Chiral orbital lasing in a twisted bilayer metasurface

Domain: physics
Published: 2026-03-12
Editorial outcome: minor_revision

Abstract
Chirality is a fundamental concept in physics, core to many phenomena in nonlinear optics, quantum physics and topological photonics. Photons are intrinsically chiral when carrying spin angular momentum, whereas orbital angular momentum can induce chirality when photons interact with structures with broken mirror symmetry. Here, we observe orbital chiral lasing from a twisted bilayer photonic structure, by leveraging its inherent structural chirality. We design and fabricate a Moiré-type optical structure consisting of two semiconductor membrane metasurfaces. By optically pumping the twisted bilayer, we achieve single-mode lasing over a broad spectral range of 250 nm. The lasing emission exhibits chiral orbital characteristics, arising from helical and non-Hermitian couplings between collective guided resonances rotating clockwise and counterclockwise and confirmed by polarization-resolved imaging and self-interference measurements. The observed chiral orbital lasing from a twisted photonic structure can contribute to diverse applications of chiral light in diagnostics, optical manipulation and communication with light.

Introduction
The emerged field of twistronics is underpinned by the pioneering observations that twisted stacking of two-dimensional materials can change their electric properties from being non-conductive1,2 to superconductive3–10, thus opening up avenues for advanced material design and inspiring research into their photonic counterparts. In twisted bilayer photonic systems11–15, interlayer coupling acts as an additional degree-of-freedom to stipulate unusual phenomena such as strong light localization16–18, enhanced optical nonlinearity19,20, and induced circular dichroism12,21, paving the way towards applications22,23. Among rich physics ranging from magic angles24 to flatbands25–29, a particularly intriguing characteristic of twisted bilayer systems lies in their intrinsic chirality12,30,31, which refers to an object or a system that cannot be superimposed with its mirror image. It was reported that a single-layer slanted metasurface can give rise to intrinsic chirality, represented by circularly polarized states with unique handedness known as spins for chiral lasing32. Despite this spin chirality, exciting advances have also been made in orbital chirality for light emission and lasing, typically by employing micro-ring cavities33–35, helical phase plates36,37, photonic crystals38,39 and metasurfaces32,40,41. Orbital chirality of photons can be understood as a twisting motion of an electromagnetic field as it travels through space, presented by the sign of optical modes’ orbital angular momentum (OAM). However, due to the law of reciprocity, optical modes in a vertical mirrored structure are always twofold-degenerate unless additional perturbations, such as helical geometry, gain, or loss, are introduced to break chiral symmetry42–44. In contrast, light traveling in twisted bilayer systems naturally couples in a non-Hermitian and helical manner, which could lead to collective oscillations possessing an intrinsically determined orbital chirality, distinct from that in single-layer systems. Consequently, twisted bilayer systems could be harnessed for on-chip chiral lasers—a long-sought goal in photonic research.

Here, we report on the observation of intrinsic orbital chiral lasing at telecom wavelengths using twisted bilayer metasurfaces. Specifically, we design a twisted photonic structure comprising two rotated metasurface membranes to form a Moiré-type lattice and employ a gain-guided mechanism to confine light. In such a system, isotropic geometry and dispersion hybridize bulk guided resonances, leading to a set of twofold degenerate collective guided resonances (CGRs) in each membrane that rotate in clockwise (CW) and counterclockwise (CCW) directions, respectively. Aided by the inherent chirality and non-Hermitian physics in the twisted photonic structure, the CW and CCW rotating modes interact to create non-Hermitian degeneracy, thus allowing one orbital chiral mode to prevail in the lasing. We further develop a wafer bonding process to fabricate the sample and achieve experimentally stable single-mode lasing over a broad spectral range of 250 nm at the lasing threshold of 73 kW/cm2. The lasing emission exhibits an intrinsic orbital chiral nature as predicted, showing a doughnut pattern in real space that carries a phase vortex with OAM quantum number l = 1, confirmed by polarization-resolved imaging and self-interference features.

Results
Principle and design
We start with a schematic of a twisted photonic structure as shown in Fig. 1a, which comprises two stacked and twisted metasurfaces with a thickness of h ~ 620 nm, separated by a gap of g ~ 100 nm, patterned on an epitaxial wafer with InGaAsP multiple quantum wells (MQWs) to provide optical gain in the telecom C-band around 1550 nm (details are provided in Supplementary Section 1). The upper and lower metasurfaces feature the same circular air holes arranged in square lattices, with a period of a ~ 540 nm and a radius of r ~ 150 nm. We numerically calculate the band structure of the single metasurface unit-cell near the 2nd order Γ point (Fig. 1b), identifying four transverse electric bands from TE-A to TE-D that lie in the continuum and radiate towards the out-of-plane direction, indicating that such a system is inherently non-Hermitian45. Nevertheless, the modes on the TE-A band still exhibit high quality factors (Qs) due to the presence of a symmetry-protected bound state in the continuum (BIC) at the Γ point46. These modes are vertically confined in the metasurface membrane and transversely extend throughout an infinite area with a specific Bloch wave vector k, and we refer to them as bulk guided resonances47. As illustrated in Fig. 1c, the TE-A band’s dispersion is quite isotropic in momentum space, showing circular iso-frequency contours near the Γ point.

Next, we rotate the upper metasurface membrane at an angle θ and place it over the lower one to form a twisted photonic system. This twist generates a Moiré pattern, leading to a supercell with a real-space size L = Na (Fig. 1d). Consequently, the supercell occupies only a fraction of the reduced Brillouin zone (BZ) of the unit cell, which spans [−π/L, π/L] in reciprocal space (green box, Fig. 1e). Moiré patterns emerge at specific twist angles θ, referred to as magic angles, which determine the size of the supercell as N = 1 / (2 sin(θ/2))48,49. This relationship implies that a larger twist angle results in a smaller supercell, and vice versa. Notably, diffraction in Moiré lattices causes out-of-plane leakage to degrade the Qs. In our design, we select θ = 22.62°, corresponding to a supercell of 5√2a × 5√2a, ensuring that only a few diffracted waves fall within the light cone. Additionally, out-of-plane leakage can be further minimized by incorporating off-Γ BICs by tuning membrane thickness. The TE-A band maintains quadratic and isotropic dispersion within this small-size supercell (Supplementary Fig. S1), which distinctly contrasts with the flat-band effect reported in the literature24. It is also important to highlight that such a twisted photonic structure is inherently chiral, meaning it cannot be superimposed onto its mirror image12. More details about the supercell design, radiation suppression, and parameter optimization are provided in Supplementary Sections 2, 3.

Further, we introduce an optical cavity to confine the bulk guided resonances to a finite region for collective oscillation50,51. To achieve this, we employ a gain-guided mechanism to create an effective cavity. Specifically, when the MQW material is optically pumped (Fig. 2a), carrier diffusion leads to spatial inhomogeneities in the gain, which in turn modify the refractive index for light confinement52. This variation can be approximated by n(ρ) = n(0) − n1 ρ2, where ρ is the radial position and n1, n2 are coefficients that depend on the material and pump power. The gain-induced refractive index evolves continuously according to the carrier density distribution that follows the profile of the pump beam. As a result, a circular pump beam naturally forms a rotationally symmetric cavity regardless of the twist angle. That is, the gain-guided effective cavity remains achiral.

As demonstrated53, trapping guided resonances with isotropic dispersion into a geometrically isotropic region can give rise to a set of CGRs that preserve continuous rotational symmetry, appearing as paired CW and CCW modes rotating in real space even without explicitly defined physical paths. We denote these CGRs’ wavefunctions as ψu,l, where the superscript indicates the metasurface position CW,CCW and the subscript denotes the rotating direction. Given that each metasurface is identical and achiral, these modes share the same complex eigenfrequency Ω0 governed by H0 ψu,l,CW,CCW = Ω0 ψu,l,CW,CCW, where H0 represents the Hamiltonian of a single metasurface membrane54. In our gain-guided cavity featuring a quadratic index changing, ψu,l’s envelopes take the form of Laguerre-Gaussian functions Ψm(r, z) (details are provided in Supplementary Section 4), characterized by quantum numbers l and p that correspond to the quantization in azimuthal and radial directions, respectively. The left panel of Fig. 2b presents the instantaneous field distributions of the collective modes for |l| = 0, 1, 2 with p = 0, where each mode exhibits twofold degeneracy due to the absence of chirality in both geometry and dispersion. The right panel of Fig. 2b illustrates the time-averaged field distributions, showing that, apart from the lowest mode |l| = 0, the average field profiles form doughnut-shaped patterns in real space. Additionally, the metasurfaces diffract CW or CCW modes into vertical radiation, producing a phase vortex beam (Fig. 2c). When observed through a linear polarizer, the diffraction results in two lobes of equal intensity appearing along the direction perpendicular to any polarizer’s major axis. By splitting and recombining the radiated beam for self-interference, a fork fringe is expected, serving as a distinct signature of chiral emission (upper panel, Fig. 2c).

We further consider a compound bilayer system, importantly, showing that the structure’s intrinsic chirality naturally breaks the twofold degeneracy in each membrane via intra and inter-layer couplings. For example, if we examine ψu residing in the upper metasurface, the lower one can be regarded as an asymmetric scatterer in real space that turns the upper one to be chiral, described by a perturbed Hamiltonian ΔH that introduces helical diffraction strengths. Consequently, the lower metasurface facilitates both CW → CCW and CCW → CW coupling paths in the upper one but with different weights, which we denoted as intra-layer couplings. Besides, the overlap of evanescent waves along the vertical direction also gives rise to inter-layer couplings. Taking into account radiation leakage as well as material gain or loss, the coupling coefficients are generally complex values that represent non-Hermitian interactions. More details about helical and non-Hermitian couplings are presented in Supplementary Section 5.

To understand the coupling dynamics in the twisted structure, we write an effective Hamiltonian Heff by including these four interacting, radiative, single-layer CGR modes ψu,l as schematically illustrated in Fig. 3a, in which the coupling paths are categorized into three distinct types, which are intra-layer cross-coupling (κintra, orange arrows), inter-layer cross-coupling (κinter, blue arrows), and inter-layer direct-coupling (κinter, purple arrow). Here, the terminology “cross" and “direct" refer to the couplings occurring between modes with opposite or same rotating directions, respectively. Besides, the subscripts 1, 2 denote the couplings direction as 1: CCW → CW and 2: CW → CCW, respectively. As a result, the effective Hamiltonian Heff can be written as a 4×4 coupling matrix, showing as an eigenvalue problem in the basis of single-layer wavefunctions V = [ψu,CCW, ψu,CW, ψl,CCW, ψl,CW]T:

Heff V = [Ω0 κintra,1 κinter,1 κinter,direct,1; κintra,2 Ω0 κinter,direct,2 κinter,2; κinter,1 κinter,direct,1 Ω0 κintra,1; κinter,direct,2 κinter,2 κintra,2 Ω0] V = Ω V (1)

Here Heff involves six independent complex parameters: Ω0, κinter,1,2, κinter,direct,1,2, κintra,1,2 from symmetry considerations. By solving this non-Hermitian eigenvalue problem (see Supplementary Section 6 for the details). Further, we identify four individual conditions for realizing exceptional points (EPs) as κinter,1 = ±κintra,1 or κinter,2 = ±κintra,2, where the eigenstates collapse at non-Hermitian degeneracy. As an example, we discuss the EP at κinter,1 = ±κintra,1 and calculate the complex eigenfrequencies as κintra,1 varies (Fig. 3b), revealing that the eigenstates ψ1–4 fall into two distinct branches, with one possessing higher loss. The EP is found on the upper branch, at which the eigenvector taking the form V = [0, 1, 0, 1]T with the highest degree of chirality, indicating that the collective mode merely rotates in the CW direction in both upper and lower layers with the aligned phases. When κintra,1 deviates from the EP within the shaded region, the real parts of the eigenfrequencies remain nearly degenerate. However, the differences in their imaginary parts (namely gains) can leverage one mode to dominate in lasing competition, ultimately prevailing over the others for single-mode lasing action. Noteworthy that, near the EPs, the degree of chirality slightly degrades but remains relatively high to enable the observation of chiral emission (Supplementary Fig. S5).

The EP condition κinter,1 = ±κintra,1 signifies a balanced interplay between intra and inter-layer coupling, manifesting that a combined symmetry of non-Hermiticity breaks the reciprocity. Notably, remaining EPs can also support chiral emission, with their corresponding eigenvectors given by V = [0, 1, 0, −1]T, [1, 0, 1, 0]T, and [1, 0, −1, 0]T. These eigenvectors differ in their transverse rotational direction and the relative phase in the two layers. Specifically, the mode can be either CW or CCW. The balance between κinter and κintra determines these characteristics as illustrated in Fig. 3c. Interestingly, the EPs in a system of two stacked dielectric metasurfaces have been theoretically investigated from the perspective of asymmetric losses55, further highlighting the rich physics of non-Hermitian bilayer systems.

To verify the principle of twisted photonic structure for orbital chiral lasing, we fabricate the samples by bonding two metasurface membranes from an InGaAsP MQW wafer on an InP substrate. The two sheets of square latticed metasurface are patterned at a twist angle of 22.62° within the same square-shaped metal out-frame for supporting and alignment. As shown in Fig. 4a, we first deposit and liftoff a thin layer of titanium/gold with a thickness of 50 nm, which can create an air gap of 100 nm after bonding to adjust the coupling strength between two metasurfaces. Then, we fabricate each metasurface that consists of 50 × 50 air-holes at a lattice constant of a = 540 nm by using e-beam lithography (EBL) and induced plasma etch (ICP). The air-hole depth and radius are given by h = 622 nm and r = 153 nm, respectively. We further undercut the scarified layer underneath the MQW layer by using wet etching, to ensure z-mirror symmetry in a single sheet. Next, the two metasurfaces are aligned via prepared marks that are visible under the microscope of the wafer bonding machine, and then we bond them through a low-temperature annealing process (see Methods for details). We present the overall and detailed side views of the fabricated sample that is cleaved by a focused ion beam (FIB) (Fig. 4b), clearly showing that the upper and lower metasurfaces sandwich an air gap and align with an evident twist angle.

We use a confocal measurement system to characterize the lasing of the twisted photonic structure (Fig. 4c). A pulsed pump laser operated at a wavelength of 1064 nm was focused on the sample surface using a 20× objective lens to drive the lasing oscillation and chiral emission. The laser emission and the reflected pump beam are collected by the same objective lens, but the latter is filtered out by using a long-pass filter with a cutoff wavelength of 1300 nm. The laser emission is then imaged in real space by using a cascade 4f system. The laser emission pattern is captured by a complementary metal-oxide-semiconductor (CMOS) camera, while the lasing spectrum is recorded using a spectrometer. To identify the phase vortex associated with chiral emission, a Mach–Zehnder interferometer was constructed for self-interference, consisting of two beamsplitters (BS) and two gold-coated mirrors (M1 and M2). The optical path difference was controlled by adjusting the angular orientation of M2. The interference patterns of lasing emission are ultimately captured by the CMOS camera. More details about the measurement setup can be found in Supplementary Section 7.

Observation of intrinsic orbital chiral lasing
To experimentally observe the orbital chiral lasing behavior of the twisted bilayer structure, we optically pump the fabricated sample using the 1064 nm pulsed laser at room temperature. The pump beam has a Gaussian intensity profile in the transverse direction, showing a circular spot of ~5.5 μm (10a) in diameter that preserves a rotational symmetry at arbitrary angles. Notably, the pump beam size is smaller than that of the metasurfaces’ footprint (50a × 50a). This size difference ensures that light confinement is primarily governed by gain-guided refractive index modification rather than the metasurfaces’ outer geometric edges. More details are presented in Supplementary Section 8.

The establishing of lasing oscillation is observed as gradually increasing the pump power (Fig. 5a). We first record the emission spectrum with a 150 g/mm grating at a pump power of 70 kW/cm2. The spectrum shows no evident resonant peaks upon the spontaneous emission background (upper panel, Fig. 5a). When the pump power reaches 73 kW/cm2, a lasing threshold is achieved, evidenced by the emergence of a distinct peak amidst the spectrum (middle panel, Fig. 5a). Further increasing the pump power to 74 kW/cm2 can suppress spontaneous emission, leading to a well-defined single peak that features a single-mode lasing behavior spanning a broad spectral range of 250 nm from 1430 nm to 1680 nm (lower panel, Fig. 5a). Additionally, we record and plot the lasing power curve (Fig. 5b), highlighting three key data points (labeled A–C) corresponding to the spectra in Fig. 5a. Notably, point B marks the power transition and represents the lasing threshold.

Further, we opt for a high-resolution grating (600 g/mm) to capture the lasing mode’s spectral details. As we stated above (Fig. 3), four hybridized modes ψ1–4 evolve in lasing process, which might appear as multiple peaks in the emission spectrum if their real eigenfrequencies split. As presented in Fig. 3c, ψ1–4 comes as two distinct branches mostly aligning with the inter-layer coupling strength κinter,direct. Due to the twisted bilayer structure’s inherent non-Hermiticity and strong inter-layer coupling from a small gap distance of g ~ 100 nm, the lower branch ψ3–4 separates ~5 nm from the high branch ψ1–2 and possesses considerable losses, thus they are not visible in our observation. Besides, the real frequencies of high branch modes ψ1–2 remain quite close, while their imaginary parts are different near the EP condition. As a result, we observe only one lasing peak rather than multiple closely spaced peaks upon the high-resolution spectrum, confirming that the twisted bilayer metasurface can support single-mode chiral lasing, despite the pump beam itself being achiral. More details are presented in Supplementary Section 9.

Next, we characterize the lasing emission from real-space imaging. a doughnut pattern is observed in real space (Fig. 5d), agreeing with our theory as laguerre-gaussian profiles in time-averaged measurements of any quantum number |l| ≠ 0. the doughnut beam size is estimated to be ~5a, smaller than that of the pump beam size (~10a) and metasurfaces’ footprint (~50a). when we shift the pump beam to a different position on the sample, the lasing beam follows the pump beam’s new location while maintaining its doughnut shape (Supplementary Video), confirming the effectiveness of the gain-guided mechanism. to verify that the lasing doughnut beam is rotationally invariant, we placed a linear polarizer in front of the cmos camera to measure polarization distributions along 0°, 45°, 90°, and 135° directions, respectively. the emission pattern is nearly uniform across all directions (Fig. 5e), aligning well with the theoretical prediction (Fig. 2c).

Finally, we evaluate the phase vortex nature of the lasing emission using the self-interference technique. Specifically, we split the lasing beam evenly into two parts and overlap them in real space using a Mach-Zehnder interferometer setup. The resulting interference fringe reveals a pair of oppositely directed forked pattern, as a hallmark of phase singularity in the wavefront. The dislocation points of these forks align with the center of laser beams, where a fringe splits into two distinct branches, confirming that the vortex beam carries an orbital angular momentum (OAM) of |l| = 1. Due to the intrinsic chirality of the twisted bilayer metasurface, the fork patterns consistently orient in the same direction, regardless of variations in the pump beam’s position or shape. This indicates that the phase vortex’s handedness remains highly stable, irrespective of external excitation. By fitting experimentally observed fringe contrast with the theoretical model53, we estimate the observed purity of chirality to be approximately αCW : αCCW = 0.7 : 0.3, suggesting that the lasing action happens near the EP condition as shown in Supplementary Fig. S5. Due to nanoscale deviations in the radius caused by etching process variations from the designed structure at the EP point, the performance deviates from the EP condition, consequently degrading the chirality purity. To verify that the observed vortex behavior intrinsically arises from the twisted-bilayer geometry, we fabricated single-layer reference samples that retain an achiral structure for comparison. In contrast to the single-layer samples, the twisted-bilayer structures consistently exhibit the same fork pattern and orientation across multiple samples and positions (see Supplementary Section 10 for further details). We also confirm that the vortex feature corresponds to a steady-state behavior rather than a transient response, based on carrier-dynamics analysis (Supplementary Section 11).

Methods
Numerical simulations
The numerical simulations were performed using the finite-element method (FEM, COMSOL Multiphysics). The bandgap structure shown in Fig. 1b and the TE-A mode’s field distribution depicted in Fig. 1c were calculated using a 3D unit-cell structure of a single-layer metasurface. This model employed Floquet periodic boundary conditions on the sidewalls and perfectly matched layers (PMLs) at the top and bottom boundaries. The results presented in Figs. 2 and 3c were obtained from calculations based on our designed Moiré superlattice structure. This computational model similarly utilized Floquet periodic boundary conditions on the sidewalls and PMLs at the top and bottom boundaries. The computational model was executed on a system equipped with two Intel(R) Xeon(R) E7-4890 v2 @ 2.80 GHz CPUs. Within the finite element model, the mesh discretization for the photonic crystal layer utilized the minimum element size of 3 nm and the maximum element size of 80 nm. Other key parameters for mesh generation include the maximum element growth rate of 1.45, the curvature factor of 0.5, and the resolution of narrow regions set to 0.6.

Sample fabrication
The sample fabrication was performed using the low-temperature annealing direct-bonding processes via a designed high-precision alignment. Following lithography and Ti/Au sputtering processes, we liftoff the metal windows of 200 μm × 200 μm on top of metasurfaces by acetone developing. The total thickness of Ti(15 nm)/Au(35 nm) is 50 nm and the thickness of the metals can be controlled precisely in sputtering processes. The sputtering metals on both sheets provide a total of 100 nm air gap, as an additional degree of freedom for inter-layer interactions. Both the auxiliary alignment marks and the periodic holes are patterned on each slab by EBL (ELX-F125), and subsequently the metasurfaces are etched by ICP. Selective dry etching of silicon dioxide is performed for 250 s under an inductive coupling power of 900 W, a RF bias power of 50 W, chamber pressure of 3.75 mTorr, and gas flow rates of 10/30 sccm for CHF3/CF4. Further, the metasurface on the InP epi wafers are fabricated using PlasmaPro 100 Cobra system (Oxford Instruments). A fast etch rate is achieved in 60 s under an inductive coupling power of 500 W, a RF bias power of 170 W, chamber pressure of 5 mTorr, gas flow rates of 12/2/30 sccm for BCl3/Cl2/Ar, and a temperature of 220 °C.

Following the etching process, the undercut InP epitaxial layers below their active regions are over-etched by 1:1 = 60% HCl:H2O solution in 11 min. Then the active die is picked-and-placed on the interposer die at the designed position by the dual-view microscope in flip-chip instrument via alignment marks. The alignment and prebonding processes are done under standard atmospheric pressure, 3 K/s heating rate, annealing temperature of 300 °C in 360 s, 2 K/s cooling rate, and 25 N pressure. Then the aligned and pre-bonded bilayers are moved into the Suss bonder machine immediately and gently. The fitted twisted bilayers are bonded based on a four-step bonding process under 0.0001 mbar atmospheric pressure. Initially, the temperature of the bonder elevates to 120 °C at a rate of 30 °C/min under no pressure, and the temperature is maintained at 120 °C in 20 min. Secondly, the bonder starts to bond under a pressure of 500 mbar in 10 min. Thirdly, the annealing temperature is enhanced to 150 °C at a rate of 10 °C/min and the annealing time is maintained at 60 min. At last, the bonder is cooled through natural cooling. The four-step bonding process is a low-pressure and low-temperature annealing method for bonding twisted bilayers, meanwhile, the thickness of TiAu can be maintained. Notably, this bonding process retains the fragile metasurface as bilayers, suspending in mid air intact after multi-step bonding.

Measurement and data processing
The experimental setup is based on a confocal microscopy system with a free-space laser as the pump light source. A height-adjustable mount is placed before the laser (MPL-N-1064-200uJ, 1064 nm, 10 kHz, 2 ns pulse) for precise alignment with the optical axis. A 20× Mitutoyo objective focuses the beam to 10 μm. The optical path includes lens L1 (f = 200 mm), confocal with the objective, and a 4f system formed by L2 (f = 250 mm) and L3 (f = 300 mm), with L2 enabling near-to-far field switching. A 1300 nm long-pass filter blocks the pump light, allowing observation of lasing emission at 1550 nm. The measurement system includes an InGaAs infrared CMOS camera (IMX990, Sony) and a monochromator (IsoPlane SCT320 with NIRvana 640, Princeton Instruments) equipped with 600 g/mm and 150 g/mm gratings for spectral analysis.

The CMOS image data is processed by subtracting the background image to extract the lasing pattern. The background image, captured when the pump light is off, reflects the CMOS camera’s idle response, including dark current, thermal noise, and stray light, and is used as a constant reference. During measurements, each image is corrected by pixel-wise subtraction of this background, effectively suppressing the camera-related noise. This enhances the signal-to-noise ratio, allowing weak features such as interference fringes and fork patterns to be clearly observed.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presented a twisted bilayer photonic system where two separate semiconductor membrane metasurfaces are bonded vertically with a twist. By optically pumping the above system, single-mode lasering with orbital chiral characteristics were observed. The experiments are of high technical quality. Nevertheless, more theoretical analyses, numerical calculations, and experimental results are expected to enrich this manuscript and help attract wider attention in the nanophotonics community. Several comments on this manuscript are:\n1. While the effective Hamiltonian is useful, the absence of a complete bandstructure calculation for the bilayer system is disappointing. A supercell analysis would directly reveal the Bloch modes and their couplings, and its omission leaves the theoretical basis less convincing.\n2. On page 4, the authors claimed that the design relies on a 620 nm membrane thickness with a 100 nm interlayer gap. However, throughout the manuscript, the authors did not provide an in-depth explanation on how these numbers were selected or optimized.\n3.The coupling constants k^intra and k^inter appear chosen to reproduce the observed spectra rather than obtained from the structure itself. Without numerical extraction or experimental calibration, the model risks being descriptive rather than predictive.\n4. The condition k^inter = −k^intra is presented as defining an exceptional point. It would be useful to clarify whether this condition has physical meaning in the realized structure, or if it is primarily a mathematical construct introduced for interpretation.\n5. The gain-guided cavity provides lateral confinement, yet its interaction with the twist-induced coupling is not explicitly considered. Treating these as separate effects simplifies the picture and leaves open the question of how the gain profile modifies the claimed chiral response.\n6. The sample fabrication details of EBL and ICP should also be included in this manuscript.\n7. OAM lasering has been experimentally demonstrated based on a variety of physics with diverse symmetry-breaking approaches and photonic cavity designs. Furthermore, one single photonic crystal slab is enough to generate OAM beams (for instance, Wang, B. et al. Nat. Photon. 14, 623–628 (2020).) Please specify the merits and advantages of the proposed twisted bilayer metasurfaces in the field of OAM generation.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of a Nature Communications initiative to promote peer review training and provide appropriate recognition to early-career researchers who co-review manuscripts.", "reviewer_3": "The authors describe the observation of a chiral orbital laser from a twisted photonic structure fabricated using a wafer bonding process. The results demonstrate a low-threshold laser with a broad spectral range of 250 nm and also show a doughnut pattern in real space.\nExperimental evidence and physical principles are strongly supported by numerical simulations and theoretical models that clarify some key concepts.\nThe sample realization is also well detailed as well as the experimental characterization.\nFinally the work is well supported by the literature.\nFor these reasons, I think the work is suitable for publication in Nature Communications.\njust a curiosity: could there be contributions due to SAM?", "reviewer_4": "The paper is undoubtedly groundbreaking; however the authors should address some points to strengthen their work and its impact.\nThese points focus on the depth of evidence, mechanistic clarity, and the claim of intrinsic chirality. In particular, I found that there is a lack of direct comparison with experiments. The most significant weakness is the absence of a direct experimental comparison with a non-twisted bilayer or a single-layer metasurface. The central claim is that the twist itself is responsible for breaking the degeneracy and enabling intrinsic orbital chiral lasing. Without a control experiment, it is difficult to rule out that the observed chiral lasing isn't simply a result of the gain-guided cavity and the dispersion of the single layer, which could also support chiral modes, albeit in a degenerate and non-preferential manner. Demonstrating that a 0° twist bilayer produces a non-chiral or degenerate lasing output would make the role of the twist irrefutable.\nThe paper claims the chirality is intrinsic and stable, but the experimental data does not conclusively prove that the system selects a specific handedness due to the twist. Did all measured devices and pump spots produce a vortex with the same topological charge? Or was it random? If it's random, the intrinsic nature is questionable; the twist creates a chiral potential, but random defects or pump fluctuations might still determine the final state. If it's always the same, what is the physical mechanism in the structure that fixes the handedness? The estimated chirality purity of 70:30 is good but not exceptional. A major question is whether this ratio is fundamentally limited by the design or if it can be improved to near 100:0 by operating precisely at the EP.\nThe gain-guided mechanism, while clever, introduces a significant and complex variable. The achiral pump creates a chiral laser, which is a fantastic result, but it also makes it difficult to disentangle the pure twist-induced effects from nonlinear interactions between the gain, carrier diffusion, and the photonic structure. It remains unclear to me how much of the mode selection is due to the twist-induced non-Hermitian coupling versus gain competition in a complex, non-uniform medium. A numerical model that incorporates spatiotemporal gain dynamics would significantly strengthen the claim that the observed phenomenon is primarily a photonic (twist) effect rather than a gain-dynamics effect.\nIn conclusion, I believe the paper should be published after the authors have considered the above criticism which does not impact on the strength and quality of the results presented here.\nMy final recommendation is: publish after minor revisions."}, "author_response": "Reply to Reviewers’ comments\nand a summary of the changes made in the revised manuscript\nResponse to Reviewer #1: ... [Full text of author response to Round 1 omitted for brevity in this example, but would be included here verbatim]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed the comments. A few minor issues could be considered for further improvement of the final manuscript:\n1. The discussion on the approximately 70:30 chirality purity ratio is insightful. The authors are encouraged to include this discussion in the main text and analyze some specific geometric parameters and/or fabrication issues that limit this chirality purity.\n2. In the Sample Fabrication section, the material \"GaInP\" is mentioned. Please doublecheck this material name.", "reviewer_4": "I have carefully examined the revised manuscript and the authors’ responses to the previous referee reports. The revision and the responses are substantial and convincing, and earlier concerns have been satisfactorily addressed.\nIn particular, the authors have significantly strengthened the theoretical framework, clarified the physical meaning of the effective Hamiltonian and coupling constants, and improved the discussion of design choices and optimization parameters.\nThe authors have clearly placed the proposed twisted bilayer metasurface laser more clearly within the broader context of orbital-angular-momentum (OAM) photonics, and the authors articulate convincingly the advantages of the twisted bilayer approach compared to single-layer implementations. The discussion of chirality, exceptional points, and gain-guided confinement is balanced and physically transparent.\nWhile some aspects of gain dynamics and chirality purity could be further explored in future works, these points no longer detract from the validity or impact of the present study.\nIn conclusion, I believe the manuscript is suitable for publication in Nature Communications, and I support its acceptance."}, "author_response": "Response to Reviewers’ Comments\nManuscript Title: Chiral orbital lasing in a twisted bilayer metasurface\nManuscript ID: NCOMMS-25-66090A\nCorresponding Author: Chao Peng (pengchao@pku.edu.cn)\nGeneral Response: We deeply appreciate the reviewers’ positive and supportive feedback, which is a great encouragement and motivation for us to continue pursuing rigorous and high-quality research.\nResponse to Reviewer #1: ... [Full text of author response to Round 2 omitted for brevity]"}]

Decision letter