vertical axes.\nNow, we also provide characterization metrics that give a more quantitative picture of our\ncompact link budget. Optical losses due to fiber-to-chip coupling, on-chip routing, and the\nmodulator section are now provided in Supplementary Notes 3,4, as addressed in Reviewer\n#1, Comment #3 in the “Supplementary information” part and two comments down in\nthis reply. Finally, as mentioned in the main text on page 6, we do use optical amplification in\nour experiments.\nAfter modulation, an EDFA amplifies the optical signal from −13 dBm to 16 dBm and a\nbandpass filter subsequently reduces out-of-band spontaneous emission noise. At the\nreceiver, the modulated signal enters a 90◦ optical hybrid module, where it interferes with\na continuous-wave local oscillator provided by a second ECL to down-convert the optical\nsignal to four electrical baseband signals.\nThe introduction fairly motivates LT over LN with lower birefringence and improved\nphotostability and DC--bias stability. However, these advantages are not yet quantified on the\npresented platform. Including time-resolv-ed bias drift under realistic optical powers and over a\nrelevant temperature range would turn a qualitative motivation into a platform-level evidence\npoint. Even a modest A/B comparison versus a representative -LN--on--Si N control device, if\navailable, would strengthen the case materially.\n₃ ₄\nReply: We thank the reviewer for his/her constructive suggestion on quantifying the DC-bias\nstability of our integrated hybrid MZMs. In the revised SI, we present a dedicated experiment\ncharacterizing the device’s long-term stability. This was realized by an in-house photonic\npackage to isolate the intrinsic bias drift of our demonstrated platform from external factors such\nas chip-to-fiber coupling efficiency drift due to unstable mounting on flexure stages. A low\noptical power shift of less than 0.5 dB over one hour showcases its superior performance\ncompared to state-of-the-art monolithic LiTaO3 devices [Optica 11, 1614-1620 (2024), Optics\nExpress 32, 44115 (2024).].\nRegarding the additional suggestion, a temperature-dependent measurement is valuable but\npresents a significant challenge to our current packaging technique. Specifically, our current\nepoxies gluing the fibers to the chip can exhibit thermal stress that can damage the device while\nheating it to temperature ranges relevant for such measurement. Similarly, a comparison with\na LN-on-Si N device was not possible as we do not have any available devices at this time.\nNevertheless, we believe that the added long-term stability data provides quantitative evidence\nfor one of t₃he₄ key advantages of our heterogeneous platform and directly addresses the core\nconcern raised by the reviewer.\nAction taken: We have updated the main text (Figure 2) and the SI (Supplementary Section\n6), which includes the result and discussion of the DC bias stability measurement.\nTo avoid repetition, the added section details can be inferred from our above response to\nReviewer #1, Comment #3 in the “main text” part.\nThe text describes fiber--to-chip coupling on the order of several dB per facet and indicates the\nneed for amplification to meet receiver power targets in some experiments, which is entirely\nplausible for -early-stage devices. Nevertheless, a clear separation of -external-coupling loss\nfrom intrinsic -on-chip insertion loss of the MZM and IQM would meaningfully assist\npractitioners. Reporting measured -mode-converter loss, -on-chip device IL, and facet coupling\nwith uncertainties would also help reconcile the -communications-experiment power budgets\nwith the -passive-loss- measurements.\nReply: We thank the reviewer for this constructive suggestion. We have performed the insertion\nloss (IL) measurements for modulators for five fields in our manufactured wafer. To extract the\nMZM device IL, our method was to first characterize the reference LiTaO3-Si3N4 hybrid\nwaveguide (fiber - edge taper - transition taper - waveguide - transition taper - edge taper - fiber)\nto determine the fiber coupling of this chip. We than measured the total loss of the MZM device\nand subtracted the reference coupling loss to extract the intrinsic on-chip MZM IL.\nFrom the measured results shown in the SI, our current device reveals an unexpectedly high\nIL (in excess of several tens of dB), however it did not affect the modulation efficiency a lot. We\nhave identified the cause as a non-optimal DUV dose for defining the Si3N4 pattern. This results\nin a partially exposed taper shape, affecting both the edge coupler and on-chips components\nlike Y-splitters. To explain that, we have also included the results measured from a previous\nsample with an identical optical design, which exhibits excellent performance with a total IL of\na few dB. However, these old samples suffer from a prominent plasma-induced loss introduced\nby the resist strip machine (Tepla 300).\nTherefore, the high loss observed on the current wafer is a process-specific issue that can be\nrectified in future fabrication runs. With a properly optimized fabrication process, we are\nconfident that our platform is highly promising for achieving high speed communication with low\npower budgets.\nAction taken: We have added a new table (Supplementary Table 2) to the SI (Supplementary\nSection 3.B.) presenting the detailed loss budget analysis:\nSupplementary Table 2. Insertion loss (IL) extraction for 6.8 mm MZMs\nField Taper-to-taper IL (dB) Device number Total IL (dB) MZM IL (dB)\nF1 (packaged) / Device 103 -23.0000 / (packaged)\nF3 -20.0736 Device 101 -35.5309 -15.4573\nF3 -20.0736 Device 102 -29.6210 -9.5474\nF3 -20.0736 Device 103 -25.904 -5.8304\nF5 -26.3650 Device 101 -44.8847 -18.5197\nF5 -26.3650 Device 102 -37.2224 -10.8574\nF5 -26.3650 Device 103 -39.7224 -13.3574\nF9 -25.7133 Device 101 -45.7785 -20.0652\nF9 -25.7133 Device 102 -46.4975 -20.7842\nF9 -25.7133 Device 103 -44.1042 -18.3909\nOld sample -6.8627 Device 101 -11.986 -5.1233\nOld sample -6.8627 Device 102 -11.5656 -4.7029\nOld sample -6.8627 Device 103 -14.6682 -7.8055\nWe have also added a new paragraph to discuss these results, explaining the fabrication\nanomaly on this fabrication run and highlighting the intrinsic low-loss potential proven by\nprevious samples:\nRegarding the insertion loss, Supplementary Table 2 details the insertion loss (IL) budgets\nfor the 6.8 mm long single MZM devices. The “Taper-to-taper IL” represents the total fiber-\nto-chip coupling loss measured from a reference straight waveguide with two mode\ntransition tapers, while the intrinsic “MZM IL” is obtained by subtracting the coupling loss\nfrom the “Total IL” of the modulator device. Missing data points in the table, such as F5,\nare due to some devices being damaged during handling or shipped away for high speed\ntelecommunication testing. The MZM devices from the primary wafer exhibit a high IL near\n20 dB as well as a total IL near 40 dB. This is attributed to a non-ideal DUV lithography\nexposure dose during the Si N fabrication step, which resulted in malformed taper\n3 4\nstructures for both the edge couplers and the on-chip Y-splitters. To demonstrate the\nplatform’s intrinsic low-loss capability, we include data attributed to chips labeled as “Old\nsample” from our group’s prior work. The devices feature identical optical designs with\nsuccessfully patterned tapers. However, their high-frequency performance is uniformly\ndegraded by parasitic capacitance loss, which we attribute to the high-power oxygen\nplasma used during the photoresist removal step. This sample shows a competitive\ncoupling loss of near 3.4 dB per facet and a reasonable on-chip MZM IL of 5-8 dB, which\nconfirms that the high loss on the current fabrication run is a correctable anomaly and not\nlimited by optical design.\nOverall methods transparency is good and likely, nevertheless the paper would still benefit from\nexplicit uncertainty estimates for VπL, loss extractions, and bandwidth fitting; precise VNA\ncalibration details and de-embedding steps; and, where applicable, references to public PD-K-\nlike parameters (layer thicknesses, refractive indices at test wavelengths). Adding micrographs\nor- cross-sections of critical transitions (e.g., vertical adiabatic converters-) would also round\nout the reproducibility story.\nReply: We agree that including further details on our modeling and data processing would\nbenefit future studies attempting to reproduce our results. As specified below, we have added\nthe information requested by the reviewer. We have uploaded the code de-embedding the\nresponse of our apparatus from that of the modulator in a data sharing repository\n(https://doi.org/10.5281/zenodo.17276861) which is now linked to our manuscript. However,\nregarding the performance uncertainty, we instead present wafer-level measurements for\nmodulation efficiency and device loss, as mentioned in the above replies, instead of reporting\nthe often negligibly small standard errors from conventional curve fits (e.g., for VπL or resonator\nintrinsic linewidth). We believe this approach more efficiently conveys uncertainties related to\nthe performance of our platform.\nAction taken:\nWe have added the description of VNA calibration and de-embedding procedure to the main\ntext (Page 5):\nA standard calibration kit first ensures two-port calibration by setting the reference plane\nat the tips of the two high-speed electrical probes. The EO measurement then relies on a\nmodification in the setup where the VNA’s output connects to the high-speed PD detecting\noptical output rather than the GSG probe. De-embedding independently characterized\nfrequency responses from the PD and one GSG probe finally yields the intrinsic EO\nresponse of the modulator.\nPDK-like parameters (layer thicknesses, refractive indices at test wavelengths) are now\nprovided in Supplementary Table 1 of the SI (Supplementary Section 2.A.):\nSupplementary Table 1. Parameter values used to model electro-optic performance.\nParameter Description Value\nλ Optical Wavelength 1.55 µm\nn_eff Effective refractive index 1.828\nε_Air Relative permittivity of air (defines ε , ε , ε ) (1)²\nε_SiO2 Relative permittivity of SiO (defines\n₁\nε\n₁\n,\n₂\nε\n₂\n,\n₃\nε\n₃\n) (1.444)²\nε_SiN Relative permittivity of Si N (defines ε , ε , ε ) (1.992)²\n₂ ₁₁ ₂₂ ₃₃\nε_o Ordinary relative permittivity of LiTaO (defines ε , ε ) (2.123)²\n₃ ₄ ₁₁ ₂₂ ₃₃\nε_e Extraordinary relative permittivity of LiTaO (defines ε ) (2.119)²\n₃ ₁₁ ₂₂\nr 13 component of LiTaO 's Pockels tensor 8.4 pm/V\n₃ ₃₃\nr 33 component of LiTaO 's Pockels tensor 30.5 pm/V\n₁₃ ₃\nE_DC(r)/V DC field to applied voltage ratio 6 µm\n₃₃ ₃\nSi N waveguide dimensions 1 µm × 0.5 µm\nLiTaO film thickness 300 nm\n₃ ₄\nInterlayer oxide thickness 30 nm\n₃\nIn Supplementary Section 4 of the SI, Supplementary figure 6(d) now provides a top-view\nSEM of the vertical adiabatic converter. We abstained from taking a cross-section of the device\nto preserve it for future studies:\nAs shown in Supplementary Figure 6(c), the 500 nm minimal width of the taper primarily\nlimits transmission. The patterning of the LiTaO layer relied on a mask-less lithography\n3\nmethod. As shown in the scanning electron micrograph of Supplementary Figure 6(d), this\ntype of lithography produces coarser features resulting in greater insertion losses. The\nsimulated data provided in Supplementary Figure 6(c) suggest that decreasing this width\ndown to 200 nm can decrease the taper’s insertion loss down to 0.08 dB. As previously\ndemonstrated for heterogeneously integrated LiNbO -Si N circuits, DUV optical\n3 3 4\nlithography can reach these widths and insertion loss figures.\nSupplementary Figure 6 Simulated transmission of silicon nitride-lithium tantalate\nwaveguide transitions. (a) Schematic diagram and corresponding FDTD simulations of the\nadiabatic coupling from the Si N waveguide to the hybrid Si N -LiTaO waveguide. (b)\n3 4 3 4 3\nSimulated transmission spectrum of the waveguide transition for various minimum LiTaO\n3\ntaper widths. (c) Simulated electric field distribution of the adiabatic taper for a 500 nm\nminimum LiTaO taper width. (d) Top-view scanning electron micrograph of the waveguide\n3\ntransition where the patterned LiTaO layer features blue false colors.\n~40-GHz-c-lass bandwidths; discrete thin--film LT modulators have reached beyond 100 GHz;\nheterogeneous LN--on--Si N has already proven the architectural value of marrying a strong\nPockels material to ultra-low-loss routing. This manuscript’s unique contribution is to combine\nthe LT material with Si N₃ a₄t wafer scale and then validate with both IM/DD and coherent\ncommunications at symbol rates competitive with the best contemporary demonstrations. Given\nthe sustained interest ₃in ₄coherent transceivers and RF-photonics, the practical impact is\ncredible, particularly if the authors add the benchmarking details outlined above and as such\nthe manuscript could be accepted for publications after the requested revisions have been\nadded.\n-\nReply: We appreciate the positive feedback and insightful suggestions from the reviewer. We\nhave addressed all the points raised and sincerely hope that the point-by-point response\nmeets the reviewer’s expectations for publication. As the reviewer noted, this work validates a\nphotonic architecture by combining the advantages of ferroelectrics and CMOS-compatible\nsilicon nitride. We believe this is a pivotal step towards next-generation photonic systems.\nReviewer #3 (Remarks to the Author):\nThis is a very good paper which addresses one of the current largest bottlenecks associated\nwith high speed data transmission. Silicon and lithium niobate based optical modulators are\nareas of active research and the competing technologies; Single lane speeds are limited by\nmodulator speeds and therefore, companies and researchers are intensely studying ways to\nachieve better on-chip modulators. In this paper authored by Cai et. al, they introduce lithium\ntantalate based modulators which are integrated with ultra-low loss silicon nitride waveguide\ndevices. I recommend publishing this paper and have some suggestions for the authors:\nReply: We sincerely thank the reviewer for the acknowledgement of our work and for the efforts\nto further improve our manuscript. As the reviewer noted, the hybrid photonic platform is of\ngreat interest to the photonic community. We believe that our work can set a new benchmark\nfor the performance of heterogeneous integrated systems for practical applications, such as\nhigh-speed telecommunication.\n1) The reported Vpi is 6V. Can the authors comment on how this compares with other state of\nthe art modulators? A commonly used figure of merit is Vpi.L. A table which compares this\nfigure of merit, bandwidth and other commonly used performance metrics with other modulators\nwould be great for readers.\nReply: We thank the reviewer for this excellent suggestion. Comparing our results with other\nstate-of-the-art modulators can provide a more intuitive understanding of the performance of\nour demonstrated heterogeneous platform, and a comprehensive benchmarking table is one of\nthe most effective ways to present above information clearly.\nAction taken: Following the reviewer’s suggestion, we have added a new section in the SI\n(Supplementary Section 9) that provides a detailed comparison of our work with other leading\nmodulator technologies. A comparison table is also attached to compare their key figures of\nmerit, which includes the modulation bandwidth, the modulation efficiency, data rate and BER.\nTo avoid repetition, the added table and discussion can be accessed from our above response\nto Reviewer #2, Comment #1.\n2) The authors demonstrate the modulator platform with both IMDD and coherent data\ntransmission. Can they comment on whether the highest rates demonstrated were limited by\nthe equipment or the modulator platform itself?\nReply: Regarding the bottleneck of the achievable rate using our LiTaO3-Si3N4 modulators, we\nwould like to offer the following discussion from two perspectives:\n1. Device limitation: The data rate of the modulator is limited by modulation bandwidth. From\nthe measured results, our 6.8 mm modulator exhibits a remarkably flat EO S21 response\nup to 110 GHz and low microwave reflections in the electrical-to-electrical S11 signals,\nindicating excellent velocity matching and impedance matching. Thus, we are confident\nthat the modulator platform itself is not the primary obstacle for us to achieve a higher data\ntransmission rate. The slight roll-off is due to the intrinsic ohmic loss of the gold electrodes.\nWhile this performance reaches state-of-the-art values (please see the comparison\nprovided in the updated Supplementary figure 7(a) in the SI), ohmic losses could be\nimproved by employing a more conductive metal, such as silver, thereby pushing a higher\ndata rate.\n2. Measurement limitation: Currently, the highest data rates in our transmission experiments\nwere significantly constrained by the bandwidth of our electrical link. As illustrated in our\nmain text:” Here, the 75 GHz electrical 3 dB-bandwidth of the AWG [46] mainly limits\nthe performance of this experiment.”, the primary bottleneck is our arbitrary waveform\ngenerator (AWG, Keysight M8199B), which greatly attenuates high-frequency signals and\ndegrades signal quality before it entering the CPW-type electrode. Meanwhile, the entire\nelectrical path, including all cables and probes, contributes to the total bandwidth limit of\nthe telecommunication system.\nThis \"Electronic Bottleneck\" is a well-known challenge in the industry, but developing ultra-high-\nbandwidth AWGs exceeding 100 GHz is extremely costly. Our demonstrated IQ modulator\n(Fig.4 in the main text) overcomes this bandwidth limited data transmission rate, thereby\nallowing us to double the data rate without requiring higher electrical bandwidth. Moreover, the\nindustry’s roadmap towards 1.6 Tb/s could be realized by parallel telecommunication channels\nwith wavelength-division multiplexing, particularly by combining multiple modulators with an on-\nchip array waveguide grating to construct monolithic optical transceivers. However, this\nrequires more efforts on dedicated designs and advanced fabrication technique, which\nrepresents a key direction for our future work.\nThese discussions mostly apply to the MZM used in the IMDD experiment. The IQ modulator\nused for coherent communications had a bandwidth of 34 GHz. Therefore, improved group\nvelocity matching consists of the best path to increase our coherent data rates.\nAction taken: Our results section already mentions that the equipment (specifically the AWG)\nprimarily constrains our IMDD rates using our highest bandwidth modulators. However, we now\nre-emphasize this point at the end of the Data Transmission part of the Results section.\nRevision to the main text (Page 8):\nAchieving greater data rates will likely require specialized driver electronics given the\nSi N -LiTaO modulator’s >100 GHz bandwidth.\n3 4 3\nFor reference, we have also added the bandwidth of the IQ modulator in Supplementary Table\n3.\n3) Because of the lack of an easily available direct bandgap material in CMOS, III-V continues\nto be relevant. In the discussion section, the authors discuss III-V active materials and their\npotential integration with the lithium tantalate-Si3N4 platform in the future through bonding. Can\nthe authors comment on the difficulties that may be associated with the adoption of this in\nCMOS foundries.\nReply: We thank the reviewer for this question. To clarify, the core of our strategy is the use of\nwafer bonding to overcome the incompatibilities between different materials and standard\nCMOS foundries. The primary difficulty in the adoption of III-V active materials is the\ncontamination they add to processing. Thus, the monolithic growth or processing of III-V thin\nfilm is strictly prohibited within a CMOS foundry. Regarding its incompatibility with the\ndemonstrated process flow for LiTaO3-on-Si3N4 platform, the high temperature required of\nepitaxy III-V growth, such as MOCVD and MBE, usually far exceeds the Curie point of lithium\ntantalate (~610–700 °C). Based on the same strategy in our work, our planned process flow for\nfull co-integration is: (1) CMOS-foundry fabrication (e.g., Photonic Damascene process or\nsubtractive process) for the silicon nitride passive photonic circuits. (2) Wafer-to-wafer bonding:\nthe optical-grade LTOI wafer and III-V epitaxial wafers are prepared on separate substrates,\nand the same wafer-scale bonding demonstrated in our work can be utilized to sequentially\ntransfer the LiTaO3 thin film and III-V thin film onto the Si3N4 wafer. (3) Process for patterning\nand metallization for the LiTaO3 and III-V devices in a separate processing line. This post-\nprocess effectively isolates the CMOS-incompatible materials and processes, avoiding any risk\nof cross-contamination. Although die-to-wafer bonding has been demonstrated for similar\nintegration tasks [Nature 620, 78–85 (2023).], our proposed approach would feature excellent\nwafer homogeneity and high-volume manufacturing, which is more scalable for co-integrating\nIII-V active devices within foundry-processed silicon photonics.\nAction taken: We have modified the description of co-integration of III-V thin film onto our\nplatform in the Discussion section to highlight the reason for the lack of III-V materials in CMOS\nfoundries.\nRevision to the main text (Page 8):\nOur Si N -LiTaO platform could potentially benefit from additional functionalities brought\n3 4 3\nby platform extensions. Namely, adapting its bonding process could lead to the integration\nof III-V active optical materials [55–58] and potentially to a multilayer platform featuring\nboth III-V components [59] and Si N -LiTaO hybrid waveguides. This wafer-scale\n3 4 3\nintegration approach offers a scalable solution that avoids the risk of cross-contamination\ninherent to processing III-V materials within a complementary CMOS foundry.\n4) In a similar vein, lithium tantalate is not currently widely used in CMOS foundries. Some\ndiscussion about what could accelerate adoption in foundries would be useful.\nReply: We would like to thank the reviewer for his/her suggestion. The challenge facing the\nadoption of lithium tantalate in CMOS foundries is quite similar to that of III-V materials, which\ncan introduce metallic ion contamination (such as lithium ions) into a standard CMOS\nfabrication line. Therefore, our response to this question is the same as the core strategy in our\nwork: The most accessible approach is not to force the material into the standard foundry, but\nrather to integrate it onto the CMOS-processed platform via wafer bonding in a separate\nprocess line. We believe this method would provide a scalable pathway for combining the\nadvantages of both platforms.\nAction taken: We have modified the description in the introduction part to clarify the current\nchallenge and frame our work as a solution, which is its co-integration with CMOS-compatible\nplatforms.\nRevision to the main text (Page 1):\nImproved economies of scale for lithium tantalate substrates due to their role in 5G/6G RF\nfilters [33] further encourages the use of this material in integrated modulators. At the\ncurrent stage, heterogeneous integration of this material offers an immediately accessible\nroute towards leveraging its features in high-volume manufacturing platforms such as\nsilicon nitride.\nReviewer #4 (Remarks to the Author):\nThe authors achieved a wafer-scale heterogeneous bonding of LiTaO3 onto Si3N4 PIC platform.\nCompared to monolithic electro-optic photonics platforms, this approach leverages the low-loss\nSi3N4 waveguides while simultaneously exploiting the electro-optic properties of LiTaO3. The\nwork demonstrates EO modulation with a Vπ of 6 V and bandwidths up to 100 GHz, as well as\nboth IMDD and coherent signal transmission, showing performance comparable to EO\nmodulators realized on other PIC platforms. The manuscript is suitable for publication, but the\nauthors should address the following points before acceptance:\nWe thank the reviewer for the appreciation of our work and our results. We have addressed\nhis/her constructive comments carefully in our response below.\n1. Compared to state-of-the-art EO modulators, this work exhibits relatively low modulation\nefficiency, with a reported Vπ·L of 4.08 V·cm. For the MZM used in coherent data transmission,\nthe electrode length is up to 13.5 mm. What is the underlying reason for this low efficiency? Is\nit due to limited optical mode overlap or insufficient electric-field overlap with the active LiTaO\nfilm?\n₃\nReply: We would like to clarify that the origin of the relatively low Vπ·L arises from the\nheterogeneous nature of our platform. Given the simulation results in the SI, we do believe that\nthe modulation efficiency is primarily limited by the optical mode distribution within the lithium\ntantalate film (approximately 50%), rather than by the strength of the microwave-optical\ninteraction owing to the design of the electrode gap. A lower Vπ·L can be easily realized by\nreaching approximately 100% mode overlap in monolithic ferroelectric waveguides. Future\ngenerations of our heterogenous platform could incorporate more advanced designs, such as\nthree-dimension distinct waveguide structures coupled with adiabatic tapers, as discussed in\nour Discussion section.\nAction taken: The first paragraph of our Discussion section already mentions tradeoffs between\npropagation losses and modulation efficiency. However, we have added additional simulations\nin the SI (Supplementary Section 3.A.) illustrating how the thickness of the hybrid waveguide’s\nsilicon nitride layer and lithium tantalate thin film affects both propagation losses and our Vπ·L\nvalue. These can be found in Supplementary Figure 4 as mentioned in our previous response\nto Reviewer #1, Comment #2 in the “main text” part.\n2. Was the EO MZM measured under a DC bias? If so, was any bias drift observed, and how\nsevere or minor was it compared to that in lithium niobate devices?\nReply: The question of the device stability under a DC bias is indeed an important one that\nshould be discussed in our work. To quantitively characterize the DC bias drift, we first set the\nMZM to its quadrature point using a DC voltage. We then monitor the optical output power in a\nfree-running state for one hour. To ensure the intrinsic device stability, the modulator was\npackaged so as to minimize environmental fluctuations attributed to fiber-to-chip coupling. The\nmeasured power drift, observed to be less than 1 dB, is comparable to previous results based\non a monolithic lithium tantalate platform [Opt. Express 32, 44115 (2024)], and is superior to\nthe results in lithium niobate devices [Nat. Commun. 11, 3911 (2020)].\nAction taken: In the main text, we have added a new paragraph and updated Figure 2 to show\nour DC bias measurement. We also added a new section to the SI (Supplementary Section\n6.), including the image of the packaged LiTaO3-Si3N4 modulator and details about this\npackaging method.\nTo avoid repetition, the added section details can be inferred from our above response to\nReviewer #1, Comment #3 in the “main text” part.\n3. In line 251, some works in other heterogeneous platforms are cited (mainly LiNbO3 or LiTaO3).\nHowever, there have also been similar demonstrations using other EO materials, such as\nbarium titanate or SiN PIC platforms, which provide comparable integration advantages but\nwith significantly larger Pockels coefficients. These works should also be discussed in\ncomparison. In addition, including other types of EO modulators and summarizing performance\nmetrics (both monolithic and heterogeneous) in a comparison table would make the advantages\nof this heterogeneous Si3N4-LiTaO3 platform over other photonic platforms clearer.\nReply: We thank the reviewer for bringing up this point. As addressed in comments raised by\nother reviewers, we have added a comparison table in what is now section 9 of the SI which\nincludes the PIC platforms mentioned by the reviewer.\nAction taken: We added Supplementary Section 9 and Supplementary Table 3 to our work.\nTo avoid repetition, the table details can be seen from our above response to Reviewer #2,\nComment #1.\n4. The advantages of LiTaO3 over lithium niobate in this context of modulators were not really\nvery clear to me – lower birefringence and reduced photorefractive effects were mentioned, but\nhow significant is the advantage (especially if we were to compare LN-with-nitride and LT-with-\nnitride side by side)? I suppose the “fast tunable LIDAR” application makes sense, but a more\nquantitative discussion would strengthen the motivation.\nReply: We would like to thank the reviewer for prompting this discussion on the motivation for\nusing thin film lithium tantalate in this work. Compared to lithium niobate, lithium tantalate does\nnot necessarily achieve a lower Vπ·L or a higher bandwidth. Both materials feature similar\nmetrics. However, lithium tantalate achieves long-term modulation stability, large-volume\nmanufacturing and less device anisotropy:\n1. Less photorefractive effect:\nThe photorefractive effect is primarily driven by iron impurities in the crystal. Congruent\nLiTaO is known to be more resistant to this photorefractive damage than congruent LiNbO\ndue to fewer defects introduced during its production process [Nature 629, 784–790 (2024)],\nthereb₃y enhancing its power handling capability for modulators [ACS Photonics ₃\n10.1021/acsphotonics.5c00159 (2025).]. Meanwhile, this intrinsic material advantage\ncontributes directly to a superior DC bias stability, as confirmed by our long-term stability\nmeasurement in the SI. For applications requiring a stable operation or high optical power\n(like analog links or LiDAR), this is a decisive advantage, as it is promising to reduce or even\nelinimate the need for complex and power-hungry electrical feedback loops.\n2. Volume manufacturing:\nDriven by its widespread applications in RF filters for 5G/6G applications, the ecosystem for\nLTOI wafers is much more mature and has been at a stage of large-volume manufacturing.\nThus, this established supply chain ensures that our LiTaO3-Si3N4 PICs can benefit from cost-\neffective and reliable production compared to lithium niobate.\nRegarding the Reviewer’s suggestion on fast tunable LiDAR, we fully agree that this is a\ncompelling application that benefits from our platform’s excellent modulation efficiency and DC\noperation stability. However, a full LiDAR demonstration is a system-level undertaking, which\nis beyond the scope of this current device-focused manuscript. We believe our new long-term\nDC bias drift measurement (now included in Figure 2 and the SI, please see the above\nComment #2.) quantitatively substantiates the advantage of using lithium tantalate. The\nsuperior stability we obtained shows intrinsically lower photorefractive effects in the unetched\nlithium tantalate thin film.\nReviewer #5 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is\npart of the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nReply: We thank the reviewer for their involvement in Nature Communications co-review\nprocess and hope that the comments they raised in the main reviewer’s referral were suitably\naddressed.\nA point-by-point response to the reviews’ comments of the manuscript\n“Heterogeneously integrated lithium tantalate-on-silicon nitride\nmodulators for high-speed communications”\nWe would like to express our sincere gratitude to all the reviewers for their detailed review and\ninsightful suggestions regarding our revised manuscript. Here, we provide point-by-point\nanswers to the reviewers’ comments.\nReviewer #1 (Remarks to the Author):\nThe authors replied my comments and revised the manuscript properly. The manuscript can\nbe published in Nature Communication. In supplementary figure 5.c, the unit of x-axis is\nincorrect. it should be MHz.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments and thank\nthe reviewer for pointing out the typographical error from Supplementary Figure 5.c. In our\nrevised manuscript, we replaced the incorrect ‘THz’ label by the correct ‘MHz’ label.\nReviewer #2 (Remarks to the Author):\nIn the updated manuscript and supplementary information, the authors have addressed my\nconcerns. One last comment would be to include the device insertion losses in the new\nSupplementary Table 3.(Performance metrics of high-speed EO modulators). In my opinion,\nthe innovation and engineering values of the work are now good for the publication.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments. As\nsuggested by the reviewer, Supplementary Table 3 now lists device insertion losses.\nReviewer #3 (Remarks to the Author):\nThe authors have addressed my comments. I recommend publishing the manuscript.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments.\nReviewer #4 (Remarks to the Author):\nI'm satisfied by the replies to the points I raised in my earlier review.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors replied my comments and revised the manuscript properly. The manuscript can be published in Nature Communication.\nIn supplementary figure 5.c, the unit of x-axis is incorrect. it should be MHz.", "reviewer_2": "In the updated manuscript and supplementary information, the authors have addressed my concerns. One last comment would be to include the device insertion losses in the new Supplementary Table 3.(Performance metrics of high-speed EO modulators).\nIn my opinion, the innovation and engineering values of the work are now good for the publication.", "reviewer_3": "The authors have addressed my comments. I recommend publishing the manuscript.", "reviewer_4": "I'm satisfied by the replies to the points I raised in my earlier review."}, "author_response": "A point-by-point response to the reviews’ comments of the manuscript\n“Heterogeneously integrated lithium tantalate-on-silicon nitride\nmodulators for high-speed communications”\nWe would like to express our sincere gratitude to all the reviewers for their detailed review and\ninsightful suggestions regarding our revised manuscript. Here, we provide point-by-point\nanswers to the reviewers’ comments.\nReviewer #1 (Remarks to the Author):\nThe authors replied my comments and revised the manuscript properly. The manuscript can\nbe published in Nature Communication. In supplementary figure 5.c, the unit of x-axis is\nincorrect. it should be MHz.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments and thank\nthe reviewer for pointing out the typographical error from Supplementary Figure 5.c. In our\nrevised manuscript, we replaced the incorrect ‘THz’ label by the correct ‘MHz’ label.\nReviewer #2 (Remarks to the Author):\nIn the updated manuscript and supplementary information, the authors have addressed my\nconcerns. One last comment would be to include the device insertion losses in the new\nSupplementary Table 3.(Performance metrics of high-speed EO modulators). In my opinion,\nthe innovation and engineering values of the work are now good for the publication.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments. As\nsuggested by the reviewer, Supplementary Table 3 now lists device insertion losses.\nReviewer #3 (Remarks to the Author):\nThe authors have addressed my comments. I recommend publishing the manuscript.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments.\nReviewer #4 (Remarks to the Author):\nI'm satisfied by the replies to the points I raised in my earlier review.\nReply: We are glad to hear that our revisions addressed the reviewer’s comments."}]

Decision letter