Highly coherent organic lasing based on etch-free metasurface

Domain: physics
Published: 2025-12-19
Editorial outcome: minor_revision

Abstract
Metasurfaces supporting bound states in the continuum (BIC) can be leveraged as lasing platforms, offering ultra-compact and highly coherent light sources. Although designs using low-index polymers that do not require etching have been demonstrated and shown to possess remarkably high quality factors for free-space radiation, these have been so far limited to passive designs. Here, we report on highly coherent BIC-based lasing, emerging from etch-free metasurfaces that are directly patterned onto organic supramolecular materials consisting of small-molecule, ionic isolation lattices (SMILES). We observe highly directional lasing with a divergence angle of 0.2°, linewidth of 0.04 nm—limited by the spectral resolution of our measurement setup—and a temporal coherence of 20.4 ± 2.4 ps. The reported performance is a direct consequence of the combination of high-quality metasurface designs with the intrinsic uniformity and stability of SMILES, offering a strategy to realize active metasurfaces and organic molecular lasers, superior to alternative approaches based on solution-processable materials.

Introduction
Metasurfaces enable versatile manipulation of light at subwavelength scales, offering remarkable control over amplitude, phase, direction and polarization1. Their applications extend across a wide range of areas, including flat optics2, holography3, lasers4,5, and quantum communications6. In particular, metasurface lasers based on bound states in the continuum (BIC)4 have drawn considerable interest as they offer compactness, narrow linewidth, high coherence, directionality, wavelength tunability, and scalability5. Metasurface lasers based on traditional III–V semiconductors have demonstrated exceptionally narrow linewidth of lasing emission and ultrahigh quality factor (Q-factor)7–10, enabled by advanced fabrication techniques and excellent material properties. Beyond this, novel material systems such as transition metal dichalcogenides11, perovskites12, colloidal nanomaterials13,14, and organic materials15–17 have also attracted growing attention to realize metasurface lasers owing to their advantages, including solution processability, synthetic tunability, and compatibility with flexible substrates. Nevertheless, these materials have a relatively low refractive index, are difficult to pattern directly, and are generally used only as the active media for pre-fabricated passive structures composed of high-index materials17,18.

Low-refractive-index materials generally exhibit poor light confinement and low diffraction efficiency, making it challenging to design efficient photonic structures19. However, novel metasurface designs have been reported that can achieve ultra-high Q-factors with low-refractive indexes20–22. For instance, the metasurface reported by Fang et al.21 achieved Q-factors exceeding one million, leveraging the etch-free process that would be typically required with commonly used polymers. This outcome was achieved with a perturbed multilayer-waveguide configuration and low-loss materials21. Etch-free designs minimize imperfections in fabrication, thereby substantially reducing unwanted scattering losses. However, these demonstrations are passive in nature, and hence require integration with active media to function as light-emitting devices.

Here, we show a highly coherent, directional, and etch-free molecular metasurface laser utilizing a novel direct patterning approach. We employ small-molecule, ionic isolation lattices (SMILES)23, a newly developed supramolecular material synthesized using self-assembly methods by combining an organic dye with a macrocycle host called cyanostar24. SMILES materials provide extraordinary brightness25 and structural stability by preventing the aggregation and quenching of organic dye molecules. Incorporating SMILES into an e-beam resist allows direct e-beam lithography (EBL) to pattern molecular nanostructures without the etching process, while preserving their brightness and optical properties. Based on this platform, we demonstrate direct EBL fabrication of molecular metasurfaces made of SMILES as shown schematically in Fig. 1a. Using optical spectroscopy, we observe lasing at the BIC. The lasing emission exhibits an ultra-narrow linewidth of 0.04 nm (corresponding to 103 μeV), which reaches the spectral resolution limit of our measurement setup. Moreover, the lasing emission is highly directional with a divergence angle of 0.2°. To further characterize the coherence properties, we measure the temporal coherence of the lasing, revealing a coherence time of 20.4 ± 2.4 ps.

Results
Direct lithography with SMILES
First, we synthesize SMILES by combining rhodamine 3B (R3B) dye and cyanostar in a solvent. To enable direct lithography of the R3B-SMILES using EBL, we incorporate PMMA into the R3B-SMILES solution, shown in Fig. 1b, together with their chemical structures. The detailed synthesis method of the SMILES resist is described in the Methods section. While PMMA is not required for SMILES to function as a bright fluorescent emitter, it has previously been used to control the precise thickness of the SMILES films during the spin-coating process by adjusting the concentration of PMMA26,27. Here, we emphasize that PMMA retains its function as E-beam resist even when mixed with SMILES.

The EBL process typically requires a baking step after resist coating to evaporate the solvent and ensure that the resist functions effectively as an etch mask. PMMA has a glass transition temperature of 105 °C28, and is generally baked at around 180 °C on a hot plate for a few minutes. However, these thermal processing can negatively affect the photophysical properties of traditional organic-based materials29–31, making the fabrication of nanostructures using such materials challenging with conventional lithography process. We note that the baking process does not degrade SMILES but rather improves its fluorescence up to a certain temperature. We spin-coat the PMMA containing R3B dye and R3B-SMILES onto substrates and bake them on a hot plate for 5 min at different temperatures. We measure the PL spectra five times at different positions on each sample and averaged the PL peak intensities. Due to sample non uniformity across positions, the measured PL intensities show slight variations. The PL intensities of baked R3B-SMILES and baked R3B dye resists as a function of baking temperature are shown in Fig. 1c, with error bars indicating the standard deviation. The R3B-SMILES resist exhibits notably stronger PL intensity than R3B dye. For R3B dye resist, the PL intensity decreases monotonically as the baking temperature increases, whereas for R3B-SMILES, the PL intensity increases up to approximately 150 °C. At this temperature, the PL intensity of baked R3B-SMILES resist is about 1.5 times higher than that of the unbaked R3B-SMILES. Beyond this temperature, R3B-SMILES films undergo thermal degradation, leading to a decrease in PL intensity. The cyanostar used in the preparation of the SMILES materials not only provides optimal spacing for organic dyes to prevent aggregation caused quenching but also enhances thermal robustness through structural stability with isolation lattices according to the reported thermal properties23.

The step-by-step process of direct lithography using the SMILES-resist is illustrated in Fig. 1d. First, a SMILES resist thin film is coated on the substrate via spin-coating. Second, the thin film is baked on a hot plate at 150 °C for 3 min. Third, EBL is performed on the SMILES-resist film using a dose value of 300 µC/cm2. Fourth, the development process is carried out using a PMMA developer, in which the sample is immersed for 1 min. Scanning electron microscope (SEM), optical microscope, and PL images of the directly patterned SMILES-resist showing the logo of the City College of New York are displayed in Fig. 1d. This shows that even with the SMILES mixed in, the patterned structure shows feature sizes on the order of 10 nm, a typical fabrication resolution of PMMA, while maintaining the bright PL of the SMILES material.

Structural design and BIC in etch-free molecular metasurface
Our molecular metasurface design is the structure used in ref. 21, but replacing PMMA with SMILES, which has a refractive index similar to PMMA, ~1.5 (See Supplementary Fig. 1 for refractive index data). The unit cell structure of the etch-free molecular metasurface is defined by a periodicity a, and a square hole with side L (Fig. 2a). We spin coat ~100-nm-thick SMILES on the 100-nm-thick SiN layer on SiO2/Si substrate. Here, the light is effectively trapped at the interface between SMILES and SiN layers21. A SEM image of the fabricated molecular metasurface is shown in Fig. 2b. For the SEM measurement, a thin gold layer (~20 nm) was deposited to prevent surface charging.

This metasurface is designed to support symmetry-protected Γ-point (k=0) BIC, where the radiative coupling is completely suppressed32,33. To visualize the electric field characteristics, we perform finite element method calculations, shown in Fig. 2c, d. The calculated electric field of x-y plane in SMILES layer at the Γ-point is shown in Fig. 2c. For the BIC, the electric field is strongly confined within the SMILES and SiN layers, confirming its non-radiative characteristic, shown in Fig. 2d (left). In contrast, the off-BIC mode shows distinct field leakage into air and substrate, shown in Fig. 2d (right).

We perform angle-resolved reflectance spectroscopy to measure the photonic dispersion of the SMILES metasurface, and calculate reflectance spectra of the molecular metasurface with a=364 nm and L=90 nm, for TE polarization (Fig. 2e). The lower guided mode resonance (GMR) band vanishes at θ=0° (k=0), indicating the formation of a BIC. The reflectance spectrum measured from the molecular metasurface exhibits a clear GMR band for TE-polarized light and qualitatively matches the calculated spectra, as presented in Fig. 2f. Compared to the metasurface of the same structure made of PMMA only21, the photonic dispersion has the same shape, but the linewidth is broader due to the absorption of SMILES. The dark and broad parabolic dispersion is attributed to Fabry-Perot interference within the thick SiO2 layer, which is independent of the SMILES layer. (See Supplementary Fig. 2) We extract the Q-factors of the two photonic bands from the angle-resolved reflectance spectra. The reflectance spectra were fitted with a Lorentzian function to obtain the peak positions Ep and linewidths ΔEp, from which the Q-factors were calculated as Q=Ep/ΔEp. For the lower band having the BIC, the Q-factor reaches approximately 1534, measured at θx=0.4°. In contrast, the upper band exhibits a lower Q-factor of about 600. This difference originates from the broad excitonic absorption centered around 2.2 eV, which increases optical loss and reduced Q factor. Numerical simulations also represent this trend, yielding a theoretical Q-factor on the order of 10³ for the lower band. (See Supplementary Fig. 3 for details.)

BIC lasing in etch-free molecular metasurface
We measure the PL spectrum of the SMILES metasurface using Fourier imaging to investigate the lasing characteristics. We excite the molecular metasurface using a 515 nm (~2.4 eV) fs-pulsed laser with a Gaussian beam of approximately 280 μm diameters (Fig. 3a). A PL image obtained from a large area of the molecular metasurface region is presented in Fig. 3b, while PL spectra measured below and above the lasing threshold are provided in Fig. 3c, d, respectively. In the weak pump power regime, PL spectrum exhibits a broad distribution of GMR band (Fig. 3c). As the pump power increases, PL spectrum exhibits the lasing emission with a very narrow linewidth as two distinct spots near θ=0, at the BIC. Due to the symmetry-protected nature of the Γ-point BIC, radiation is completely suppressed at k=0, while lasing emission emerges from near-Γ states with finite radiative coupling. Notably, the measured lasing emission appears at an angle of under 0.2° in the momentum space, indicating that it is highly directional along the out-of-plane axis of the metasurface.

The threshold of pulse fluence Pth for lasing is found to be approximately 2.08 μJ/cm², corresponding to a pulse energy of 1.28 nJ per pulse and an instantaneous pump power of about 6.4 kW. The angle-integrated PL spectra as a function of pulse fluence is shown in Fig. 3e. As the pulse fluence increases, a sharp peak suddenly emerges above Pth. We measure the PL spectrum near the threshold power by narrowing the entrance slit of the spectrometer as much as possible. The narrowest linewidth of the lasing emission obtained by fitting is 0.031 ± 0.0005 nm, which is on the order of the instrument limit of our measurement setup (~0.04 nm), as shown in the inset of Fig. 3e. We plot the pulse fluence dependence of PL peak intensity (black square), and linewidth (blue circle) of lasing emission in Fig. 3f. The PL peak intensity shows nonlinear behavior, while the linewidth of PL emission decreases abruptly above Pth, as clear evidence of lasing behavior (We also obtained data over a wider pump-power range, provided in the Supplementary Fig. 4). A slight blue shift of the PL peak is observed above the threshold, originating from Kerr-type nonlinearities and saturation of molecular optical transitions inherent to the organic molecules34.

We also measure the coherence properties of lasing emission from the SMILES metasurface using a Michelson interferometer and a retroreflector. A superimposed PL image from the interferometer is presented in Fig. 4a. The PL emissions from each arm are marked by white dashed circles, which also indicate the pumped regions. Here we excite the metasurface with a beam diameter of ~50 μm. Below Pth, the superimposed PL image represents simply the sum of two PL images, showing no interference features. In contrast, above Pth, the superimposed PL image exhibits clear fringes in the overlapped region, indicating coherent emission. Moreover, fringe patterns are observed beyond the pumping area. The lasing emissions appear broad in both vertical and horizontal directions, which are the leaked emissions from the propagating GMRs. We observe that the lasing emission propagates beyond 100 μm (See Supplementary Fig. 5).

We evaluate the temporal coherence of the lasing emission by changing the relative optical path length between the two interferometer arms. The visibility of the interference as a function of the time delay τ is shown in Fig. 4c. The measured data fit with a Gaussian curve, yielding a temporal coherence time of 20.4 ± 2.4 ps. The interference fringes are clearly visible at τ=0 ps but almost disappear at τ=–26.4 ps, as shown in the insets, indicating the loss of temporal coherence with increasing delay time.

Polarization vortex of BIC
We investigate the topological properties of BICs in momentum space using polarization-resolved measurements. PL images in momentum space below Pth with two orthogonal linear polarizations are shown in Fig. 5a, b, where the red arrows indicate the polarization direction. The PL images show broad intensity distributions in all angles, with weakly visible photonic modes. However, above Pth, two-lobe emission patterns are observed (Fig. 5c–f). As the measured polarization angle is rotated, the orientation of two lobes also rotates in momentum space. Without the linear polarizer, the PL emission exhibits a donut-shaped pattern above the threshold power (See Supplementary Fig. 6). These data present the vortex feature of BICs, consistent with previously reported results for BIC lasing12,33,35. To further reveal the topological characteristics of the BIC, we calculated the polarization vortex in momentum space, as shown in Fig. 5g. The phase singularity around Γ point indicates a topological charge, confirming the vortex nature of the BIC in momentum space.

Methods
Sample fabrication
To synthesize R3B-SMILES resist, we dissolve the rhodamine 3B dye as the perchlorate (ClO4–) salt and cyanostar (Halophore, Inc.) with a molar ratio of 1:2 in a PMMA C2 solution (2% w/w PMMA in chlorobenzene, Kayaku Chem). The solution undergoes 48 h of stirring at room temperature.

For measuring baking-temperature dependence, R3B-SMILES and R3B dye films are prepared by spin-coating on SiO₂ substrates at 2000 RPM for 60 s. The spin-coated samples are then placed on a hot plate at various temperatures for five minutes.

To make a metasurface structure, we spin-coat the SMILES-resist on a SiN/SiO2/Si substrate at 2000 RPM for 60 s. The thicknesses of SiN and SiO2 are 100 nm and 1470 nm, respectively. We place the spin-coated SMILES resist substrate on a hot plate at 150 °C for 3 min for thermal annealing. We write the metasurface patterns using E-beam lithography, followed by the development process for 1 min.

Simulation
We perform rigorous coupled-wave analysis with the Lorentz oscillator model to calculate reflectance spectra of the molecular metasurfaces. We assume the permittivity of SMILES resist using the following Lorentz oscillator model: ε(E)=ε∞ + Σ j Ej2 / (Ej2 - E2 - iγj E) where E is an energy, ε∞ is the high-frequency dielectric constant, Ej is resonance energy of the j-th oscillator, fj is oscillator strength of the j-th oscillator, γj is damping coefficient of the j-th oscillator, ε∞=2.5, E1=2.19 eV, E2=2.36 eV, E3=3.5 eV, f1=0.036 eV2, f2=0.01 eV2, f3=0.04 eV2, γ1=0.12 eV, γ2=0.19 eV, γ3=0.5 eV. We assume the refractive index of SiO2 as n=1.45, the index of SiN as n=2.0, and the index of silicon n=4.0 in the calculation.

We also perform three-dimensional finite element method (FEM) simulations using COMSOL Multiphysics to calculate the electric field distributions of the molecular metasurface and polarization vortex of BIC. The simulations are carried out with the eigenfrequency solver module.

Measurements
Optical measurement and interferometry: angle-resolved reflectance measurements were conducted using a broadband tungsten halogen lamp. The reflected signal was collected using a 5× objective lens with numerical aperture 0.15. The signal was measured with a spectrometer (Princeton Instruments) and a charge-coupled device camera (Princeton Instruments). PL spectrum measurements were conducted using a fs-pulsed laser (Light Conversion), emitting at 515 nm wavelength and 10 kHz repetition rate.

PL spectra of R3B-SMILES and R3B dye on SiO2 substrates as a function of baking temperature were measured under identical excitation power and exposure time. Five different positions on each sample were measured, the PL peak intensities were averaged, and the standard deviation of these values was obtained.

The excitation beam had a Gaussian profile with a diameter of approximately 280 µm on the sample surface. For the angle-resolved PL spectrum measurements, the entrance slit of the spectrometer was set to 50 µm, while a narrower slit width of 10 µm was used to achieve higher spectral resolution. PL spectra and images were collected using a 5× objective lens with numerical aperture 0.15. A 550 nm long pass filter was used to eliminate the laser signal in PL measurement. Interference fringes were measured with a Michelson interferometer and a retroreflector, where the excitation beam was focused to a diameter of approximately 50 µm. Visibility is calculated as V=(Imax - Imin)/(Imax + Imin), where maximum intensity Imax and minimum intensity Imin within the interference fringes, respectively. One arm of the interferometer was placed on a computer-controlled stage (Thorlabs), enabling measurement of temporal coherence by varying the optical path length. The resulting visibility as a function of delay time was fitted with a Gaussian profile to extract the coherence time. The PL images in momentum space were measured with an excitation beam of approximately 50 µm diameter.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the manuscript, “Highly coherent organic lasing based on etch-free metasurface”, Daegwang Choi et al. investigate lasing in resonant organic metasurfaces. The authors design a low-refractive-index metasurface composed of a square lattice of square holes that supports guided-mode resonances (GMRs) and bound states in the continuum (BICs) at energies around 2.05 eV at normal incidence. They measure photoluminescence (PL) below and above threshold and observe lasing at quasi-normal incidence, which they attribute to excitation of BIC modes. The authors further demonstrate spatial and temporal coherence of the emitted beam above the lasing threshold. Finally, they observe radial polarization of the emission in the polarization-resolved spectra in the reciprocal space that confirms the BIC nature of the photonic mode enhancing the emission.\nReasons to accept:\nThe demonstrated concept offers a compact planar etch-free photonic system for observation of lasing without the use of distributed Bragg reflectors (DBRs). The lasing behaviour is confirmed experimentally in spatial coherence, temporal coherence and polarization-resolved measurements. The demonstrated lasing linewidth of ~0.031 nm and lasing threshold of ~20 uJ/cm^2 is relatively low compared to other planar organic and semiconductor lasers.\nReasons to reject:\nNovelty concerns: The previous publication [Ref.(27)] by the same authors reported on polariton lasing from the same organic material sandwiched between DBRs, with the lasing energy of 1.925 eV, lasing threshold of ~130 uJ/cm^2, and lasing linewidth of 0.058 nm. The lasing threshold in such system is 6.5 times higher than that in the current manuscript, while the linewidth is ~2 times larger. Therefore, the main novelty of the current manuscript is nanostructuring the material to support high-quality cavity modes inside the metasurface eliminating the need for bulky DBRs.\nOverall assessment:\nThe overall concept presented in the current manuscript is of potentially high interest to people from distinct physical communities. The current manuscript contains several major technical inconsistencies discussed below. If properly explained and resolved, the manuscript may be publishable in Nature Communications. Below, I outline a detailed list of comments and criticisms.\nMajor comments.\n(1) In Ref.(27), the same authors show hybridization of cavity modes and excitons into polaritons with energies around 2 eV. In the current manuscript, the cavity modes are realized at 2.05 eV, however, no discussion of the role of excitons is provided. Do the observed GMR and BIC mode carry the polariton properties because of proximity of excitonic resonances at 2.2 eV? If yes, how does this affect the lasing properties?\n(2) It is unclear why the lasing is driven by the BIC modes, while their Q factor is lower than for GMR modes even at the lasing angle of 0.02 degrees (Fig.2g). Could the authors comment on this?\n(3) It is also unclear why the lasing is dominated by the specific angle of 0.02 and is so narrowband in angular range (Fig.3d and Fig.5c-f). The BIC Q factor changes weakly with the angle variation and should be even higher for angles smaller than 0.02. Could the authors comments on this?\n(4) The GMR-driven PL is not visible in Fig.3e for the pump powers below the threshold, while visible in Fig.3c. Could the authors clarify this?\n(5) What are measured Q factors of GMR and BIC modes? Not the lasing Q factor.\n(6) Please add more description to the polarization vortex property of BIC in addition to supplemental calculations of the phase singularity of the designed mode in the reciprocal space.\nMinor comments.\n(7) The BIC PL peaks in Fig.3d above the threshold are blue-shifted with respect to those in Fig.3c below the threshold. I recommend the authors to add a relevant discussion, as was done in Ref.[27].\n(8) Could the authors specify the angle \\theta in Figs.3(e,f)? Or, they show the angle averaged signal?\n(9) The angle \\theta values marked with dashed lines in Fig.2e do not correspond to the values listed in the legend of Fig.2f. Please correct this and add the dimension of \\theta (degrees, radians)\n(10) Please change the description of the agreement between experiment and theory in Figs.2(c,d) from “perfect” to “qualitative”, as the experimental data does not show the BIC features and has large distortion near the normal incidence.", "reviewer_2": "In this manuscript, the authors report highly coherent BIC lasing emerging from etch-free metasurfaces directly patterned onto SMILES materials and observe BIC lasing with a linewidth of 0.04 nm. The authors claim there are two key factors that enabled such an exceptionally narrow linewidth for BIC lasing. The combination of high-quality metasurface designs and the intrinsic uniformity and stability of SMILES leads to highly coherent lasing. If these claims and results are well-supported and clearly justified, the manuscript could be considered for publication in Nature Communications. However, there are several issues in the current manuscript that need to be addressed or clarified, which I list below.\n1. Figure 3d shows two bright spots of the angle-resolved PL spectrum in one direction above the threshold power. Here, the metasurface has a BIC at the θ=0, and the Q-factor of the lower band increases as the angle approaches zero according to Fig. 2. Usually, this is a symmetry-protected BIC at Г=0 (θ=0), and it has been observed the doughnut-shaped far-field radiation pattern in BIC-based lasing before (NanoLett.2020,20,6005−6011; Science 2020, 367, 1018−1021; Nat. Commun. 2017, 8, 13687; Phys. Rev. Lett. 2013, 110, 206802). The angle-resolved PL spectrum in Fig. 3d is along one direction and may indicate that the far-field radiation pattern is doughnut-shaped. To completely describe the lasing characteristics, I suggest the authors measure the angle-resolved PL images in two directions below and above the threshold power (like Fig. 5 except without polarizations). Furthermore, please discuss and compare them with the doughnut-shaped far-field radiation pattern in the reported work.\n2. The authors measured the PL spectrum near the threshold power by narrowing the entrance slit as much as possible (Line 113). What is the width of the entrance slit? Where is the entrance slit? Is the slit used to restrict the pumping light? If so, the authors are encouraged to further clarify its specific role and impact in the experimental measurements. Furthermore, could the authors measure the PL spectrum near the threshold power using a 515 nm fs-pulsed laser with a Gaussian beam of approximately 280 μm diameters without the entrance slit?\nAs far as I know the entrance slit restricting the pumping light has a significant impact on the emission peak width. Hence, it is essential that the authors investigate how the lasing peak width varies with the entrance slit width and discuss the underlying physical mechanisms. Please measure the PL spectrum near the threshold power under the different entrance slits.\n3. The authors also excite the molecular metasurface using a 515 nm (~ 2.4 eV) fs-pulsed laser with a Gaussian beam of approximately 280 μm diameters (Fig. 3a) (Line 104). The pumping light on the metasurface is a spot with 280 μm diameters or a slit of several nanometers for different cases? The authors should clarify the shape of the incident light spot for the different figures such as Fig. 3, 4 and 5. In addition, at what angle is the pumping light on the metasurface, and how is the lasing emission collected?\nIn a word, the authors need to describe the optical measurement setup in detail so that the readers can clearly understand the experimental conditions and results.\n4. The threshold pumping power Pth for lasing is found to be ~ 20.8 μJ/cm² (Line 110). How is the incident power measured, and lasing threshold calculated for fs-pulsed excitation at 10 kHz repetition rate? The pumping power is the peak power, instantaneous power, average power per second or other else of the fs-pulsed laser? What are the pulse width and pulse energy of the laser? Since lasing action is generally governed by the instantaneous power of the pump, this distinction is essential. The pumping average power may be very small, but the instantaneous power can be very high for the fs-pulsed laser.\n5. The superimposed PL image of Fig. 4b is a little obscure and seems different from fork-shaped interference patterns in the reported work (Fig. 2D in Science 2020, 367, 1018−1021). It appears that the fork-shaped interference pattern was not observed. Could the authors discuss the differences and the relationship between this work and the reported work?\n6. Figure 4c shows temporal coherence of BIC lasing. The visibility of the interference fringes is plotted as a function of the delay time between the two arms of the Michelson interferometer. I suggest the authors provide images of these interference fringes so that readers can have a clearer understanding of the visibility.\n7. The authors claim there are two key factors that enabled such an exceptionally narrow linewidth for BIC lasing. One is the metasurface design enables very high Q-factors. However, the Q-factor of ~700 in Fig. 2g is really not high, compared to the Q-factor of 5.0 × 105 or 3.0× 103 in Ref. 7 and the million-Q reported in Ref. 21. The authors also speculate the intrinsic properties of the SMILES materials support the narrow linewidth and long temporal coherence. If the SMILES materials support narrow linewidth and long temporal coherence, the authors need to present evidence or some discussions? I suggest the authors should give some further explanations for the exceptionally narrow linewidth.\nTo better illustrate the role of the BIC mode, the authors should measure PL spectra as a function of the pumping power at the planar SMILES region similar as Fig. 3e.\n8. This work has a similar frame with the reported work (Nano Lett. 2025, 25, 1653−1659), which only shows the emission with the Q-factor of 308. I suggest the authors compare this work with them.\n9. We could distinguish amplified spontaneous emission and lasing emission with photon statistics. One of the best indications of temporal coherence is the second-order correlation function g(2)(t,τ). If possible, the authors could measure the second-order correlation function, which can rigorously quantify the quantum nature of the phase transition from spontaneous emission to stimulated emission (Sci. Adv. 8, eade8817 (2022)).\n10. Line 18, “an” should be “a”?\nLine 80, Ref. 20 should be Ref. 21?", "reviewer_3": "The manuscript reports highly efficient BIC lasing in SMILES etch-free metasurfaces. This is a very promising concept. The work is sound and novel enough. I support it.\nA few points for improvement:\n1) why Q-factor is only 700 in Fig. 2g? The authors report up to 10^6 in ref. 21.\nAny reason for why not going to ultrahigh Qs in this study? Explain.\n2) at which angle BIC lasing occurs? It seems it is off-normal in Fig. 3d. Any specific reason for that? explain in more detail.\n3) in Fig. 4, fringe patterns are observed beyond the pumping area. Not clear why. Explain in more detail."}, "author_response": "Author Response Letter to the Reviewer Comments\nManuscript #: NCOMMS-25-59675-T\nTitle: Highly coherent organic lasing based on etch-free metasurface\nWe are thankful to the reviewers for their critiques and thoughtful comments which have helped to\nmake the manuscript more coherent and comprehensive. In response to the referee comments, we have\nsubstantially improved our manuscript as delineated below. Therein, please note that the reviewer\ncomments are in black while our responses are in blue. In addition, all changes in the revised manuscript\nare highlighted in yellow and indication of positions is based on the revised manuscript.\nTo Reviewer #1:\nComment 1) In Ref.(27), the same authors show hybridization of cavity modes and excitons into\npolaritons with energies around 2 eV. In the current manuscript, the cavity modes are realized at 2.05\neV, however, no discussion of the role of excitons is provided. Do the observed GMR and BIC mode\ncarry the polariton properties because of proximity of excitonic resonances at 2.2 eV? If yes, how does\nthis affect the lasing properties?\nResponse 1) We thank the reviewer for raising this point. The works in Ref. 27 demonstrated strong\ncoupling between cavity modes and SMILES excitons, leading to the formation of exciton–polaritons.\nHowever, we could not observe the characteristic anti-crossing behavior in either reflection or\nphotoluminescence (PL) measurements in the metasurface structure. We speculate that is because the\ncoupling strength between the guided-mode resonance (GMR) band and excitons in this metasurface\nstructure is not sufficient to reach the strong coupling regime, unlike in microcavity systems. While the\nbound state in the continuum (BIC) is strongly confined within the structure, the GMR band is\nintrinsically a leaky mode unlike cavity mode. And the electric field of GMR band is distributed not\nonly in the SMILES layer but also within the SiN layer. As a result, the spatial overlap between the\nelectric field and the excitons is relatively lower than microcavity systems. Moreover, the SMILES\nlayer in this work is about half as thick as that in Ref. 27, which may further reduce the overall coupling\nstrength. Therefore, this work focuses on photonic lasing, not polariton lasing. The excitons here\nprimarily act as a bright gain medium.\nComment 2) It is unclear why the lasing is driven by the BIC modes, while their Q factor is lower than\nfor GMR modes even at the lasing angle of 0.02 degrees (Fig.2g). Could the authors comment on this?\nResponse 2) To clarify this point, we have performed additional measurements and improved the\nreflectance data. We have extracted the experimental Q-factors of both the upper and lower bands\naccording to the angle shown below.\noverall Q-factors of the lower band is higher than that of upper band, which is attributed to the\nabsorption band of excitons (~ 2.2eV).\nMoreover, we found that the absorption of the SMILES layer had been overestimated in the initial\nsimulations. Since the excitonic absorption can vary depending on the concentration of the fabricated\nSMILES film, we recalculated the simulations by adjusting the absorption accordingly. In the\nsimulation, the lower band containing the BIC exhibits a Q-factor of about 1,020 at 𝜃 = 0.1°.\n𝑥\nWe have revised and included these changes in the revised manuscript. (Figure 2, Lines 96-107)\nComment 3) It is also unclear why the lasing is dominated by the specific angle of 0.02 and is so\nnarrowband in angular range (Fig.3d and Fig.5c-f). The BIC Q factor changes weakly with the angle\nvariation and should be even higher for angles smaller than 0.02. Could the authors comments on this?\nResponse 3) The lasing occurs at the angle of approximately 0.2° (not 0.02°) because this lasing mode\noriginates from the symmetry-protected Γ-point (k = 0) BIC, where the radiative coupling is completely\nsuppressed, resulting in no out-coupling to free space [1,2]. Therefore, the emission cannot occur\nexactly at k = 0 (𝜃 = 0°). However, near 𝑘 = 0 (non-zero and small k), the mode has a finite radiative\nchannel, allowing lasing emission to occur close to the normal direction. In addition, the narrow angular\ndivergence of the lasing emission is closely related to the large emission area of a device. According to\nthe Fourier relation between the spatial mode profile and the far-field distribution, a larger emission\narea in real space leads to a narrower momentum-space (angular) spread. [3,4] We fabricated a\nmetasurface cavity with a lateral size of 300 µm on each side and optically pumped it using a laser beam\nwith a diameter of 280 µm.\nTherefore, the combination of the Γ-point BIC mode and the large-area cavity results in the highly\ncollimated emission observed within a very small angular range around 𝜃 ~ 0.2°. We have included\nthese in the revised manuscript. (Lines 91-95, 115-116)\n[1] Nature 541, 196–199 (2017)\n[2] Nature Communications 12, 4135 (2021)\n[3] Nature Photonics 16, 279–283 (2022)\n[4] Nature 618, 727–732 (2023)\nComment 4) The GMR-driven PL is not visible in Fig.3e for the pump powers below the threshold,\nwhile visible in Fig.3c. Could the authors clarify this?\n3c shows angle-resolved PL data. For the power–dependent measurements (Fig. 3e), we used the angle-\nintegrated PL configuration to reduce the acquisition time. The reasons are as follows: (1) In angle-\nresolved measurements, data collection below the lasing threshold can take several hundred seconds to\nachieve a sufficient signal-to-noise ratio sometime, whereas the angle-integrated configuration allows\nacquisition within a few seconds. (2) Longer exposure time to the pump laser may cause sample\ndegradation, so the angle-integrated configuration also helps prevent potential degradation and ensures\nconsistent data quality from a single sample.\nWhen the data in Fig. 3c are integrated over all angles, the resulting spectrum appears very similar\nto Fig. 3e. Although the GMR-related emission band appears flattened and barely visible in Fig. 3e,\nGMR-related emissions are present in the data. To clarify this point, we additionally plotted both\ndatasets for direct comparison below.\nWe have clarified this in the revised manuscript. (Line 118)\nComment 5) What are measured Q factors of GMR and BIC modes? Not the lasing Q factor.\nResponse 5) In the improved reflectance data, we obtained a Q-factor of approximately 1,530 for the\nlower GMR band, where the BIC mode is located, at 𝜃 = 0.41 °. And there is no reflectance dip\nmeasured in the lower band at 𝜃 = 0° due to the BIC.\nComment 6) Please add more description to the polarization vortex property of BIC in addition to\nsupplemental calculations of the phase singularity of the designed mode in the reciprocal space.\nResponse 6) We thank the reviewer for the valuable suggestion. We have performed additional Finite\nElement Method (FEM) simulations using COMSOL Multiphysics 6.2 to calculate the E-field\ndistribution in momentum space. We extracted the polarization information around k = 0 associated\nwith the BIC.\nsimulation and analysis, we referred to the following works [5,6,7].\nThe corresponding data and analysis have been added to the revised manuscript. (Figure 2 and 5, Lines\n91-95, 147-149)\n[5] Physical Review Letters. 113, 257401 (2014)\n[6] Nature Photonics 12, 397–401 (2018)\n[7] Nature Photonics 14, 623–628 (2020).\nComment 7) The BIC PL peaks in Fig.3d above the threshold are blue-shifted with respect to those in\nFig.3c below the threshold. I recommend the authors to add a relevant discussion, as was done in\nRef.[27].\nResponse 7) We appreciate the reviewer’s comment. We have revised the relevant discussion in the\nrevised manuscript. (Lines 125-126, Reference 34)\nComment 8) Could the authors specify the angle \\theta in Figs.3(e,f)? Or, they show the angle averaged\nsignal?\nResponse 8) The data in Figs. 3e and 3f were obtained from angle-integrated PL measurements, and\ntherefore represent the sum over all detection angles. We have clarified this point in the revised\nmanuscript. We have revised the corresponding sentence in the revised manuscript. (Line 118)\nComment 9) The angle \\theta values marked with dashed lines in Fig.2e do not correspond to the values\nlisted in the legend of Fig.2f. Please correct this and add the dimension of \\theta (degrees, radians)\nQ-factor can now be experimentally extracted from the updated reflectance spectra, the simulated data\nhave been moved to Supplemental Information. And we have corrected dimension in the data.\nComment 10) Please change the description of the agreement between experiment and theory in\nFigs.2(c,d) from “perfect” to “qualitative”, as the experimental data does not show the BIC features and\nhas large distortion near the normal incidence.\nResponse 10) We appreciate the reviewer’s comment. We have revised the description. (Line 100)\nComment 1)\nFigure 3d shows two bright spots of the angle-resolved PL spectrum in one direction above the threshold\npower. Here, the metasurface has a BIC at the θ=0, and the Q-factor of the lower band increases as the\nangle approaches zero according to Fig. 2. Usually, this is a symmetry-protected BIC at Г=0 (θ=0), and\nit has been observed the doughnut-shaped far-field radiation pattern in BIC-based lasing before\n(NanoLett.2020,20,6005−6011; Science 2020, 367, 1018−1021; Nat. Commun. 2017, 8, 13687; Phys.\nRev. Lett. 2013, 110, 206802). The angle-resolved PL spectrum in Fig. 3d is along one direction and\nmay indicate that the far-field radiation pattern is doughnut-shaped. To completely describe the lasing\ncharacteristics, I suggest the authors measure the angle-resolved PL images in two directions below and\nabove the threshold power (like Fig. 5 except without polarizations). Furthermore, please discuss and\ncompare them with the doughnut-shaped far-field radiation pattern in the reported work.\nResponse 1) We appreciate the reviewer’s valuable comment. Our far-field PL data also shows a\ndoughnut-shaped emission pattern, consistent with the characteristics of a BIC-based lasing mode.\nThese results are in good agreement with the previously reported doughnut-shaped far-field radiation\npatterns in BIC-based lasers.\nIn addition, we have performed numerical simulations to further confirm the presence of polarization\nvortices associated with this mode, and we have included a detailed discussion of these results in the\nrevised manuscript. (Supplemental figure 5, Figure 5, Lines 145-149)\nComment 2) The authors measured the PL spectrum near the threshold power by narrowing the\nentrance slit as much as possible (Line 113). What is the width of the entrance slit? Where is the entrance\nslit? Is the slit used to restrict the pumping light? If so, the authors are encouraged to further clarify its\nspecific role and impact in the experimental measurements. Furthermore, could the authors measure the\nPL spectrum near the threshold power using a 515 nm fs-pulsed laser with a Gaussian beam of\napproximately 280 μm diameters without the entrance slit?\nAs far as I know the entrance slit restricting the pumping light has a significant impact on the emission\npeak width. Hence, it is essential that the authors investigate how the lasing peak width varies with the\n6\nentrance slit width and discuss the underlying physical mechanisms. Please measure the PL spectrum\nnear the threshold power under the different entrance slits.\nResponse 2) We apologize for the confusion. The “slit” mentioned in Line 113 refers to the entrance\nslit of the spectrometer, as illustrated in the measurement setup below. This slit is not related to the\nexcitation (pump) beam path but is located at the entrance of the spectrometer used to collect the emitted\nPL signal.\nThe role of this entrance slit is to define the spectral resolution of the spectrometer. As well known,\nthe resolution (Δλ) is approximately proportional to the slit width (Δλ∝ w). A narrower slit provides\nbetter spectral resolution. To obtain the inset data in Fig. 3e, we set the entrance slit of the spectrometer\nto 10 μm in order to achieve the maximum spectral resolution. Under this condition, we achieved a\nmeasured linewidth of 104 μeV for the lasing PL peak. Therefore, while the slit width indeed influences\nthe measured emission linewidth, this effect originates from the spectrometer resolution and does not\nphysically alter the lasing process or the sample emission itself.\nTo clarify this point, we show PL spectra near the threshold power under different entrance slit widths\nbelow.\nWe have clarified these in the revised manuscript. (Lines 120, 341-342)\nComment 3) The authors also excite the molecular metasurface using a 515 nm (~ 2.4 eV) fs-pulsed\nlaser with a Gaussian beam of approximately 280 μm diameters (Fig. 3a) (Line 104). The pumping light\non the metasurface is a spot with 280 μm diameters or a slit of several nanometers for different cases?\nThe authors should clarify the shape of the incident light spot for the different figures such as Fig. 3,\n4 and 5. In addition, at what angle is the pumping light on the metasurface, and how is the lasing\nemission collected?\nIn a word, the authors need to describe the optical measurement setup in detail so that the readers can\nclearly understand the experimental conditions and results.\nconditions used for each measurement:\nPump spot size and shape\n- In Fig.3, we used a large Gaussian spot of ~280 μm diameter to excite a wide area of the\nmetasurface in order to highlight the uniformity and large area of lasing emission across the\ndevice, which is well displayed in Fig.3a and 3b.\n- In Fig.4, we used a smaller excitation spot with a diameter of approximately 50 μm. This is\nbecause larger pump beams make it difficult to measure interference in our setup.\n- In Fig.5, we also used a smaller excitation spot as the same as Fig.4.\nEntrance slit in the spectrometer\n- For most spectral measurements, the spectrometer entrance slit was set to ~50 μm. Only when\nwe measured the lasing spectrum for an inset of Fig.3e, we narrowed the slit to ~10 μm to\nmaximize spectral resolution ( ~0.04 nm).\nIncidence and collection geometry\n- The pump beam was incident normal to the metasurface (θ = 0°) in all measurements. PL\nemission was collected in reflection geometry (same side as the pump) through an objective lens.\nThe simplified setup schematic is provided in the response 2 above.\nWe have included this clarification in the revised version of the manuscript. (Measurements section)\nComment 4)\nThe threshold pumping power Pth for lasing is found to be ~ 20.8 μJ/cm² (Line 110). How is the incident\npower measured, and lasing threshold calculated for fs-pulsed excitation at 10 kHz repetition rate?\nThe pumping power is the peak power, instantaneous power, average power per second or other else of\nthe fs-pulsed laser? What are the pulse width and pulse energy of the laser? Since lasing action is generally\ngoverned by the instantaneous power of the pump, this distinction is essential. The pumping average\npower may be very small, but the instantaneous power can be very high for the fs-pulsed laser.\nResponse 4)\nWe appreciate the reviewer’s comment. First, the incident pump power was measured at the sample\nposition using a commercial optical power meter. The measured pump power at the lasing threshold\nwas 12.80 μW. Using this value and the pump spot area (Gaussian beam with a diameter of 280 μm),\nthe pulse fluence per pulse was calculated with 10 kHz repetition rate.\n𝑃 12.8×10−6 𝑊 2.08×10−6\n𝐹 = 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 = = 𝑊/𝑚2𝑠\n𝑝𝑢𝑙𝑠𝑒 𝑓 ×𝐴𝑟𝑒𝑎 2.8 2 104×10−8\n𝑟𝑒𝑝 104/𝑠×𝜋×( ×102×10−6 𝑚)\n2\n= 2.08×10−2 𝐽/𝑚2 =2.08 ×10−2×106𝜇𝐽/(102𝑐𝑚)2 = 2.08 𝜇𝐽/𝑐𝑚2\nWe found that the order of magnitude was previously miscalculated, and the correct pulse fluence\n12.8×10−6\nvalue is one order lower as 2.08 μJ/cm². And pulse energy of the laser is 𝐸 = = 1.28 𝑛𝐽\n𝑝𝑢𝑙𝑠𝑒 104\nper pulse.\nWe did not directly measure the pulse width of the fs-pulsed laser; however, according to the\nspecification sheet of the laser equipment, the pulse width is approximately 200 fs. We clarified these\nterminologies for better understanding. We have revised these changes in the revised manuscript.\n(Figure 3, Lines 118-119, 123)\nshaped interference patterns in the reported work (Fig. 2D in Science 2020, 367, 1018−1021). It appears\nthat the fork-shaped interference pattern was not observed. Could the authors discuss the differences\nand the relationship between this work and the reported work?\nResponse 5) We note that the fork-shaped interference fringes reported that paper [Science 2020] were\nobtained in the far-field (k-space), where the presence of polarization vortices and phase singularities\nto prove vortex microlaser. In contrast, our Michelson interferometer measurement in Fig. 4 was\nperformed in real space, directly imaging the overlapped emission area from the metasurface.\nIn real-space interferograms, the lasing mode does not exhibit a phase singularity, and therefore no\nfork-shaped pattern is expected. Our real-space interference measurement was conducted to reveal the\nspatial coherence over a broad emission area, rather than to investigate the vortex behavior of the lasing\nemission.\nComment 6)\nFigure 4c shows temporal coherence of BIC lasing. The visibility of the interference fringes is plotted\nas a function of the delay time between the two arms of the Michelson interferometer. I suggest the\nauthors provide images of these interference fringes so that readers can have a clearer understanding of\nthe visibility.\nResponse 6) We thank the reviewer for the helpful suggestion. We have added representative images\nof the interference fringes at different delay times in the revised manuscript. (Figure 4, Lines 137-138)\nComment 7) The authors claim there are two key factors that enabled such an exceptionally narrow\nlinewidth for BIC lasing. One is the metasurface design enables very high Q-factors. However, the Q-\nfactor of ~700 in Fig. 2g is really not high, compared to the Q-factor of 5.0 × 105 or 3.0× 103 in Ref. 7\nand the million-Q reported in Ref. 21. The authors also speculate the intrinsic properties of the SMILES\nmaterials support the narrow linewidth and long temporal coherence. If the SMILES materials support\nnarrow linewidth and long temporal coherence, the authors need to present evidence or some\ndiscussions? I suggest the authors should give some further explanations for the exceptionally narrow\nlinewidth.\nTo better illustrate the role of the BIC mode, the authors should measure PL spectra as a function of the\npumping power at the planar SMILES region similar as Fig. 3e.\nResponse 7) We agree that the simulated Q-factor is relatively lower compared to other BIC-based\nsystems reported in Refs. 7 and 21. This is mainly attributed to the absorption loss in our active SMILES\nlayer. In contrast, the structures in Ref. 21 employed nearly lossless materials such as PMMA and SiN\nin their spectral range. In the case of Ref. 7, the extremely high Q-factors were achieved in\ntechnologically mature III–V semiconductor platforms. Moreover, the gain medium (quantum wells) is\n9\nspatially separated from the lasing region. The BIC lasing occurs in the passive region (nanocavity\nregion), enabling much higher Q-factors. In our work, SMILES metasurface integrates the absorbing\ngain medium directly within the resonant layer, which intrinsically limits the achievable Q.\nWe have improved our measurements as shown below and experimentally confirmed a Q-factor of\napproximately 1,534 in the lower GMR band.\nMoreover, we found that the absorption of the SMILES layer had been overestimated in the initial\nsimulations. Since the excitonic absorption can vary depending on the concentration of the fabricated\nSMILES film, we recalculated the simulations by adjusting the absorption accordingly. In the improved\nsimulation, the lower GMR band containing the BIC exhibits a Q-factor of about 1,026 at 𝜃 = 0.1°.\n𝑥\nWe agree that there is no direct experimental evidence supporting that the intrinsic properties of\nSMILES materials are responsible for the narrow linewidth and long temporal coherence. Our\ndescription was to point out that, compared to previously reported organic and solution-processed BIC\nlasers, our SMILES system phenomenologically exhibits a narrow linewidth of lasing emission. We\nrecognized that such an expression could be considered an overstatement. We have revised the\nmanuscript accordingly. (Lines 156-159)\nThe planar SMILES region corresponds to a uniform SMILES film without any patterning. In this\ncase, the PL intensity increases linearly with the pump power and then shows a typical saturation\nbehavior, rather than any lasing-like nonlinear increasing. This characteristic has been well\ndemonstrated in our previous R3B-SMILES work [Ref. 25].\nComment 8) This work has a similar frame with the reported work (Nano Lett. 2025, 25, 1653−1659),\nwhich only shows the emission with the Q-factor of 308. I suggest the authors compare this work with\nthem.\nResponse 8) We appreciate the reviewer for pointing out the related work. In that study, although the\ngain medium with polymer was directly patterned on the structure, the use of a silver substrate\nintroduced significant plasmonic losses, which limited the achievable Q-factor and prevented lasing.\nMoreover, the observed sharp emission in that work does not originate from BIC modes.\nimproved experimental data show that the Q-factor reaches approximately 1,530, exhibiting a much\nsharper emission peak and distinct BIC-related features.\nComment 9) We could distinguish amplified spontaneous emission and lasing emission with photon\nstatistics. One of the best indications of temporal coherence is the second-order correlation function\ng(2)(t,τ). If possible, the authors could measure the second-order correlation function, which can\nrigorously quantify the quantum nature of the phase transition from spontaneous emission to stimulated\nemission (Sci. Adv. 8, eade8817 (2022)).\nResponse 9) We appreciate the reviewer’s valuable suggestion. Unfortunately, measuring the second-\norder correlation function 𝑔(2)(𝜏)is technically challenging in our measurement setup due to the lack\nof suitable single-photon detectors and time-correlated counting modules. Nevertheless, we can\nconfidently distinguish lasing emission from amplified spontaneous emission based on the following\nobservations:\n- Spectral linewidth narrowing – the emission linewidth sharply decreases to ∼100 µeV above\nthe threshold, which is a clear signature of lasing behavior.\n- Nonlinear intensity increase – the output intensity exhibits a distinct threshold behavior\naccompanied by a superlinear power dependence.\n- Temporal and spatial coherence – Interference fringes confirm the buildup of coherence\ncharacteristic of laser emission.\nThese features collectively verify that the observed emission originates from lasing rather than\namplified spontaneous emission.\n- Nature Checklist on Lasing: The experimental data presented in this work agrees with the\nchecklist provided by Nature Journals to establish lasing.\nhttps://www.nature.com/documents/nr-lasing-reporting-flat.pdf\nComment 10) Line 18, “an” should be “a”?\nLine 80, Ref. 20 should be Ref. 21?\nResponse 10) We appreciate the reviewer’s corrections. We have corrected the mentioned typos.\nComment 1)\nwhy Q-factor is only 700 in Fig. 2g? The authors report up to 10^6 in ref. 21.\nAny reason for why not going to ultrahigh Qs in this study? Explain.\nResponse 1) We have improved our measurements as shown below and experimentally confirmed a Q-\nfactor of approximately 1,530 in the lower GMR band.\nMoreover, we found that the absorption of the SMILES layer had been overestimated in the initial\nsimulations. Since the excitonic absorption can vary depending on the concentration of the fabricated\nSMILES film, we recalculated the simulations by adjusting the absorption accordingly. In the improved\nsimulation, the lower band containing the BIC exhibits a Q-factor of about 1,026 at 𝜃 = 0.1°, and\n𝑥\nconsistently higher Q-factors across all angles compared to the upper band, which lies in a stronger\nabsorption region. These improvements have been added to the main text and the Supplementary\nInformation.\nComment 2) at which angle BIC lasing occurs? It seems it is off-normal in Fig. 3d. Any specific reason\nfor that? explain in more detail.\nResponse 2) The lasing occurs at the angle of 0.2° because this lasing mode originates from the\nsymmetry-protected Γ-point (k = 0) BIC, where the radiative coupling is completely suppressed,\nresulting in no out-coupling to free space [1,2]. Therefore, the emission cannot occur exactly at k = 0\n(𝜃 = 0°). However, near 𝑘 = 0 (non-zero and small k), the mode has a finite radiative leakage channel,\nallowing lasing emission to occur close to the normal direction.\n[1] Nature 541, 196–199 (2017)\n[2] Nature Communications 12, 4135 (2021)\nComment 3) in Fig. 4, fringe patterns are observed beyond the pumping area. Not clear why. Explain\nin more detail.\nResponse 3) The fringe patterns observed beyond the pumping area originate from the leaked emissions\nof the propagating GMR modes. These lasing emissions spread broadly in both vertical and horizontal\ndirections and propagate beyond the excitation spot. The propagating modes interfere with each other\n(interference between right and left & up and down), resulting in the formation of the observed fringe\npatterns.\nand the editor. We have carefully considered all comments and revised our manuscript accordingly to\nenhance its clarity. We hope that the improvements in response fulfill the expectations for publication,\nand we remain grateful for your time and consideration.\nOct. 17, 2025\nDaegwang Choi, Serena Zachariah, Rishabh Kaurav, Jie Fang, Junsheng Chen, Bo W. Laursen, Amar\nH. Flood, Arka Majumdar, Andrea Alù, Vinod M. Menon\n13"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors carefully revised the manuscript and addressed all relevant questions. In my view, the revised version meets the criteria of novelty, impact and technical quality of Nature Communications. Therefore, I support publication of the paper in the current form.\nAn optional comment to consider discussing in the text is the following. In Question 3 of the previous round, I asked about the reason of specific emission angle of 0.2 degrees for the BIC mode. The authors replied that the emission cannot happen at 0 degrees due to the symmetry protection, which is valid, and should happen at a very small angle instead. It is still unclear how small the emission angle can be and what limits it. From a theory perspective, the BIC Q factor in an ideal metasurface will be exceptionally large at arbitrarily small angles, say, of 0.0001 degrees. My guess is that the emission occurs at an angle satisfying the critical coupling condition, where the intrinsic radiative losses of the BIC match the absorptive loss due to coupling with the excitonic material. For this particular regime, the electric field is maximized while the radiative Q factor may be not the highest. Since the effect of absorption losses is very weak for the BIC mode, the emission angle is small ~0.2 degrees, but not less than than, despite the BIC radiative Q factor only grows for angles smaller than 0.2 degrees.", "reviewer_2": "The authors have responded to all my concerns about this manuscript. However, there are still some issues in the revised manuscript.\n1. Authors have measured far-field radiation patterns as in Supplemental Fig. 5. Please describe or discuss it in main text. And please discuss and cite all Figures in main text.\n2. Authors have discussed the calculation of threshold pumping power, please give the instantaneous power of the pump, which governs the lasing action. And discuss it.\n3. How did authors calculate the Q factor of GMR modes and lower band around BIC? The peak wavelength was divided by the full width at half of the maximum (FWHM)? Authors claim that the lower GMR band containing the BIC exhibits a Q-factor of about 1,026 at θ_x = 0.1° in the improved simulation in Supplemental Fig. 3. From Supplemental Fig. 3f, the Q-factor of lower band almost decreases with the angle, and the Q factor of lower band is larger than the Q factor of the upper band. However, it looks that the width of peak 5 (θ_x = 1°, green arrow pointing) is narrower than peak 1 (θ_x = 0.2°, green arrow pointing) at the lower GMR band in Supplemental Fig. 3d, therefore, the Q factor of peak 1 (θ_x = 0.2°) is smaller than the Q factor of peak 5 (θ_x = 1°), which contradicts Supplemental Fig. 3f. To ensure the result is credible, authors need to tell readers how to calculate the Q factor in detail. And give the accurate wavelengths and peak width of all peaks.\nFrom Supplemental Fig. 3e, the Q factor of lower band is almost 2 times that of the upper band. However, both peak widths present no obvious difference from the spectra in Supplemental Fig. 3c. How to calculate the Q factor in Supplemental Fig. 3e from the spectra in Supplemental Fig. 3c? Authors claim that the Q-factor of approximately 1,534 in the lower GMR band from the measured spectra in Supplemental Fig. 3c, authors need to describe the calculation in detail.\nWhy not calculate the same angle as the measured one for the convenience of comparison?"}, "author_response": "Author Response Letter to the Reviewer Comments\nManuscript #: NCOMMS-25-59675-T\nTitle: Highly coherent organic lasing based on etch-free metasurface\nWe are thankful to the reviewers for their critiques and thoughtful comments which have helped to\nmake the manuscript more coherent and comprehensive. In response to the referee comments, we have\nimproved our manuscript as delineated below. Therein, please note that the reviewer comments are in\nblack while our responses are in blue. In addition, all changes in the revised manuscript are highlighted\nin yellow and indication of positions is based on the revised manuscript.\nTo Reviewer #1:\nComment 1) The authors carefully revised the manuscript and addressed all relevant questions. In my\nview, the revised version meets the criteria of novelty, impact and technical quality of Nature\nCommunications. Therefore, I support publication of the paper in the current form.\nAn optional comment to consider discussing in the text is the following. In Question 3 of the previous\nround, I asked about the reason of specific emission angle of 0.2 degrees for the BIC mode. The authors\nreplied that the emission cannot happen at 0 degrees due to the symmetry protection, which is valid,\nand should happen at a very small angle instead. It is still unclear how small the emission angle can be\nand what limits it. From a theory perspective, the BIC Q factor in an ideal metasurface will be\nexceptionally large at arbitrarily small angles, say, of 0.0001 degrees. My guess is that the emission\noccurs at an angle satisfying the critical coupling condition, where the intrinsic radiative losses of the\nBIC match the absorptive loss due to coupling with the excitonic material. For this particular regime,\nthe electric field is maximized while the radiative Q factor may be not the highest. Since the effect of\nabsorption losses is very weak for the BIC mode, the emission angle is small ~0.2 degrees, but not less\nthan than, despite the BIC radiative Q factor only grows for angles smaller than 0.2 degrees\nResponse 1) We appreciate the reviewer’s insightful comment. We agree that the emission angle of\n~0.2° is likely determined by the balance between radiative and absorptive losses, as described by some\ncoupling conditions. From a theoretical perspective, the emission angle is influenced by the properties\nof the guided-mode resonance (GMR) hosting the BIC, particularly its Q-factor, group velocity near\nthe Γ-point, and depth of reflectance dip. As the emission angle approaches zero, 𝜃 = 0 (closer to the\n𝑥\nsymmetry-protected BIC), the radiative loss decreases and the reflectance dip in the spectrum becomes\nshallower, indicating weaker out-coupling. The material absorption loss should also be taken into\naccount in this consideration. Therefore, the observed lasing angle likely corresponds to the condition\nwhere sufficient radiative leakage is maintained while maximizing the optical field enhancement near\nthe BIC resonance. In previously reported BIC lasing studies, this aspect has rarely been discussed\nexplicitly, and we appreciate that the reviewer’s comment identifies an important aspect that merits\nfurther theoretical consideration.\nA relevant discussion can be found in Nature Communications 12, 4135 (2021), where a similar point\nto the reviewer’s comment was discussed, albeit in a limited scope. The far-field emission angle was\nqualitatively attributed to the mode size of lasing emission, which is phenomenologically governed by\nthe excitation geometry or the metasurface structure.\nComment 1) Authors have measured far-field radiation patterns as in Supplemental Fig. 5.\nPlease describe or discuss it in main text. And please discuss and cite all Figures in main text.\nResponse 1) We appreciate the reviewer’s comment. As suggested, we included a description and\ndiscussion of the far-field radiation patterns of Supplementary Fig. 5 in the previous revised manuscript\n(Lines 147–148). But, we revised the expression for clarity.\nComment 2) Authors have discussed the calculation of threshold pumping power, please give\nthe instantaneous power of the pump, which governs the lasing action. And discuss it.\nResponse 2) In the previous response letter, we calculated the pulse energy to be 1.28 nJ per pulse, and\nthe pulse width of our pump laser is approximately 200 fs. Accordingly, the instantaneous power of\npump can be obtained as,\n𝐸 1.28×10−9 𝐽\n𝑃 = 𝑝𝑢𝑙𝑠𝑒 = = 6.4×103 𝑊\n𝑖𝑛𝑠𝑡 𝜏 200×10−15 𝑠\n𝑝𝑢𝑙𝑠𝑒\nWe have included this instantaneous power value (6.4 kW) in the revised manuscript (Lines 119-120).\nComment 3) How did authors calculate the Q factor of GMR modes and lower band around\nBIC? The peak wavelength was divided by the full width at half of the maximum (FWHM)?\nAuthors claim that the lower GMR band containing the BIC exhibits a Q-factor of about 1,026\nat θ_x = 0.1° in the improved simulation in Supplemental Fig. 3. From Supplemental Fig. 3f,\nthe Q-factor of lower band almost decreases with the angle, and the Q factor of lower band is\nlarger than the Q factor of the upper band.\nHowever, it looks that the width of peak 5 (θ_x = 1°, green arrow pointing) is narrower than\npeak 1 (θ_x = 0.2°, green arrow pointing) at the lower GMR band in Supplemental Fig. 3d,\ntherefore, the Q factor of peak 1 (θ_x = 0.2°) is smaller than the Q factor of peak 5 (θ_x = 1°),\nwhich contradicts Supplemental Fig. 3f. To ensure the result is credible, authors need to tell\nreaders how to calculate the Q factor in detail. And give the accurate wavelengths and peak\nwidth of all peaks.\nFrom Supplemental Fig. 3e, the Q factor of lower band is almost 2 times that of the upper band.\nHowever, both peak widths present no obvious difference from the spectra in Supplemental\nFig. 3c. How to calculate the Q factor in Supplemental Fig. 3e from the spectra in Supplemental\nFig. 3c? Authors claim that the Q-factor of approximately 1,534 in the lower GMR band from\nthe measured spectra in Supplemental Fig. 3c, authors need to describe the calculation in detail.\nWhy not calculate the same angle as the measured one for the convenience of comparison?\nResponse 3) We apologize for any confusion caused by our previous explanation and thank the reviewer\nfor the careful observation. First of all, the Q-factors were obtained by fitting each resonance peak using\na Lorentzian fitting function as\n2𝐴 Δ𝐸\n𝑦 = 𝑦 + ∗\n0 𝜋 4∗(𝐸−𝐸 )2+Δ𝐸2\n𝑐\n2\nwhere 𝑦 is offset, 𝐸 is peak position, Δ𝐸 is FWHM (Linewidth of peak), 𝐴 is area. The Q-factor\n0 𝑐\nfor each mode was then calculated as 𝑄 = 𝐸 /Δ𝐸.\n𝑐\nFor the calculated data, the peak parameters at 1° and 0.2° are as follows. It may appear narrower, but\nthis is due to the depth of the dip rather than the actual linewidth. Furthermore, as requested, we provide\naccurate resonance energies and their linewidths of all peaks in the table below.\nAngle Peak energy Linewidth of Q-factor of Peak energy Linewidth of Q-factor of\n(Degree) of upper band upper band upper band of lower band lower band lower band\n(eV) (eV) (eV) (eV)\n0 2.0357 0.00323 629.493\n0.1 2.03615 0.00277 735.345 2.0272 0.00197 1026.663\n0.2 2.03709 0.00255 797.834 2.02625 0.00198 1025.362\n0.3 2.03848 0.00253 805.198 2.02491 0.00215 943.67\n0.4 2.03997 0.00263 774.23 2.02342 0.00205 985.754\n0.5 2.04147 0.00292 699.959 2.02174 0.00204 989.01\n0.6 2.04324 0.00287 710.854 2.02017 0.00216 934.857\n0.7 2.0449 0.00301 678.561 2.01873 0.00226 892.444\n0.8 2.0463 0.00307 665.715 2.01671 0.00222 908.689\n0.9 2.04839 0.00312 656.436 2.01522 0.00228 885.491\n1 2.0502 0.0031 660.819 2.0137 0.00234 859.496\nenergy of 2.0253 eV and a fitted linewidth of 0.00132 eV, corresponding to a Q-factor of approximately\n1534.\n2.0253\n𝑄 = = 1534.31\n0.00132\nThe calculation was not performed at the exact same angles as the measurements because the Q-factor\nvaries significantly only near the BIC region and changes much less at larger angles. However, in the\nexperiment, the measurement was performed over a broader angular range due to the limited resolution\nof the setup. Therefore, the calculated and measured data appear to cover slightly different angular\nregions.\nTo clarify this point, we have added a description of the fitting method in the main text. (Lines 103-\n105)\nWe sincerely appreciate the valuable feedback and insightful suggestions provided by the reviewers\nand the editor. We have carefully considered all comments and revised our manuscript accordingly to\nenhance its clarity. We hope that the improvements in response fulfill the expectations for publication,\nand we remain grateful for your time and consideration.\nNov. 5, 2025\nDaegwang Choi, Serena Zachariah, Rishabh Kaurav, Jie Fang, Junsheng Chen, Bo W. Laursen, Amar\nH. Flood, Arka Majumdar, Andrea Alù, Vinod M. Menon\n4"}]

Decision letter