Vertical chiral emission from an intrinsically achiral metasurface enabled with anisotropic continuum

Abstract

Manipulating emission polarization is crucial for advanced applications such as polarized fluorescence and lasing, thermal radiation control, stereochemistry and spintronics. Existing approaches for generating circularly polarized emission primarily rely on nanostructures with true chirality, where the far-field polarization at the Γ point is believed to require out-of-plane symmetry breaking. This raises a question: can an intrinsically achiral metasurface that preserves out-of-plane symmetry generate chiral emission? Here, we introduce the concept of anisotropic continuum, defined as a continuum state exhibiting distinct responses for orthogonal polarizations of incidence, as a new degree of freedom for intrinsically achiral metasurfaces. In such metasurfaces, chiral emission arises from manipulating the anisotropic continuum together with the metasurface’s in-plane perturbations. Degrees of circular polarization of 0.83 for upward emission and −0.9 for downward emission are achieved experimentally by integrating the fabricated silicon metasurface with fluorescent organic dyes. Our findings not only address a long-standing gap in the understanding of structural chirality, but also open new opportunities for high-performance applications in polarized fluorescence and thermal emission.

Chiral light emission usually requires twisted or asymmetric nanostructures. Here, the authors show that even out-of plane symmetric metasurfaces can emit circularly polarized light, achieved by tuning an anisotropic continuum rather than breaking symmetry.

Introduction

Electromagnetic chirality refers to the phenomenon in which an object interacts differently with right and left circularly polarized (RCP/LCP) light^1–4, a property commonly observed in natural materials with three-dimensional asymmetric structures. This chiral behavior gives rise to a distinct chiroptical response, which can be characterized by circular dichroism (CD) or optical rotation⁵. While natural materials have been used to generate chiral emission, their low chirality and limited tunability constrain practical applications^6,7. Metasurfaces, however, offer a powerful platform to enhance and control chiral emission, enabling new opportunities in polarized emission^8–13, chirality tuning^14,15, chiral sensing^16,17, and photodetection^18,19.

Recent advances in chiral metasurfaces and their ability to generate chiral emission have been driven by the incorporation of nonlocal resonances, especially bound states in the continuum (BIC)^20–29 and guided modes³⁰. In such systems, light remains confined within the metasurface while coexisting with a continuum of electromagnetic waves. Typically, intrinsic chirality in metasurfaces, characterized by chiroptical responses at the Γ point, is achieved in geometrically chiral structures through symmetry breaking across all mirror planes^6,31–34. To break the out-of-plane mirror symmetry (σ_z symmetry) and achieve chiral quasi-BIC, various methods have been employed, including slant-etching^9,24, grayscale lithography^10,25 and bi-layer design³⁵. In these structures, chiral emission arises when the quasi-BIC aligns with a high-transmission background. With mirror symmetry breaking in all planes, chiral emission occurs when near-unity CD is observed (indicated in the left panel of Fig. 1a)^24,28. Here, chiral emission refers to a radiative process in which the far-field degree of circular polarization (DCP = (I_R – I_L)/(I_R + I_L)) approaches unity. Planar chiral metasurfaces represent another route, where near-unity CD^29,36,37 at normal incidence has also been reported. However, these designs primarily focus on transmission properties, and their potential for controlling emission remains largely unexplored (see Supplementary Section 1). Moreover, recent studies have revealed that a large CD in transmission does not necessarily correspond to chiral emission²⁴, highlighting a gap in understanding the emission mechanism of planar chiral systems. In our earlier work, chiral emission was observed in the presence of a dielectric substrate that perturbed σ_z symmetry¹². Conventionally, σ_z-symmetric metasurfaces are believed to support only extrinsic chiral emission away from the Γ point, typically enabled by breaking in-plane inversion symmetry^38–41. The far-field polarization at the Γ point, by contrast, is generally considered nearly linear due to the absence of geometric chirality^21,24,28,42. An interesting question arises: can a metasurface with σ_z symmetry, which is intrinsically achiral, generate circular or even arbitrary polarization at the Γ point? Our answer is yes.

Fig. 1. Illustration of chiral emission in geometrically non-chiral metasurfaces.

a Schematic illustration of chiral emission from metasurfaces without (left) and with (right) σ_z symmetry. The unit cell consisting of a silicon dimer is shown in the top-right corner. b The conceptual relationship between transmission spectra in σ_z-symmetric metasurface (top), eigenstate emission phase difference φ = arg(E_y/E_x) (middle) and emission intensity of RCP and LCP (bottom). T_x and T_y denote the transmission spectra under x- and y-polarized incidence, respectively. I_L and I_R represent the LCP and RCP components of the emission. c Transmission and reflection properties for σ_z-symmetric metasurfaces under RCP/LCP incidence. d Steps of structure transformations for the generation of chiral emission. The condition of φ = π/2 and η = 1 yields circularly polarized emission at the Γ point. e The Poincare sphere and corresponding polarization map for continuum-assisted eigenstates.

Here, we demonstrate theoretically and experimentally that breaking σ_z symmetry is not required to generate chiral emission at the Γ point, using a vertically etched silicon metasurface embedded in a uniform environment (right panel of Fig. 1a). An additional degree of freedom is introduced by leveraging anisotropic continuum, characterized by distinct responses for orthogonal polarizations in the continuum, e.g., x-polarized and y-polarized light. The term continuum refers to a range of states that are closely connected and form a smooth, uninterrupted spectrum⁴³. In metasurfaces, the continuum is characterized by a slowly-varying reflection/transmission spectrum, induced by Fabry-Perot resonances⁴⁴, Mie resonances⁴⁵, high-contrast grating effects⁴⁶, and other mechanisms. We show that when a bound mode is embedded within an anisotropic continuum, in-plane perturbations can induce radiative coupling. This leads to chiral emission within a specific continuum, where the radiated polarization exhibits opposite chirality in the upward and downward directions. While ultrahigh-Q modes in metasurfaces, such as quasi-BIC^47,48 and leaky guided modes^30,49,50, have been extensively studied and widely applied in photonics, the specific influence of the continuum on emission polarization remains largely unexplored. Related work has examined its effect on linearly polarized emission⁵¹, whereas its contribution to chiral emission remains an open and critical question. Understanding the role of these states could open new avenues for designing metasurfaces with customized polarization properties.

Results

To show the importance of anisotropic continuum in realizing chiral emission in σ_z-symmetric metasurfaces, we decompose the far-field polarization of a resonant mode in the σ_z-symmetric metasurface into Cartesian components |E_x|e^iθx and |E_y|e^iθy. It is determined by two critical parameters: (i) the phase difference between the x- and y-components E_x and E_y, expressed as φ = θ_y− θ_x = arg(E_y/E_x) and (ii) the magnitude ratio of E_x and E_y, denoted as η = |E_y|/|E_x|. We start with a structure that exhibits no polarization conversion in either transmission or reflection between the x- and y-polarized channels to simplify the analysis. For a discrete localized state, the phase difference φ can be expressed in terms of background reflection and transmission coefficients (r_x, r_y, t_x, t_y) as follows (Supplementary Section 2)^44,52

$$\phi =\frac{\arg \left(r_y+{\alpha }_z{t}_y\right)-\arg \left(r_x+{\alpha }_z{t}_x\right)}{2}+n\mathrm{\pi },$$ 1

where n is an integer and α_z = ±1 is a parity number. This equation establishes a connection between the eigenstate far-field polarization and the scattering coefficients of the continuum.

The relation between the eigenstate phase difference φ and the continuum transmission spectra in a σ_z-symmetric structure is conceptually demonstrated in Fig. 1b. For metasurfaces with a transmissive background or an isotropic continuum (left panels of Fig. 1b), φ is always zero and the vertically radiated polarization remains linearly polarized. For instance, metasurfaces with the unit cell composed of cylinders or circular apertures are unable to generate an arbitrary phase difference φ as they inherently satisfy arg(r_y + α_zt_y) = arg(r_x + α_zt_x). In contrast, a nonzero phase difference between the x- and y-components of the radiated electric field can be generated with the assistance of the anisotropic continuum (right panels of Fig. 1b). To achieve chiral emission, φ = π/2 should be satisfied first. This requires that the metasurface should generally be geometrically anisotropic in a certain direction.

Due to the presence of anisotropic continuum, the transmission and reflection spectra of σ_z-symmetric metasurfaces exhibit unique features compared with σ_z-asymmetric ones. As shown in Fig. 1c, under the condition of DCP = 1, RCP incidence excites a strong resonance; however, its transmission remains identical to the non-resonant direct transmission under LCP incidence, due to destructive interference with the background (Supplementary Section 3). Consequently, the transmission CD at the C point ideally vanishes when the resonant and background scatterings are exactly out of phase.

We consider a σ_z-symmetric dielectric metasurface, schematically shown in Fig. 1d. The unit cell consists of a silicon dimer embedded in a silica environment, preserving σ_z symmetry regardless of in-plane variations. This adopted structure follows our earlier work to isolate and emphasize the role of out-of-plane mirror symmetry¹². The phase difference φ of the structure is governed by anisotropic continuum modulation, while the ratio η = |E_y|/|E_x| is influenced by in-plane perturbations. Initially, with α = 0 and δt = 0, no symmetry breaking occurs, and the mode remains embedded in an anisotropic continuum with φ = π/2. Introducing a nonzero δt renders the mode radiative, allowing coupling to x-polarized waves. Further introducing nonzero α induces simultaneous non-zero |E_x| and |E_y|, producing circular polarization. In this step, breaking the in-plane mirror symmetry to make the structure “2D-chiral” is essential for generating both |E_x| and |E_y|, as the in-plane mirror symmetry guarantees linear polarization. Figure 1e provides a clearer illustration of the role of the anisotropic continuum. The polarization of the far-field radiation can be fully described by a set of Stokes parameters: S₁ = (|E_x|²−|E_y|²)/(|E_x|²+|E_y|²), S₂ = 2|E_x||E_y|cosφ/(|E_x|²+|E_y|²), S₃ = 2|E_x||E_y|sinφ/(|E_x|²+|E_y|²). Once φ is fixed, the Stokes parameters trace an arc on the Poincare sphere connecting two opposite points on the S₁ axis as |E_x| and |E_y| vary. In this case, the phase difference φ is the angle between the arc and the equator, while |E_y|/|E_x| controls the distance between the observed point and the pole along the S₁ axis. Without the contribution of anisotropic continuum, radiative modes induced by in-plane symmetry breaking trace a line along the equator of the Poincare sphere (gray line in Fig. 1e). When anisotropic continuum is introduced, this line is lifted toward either the northern or southern hemisphere. Under specific structural parameters, the trajectory intersects the pole, marked by a red star. This evolution is further depicted in the right panel of Fig. 1e, highlighting the changes in the radiated polarization as the structural parameters vary.

To demonstrate control over the phase difference φ, we select an unperturbed metasurface with α = 0 and δt = 0, and then vary the pillar length L. This variation induces anisotropy of the background response, which results in nontrivial values of φ according to Eq. (1). Note also that the phase difference φ depends on the relative positions between the bound mode and the broad transmission dip of T_y. The variation of φ with L is shown in Fig. 2a, where φ decreases with increasing L and reaches φ = π/2 when L = 190 nm. The corresponding transmission spectra T_x and T_y for L = 190 nm are shown in the inset of Fig. 2a, with the position of the bound mode marked as a red star. Under this condition, further modulation of the asymmetric parameters α and δt transforms the bound mode into a bright mode that radiates into the far-field. The far-field polarization distribution in k-space is shown on the right side of Fig. 2b, revealing a C point at the Γ point.

Fig. 2. Operating principle and characterization of the proposed silicon metasurface.

a The variation of the phase difference φ as a function of the pillar length L, with the point where φ = π/2 marked by a red star. The z-component of the electric field (E_z) distribution for the observed guided mode is shown in the inset. The other size parameters are: p = 350 nm, w = 90 nm, h = 210 nm. b The band folding process and polarization map in the vicinity of the Γ point. c Overall SEM image of the fabricated silicon metasurface, with a schematic illustration in the x-z plane shown in the lower left. Scale bar: 200 nm. d Simulated (top) and experimental (bottom) transmission spectra of the silicon metasurface for TE and TM incidence in the x-z plane.

A silicon metasurface with α = 0.09 and δt = 11 nm is fabricated on a silica substrate to experimentally verify our theoretical predictions. The metasurface is subsequently covered with a 50 μm layer of PMMA to achieve refractive index matching with the silica substrate²⁴ (so that the silicon metasurface is symmetric along the vertical direction) and to minimize the impact of Fabry-Perot resonance on the spectrum. Figure 2c presents the overall scanning electron microscope (SEM) images of the silicon metasurface together with a schematic illustration in the x-z plane, revealing vertically etched structures that preserve the out-of-plane mirror symmetry. Angle-resolved transmission spectra for transverse electric (TE) and transverse magnetic (TM) polarized incident light, denoted by T_TM and T_TE, respectively, are confirmed by simulations and experiments in Fig. 2d. Under TM-polarized incidence, only the TM₁ mode is excited, whereas TE-polarized incidence excites TM₁, TE₁, and TM₂ modes (note that TE/TM incidence refers to the incident polarization, while TE/TM modes refer to the eigenmodes). The TM₁ mode couples to TE and TM components, residing within the broad transmission dip observed under TE incidence. The experimental results closely align with the simulations, confirming the coupling of the TM₁ mode to both the x- and y-polarization components and its modulation by the continuum. While the primary discussion in the main text focuses on the TM mode, the TE mode also holds potential for chiral emission, as elaborated in the Supplementary Section 4.

To investigate the evolution of radiated polarization, we fabricated a series of metasurfaces with varying configurations: zero α and δt, nonzero δt only, and both nonzero α and δt. Transmission spectra of these metasurfaces were simulated (solid lines in Fig. 3), theoretically derived with TCMT (indicated by “×” in Fig. 3), and experimentally measured (indicated by “Δ” in Fig. 3) to provide insight into the continuum and bound modes. For α = 0 and δt = 0, the transmission spectrum T_y exhibits a broad dip between 700 nm and 900 nm, while T_x maintains high transmission (Fig. 3a). In this configuration, no bound mode is excited due to symmetry protection. The phase difference φ is calculated through Eq. (1) with α_z = −1, and is depicted as a dotted gray line in the upper panel of Fig. 3a. Although a nonzero φ is generated across the wavelength range, we have |E_x| = |E_y| = 0. For α = 0 and δt = 11 nm, the bound TM₁ mode becomes leaky, experimentally resulting in a sharp transmission dip at λ = 778 nm for T_x, while the line shape of T_y remains nearly unchanged (Fig. 3b). This leakage, independent of C₂ symmetry breaking, indicates that the mode is a quasi-guided mode arising from band-doubling rather than a symmetry-protected BIC. For α = 0.09 and δt = 11 nm, the transmission spectra exhibit resonant features under both x-polarized and y-polarized light illumination. At λ = 775 nm, a transmission dip appears for T_x while a transmission peak emerges for T_y, located near the broad dip of T_y (Fig. 3c). The simultaneous presence of the dip and peak signifies coupling to both x- and y-polarizations. The TCMT-fitted spectra show excellent agreement with numerical simulations, validating that the model accurately describes the key features of vertical chiral emission.

Fig. 3. Evolution of the transmission spectra for the perturbed metasurfaces in Fig. 1d.

a α = 0 and δt = 0. b α = 0 and δt = 11 nm. c α = 0.09 and δt = 11 nm. For comparison, the transmission spectra for x- and y-polarized incidence (T_x and T_y) are obtained by three approaches, including fitting based on the TCMT, numerical simulation and experimental measurement. The upper panels show simulated (solid) and fitted curves (marked by “×”), and the lower panels show experimentally measured curves.

In terms of the resonant mode field distribution, the coupling behavior can be directly understood from the overlap integral between the incident field and the resonant mode: symmetry enforces zero overlap, δt alone allows coupling only to x-polarization, and the combination of δt and α yields finite overlap for both x- and y-polarizations with a phase difference close to π/2. Full derivations and supporting figures are provided in Supplementary Section 5.

The chiral emission capabilities of metasurfaces play a significant role in modulating fluorescence, laser emission and thermal radiation. Here, we further examine the fluorescence properties enabled by the σ_z-symmetric dielectric metasurface, with the absorption characteristics shown in Supplementary Section 6. As shown in Fig. 4a, in the case where L = 190 nm, the long and short axes of the radiated polarization consistently align with the x or y axis, with only the ellipticity changing as α and δt vary. Thus, the phase difference φ remains nearly π/2 for small values of α and δt. This variation demonstrates that the phase difference φ induced by continuum scattering coefficients is robust against in-plane perturbations. Since δt and α control |E_x| and |E_y| independently, there are multiple instances where |E_y|/|E_x| = 1, resulting in C points (Fig. 4b). Larger values of α and δt introduce greater symmetry breaking, which leads to lower Q-factors. In the experiment, a thin layer of IR140 organic dye is first spin-coated onto the fabricated metasurface, followed by a 50 μm PMMA coating. The sample is then excited with a 532 nm laser, and its photoluminescence (PL) characteristics are analyzed (Supplementary Section 7). The PL from the dye is strongly modulated by the TM₁ mode supported by the metasurface, with its polarization determined by the eigenstate. Figure 4c displays the polarization-resolved upward PL spectra for three cases with different α values at a fixed δt of 11 nm. For α = 0, the RCP and LCP components of the PL spectrum are nearly equal, indicating linearly polarized emission. As α increases, the proportion of the LCP component decreases, resulting in an increased DCP.

Fig. 4. Fluorescence modulation with the fabricated metasurfaces.

a Evolution of the radiated polarization with the asymmetric parameters α and δt. b Relation between α and δt corresponding to the formation of C points. c Polarization-resolved upward PL spectra for cases with α = 0, 0.04, and 0.09, with δt fixed at 11 nm. d Circular polarization components of the x-z cross section, showing the opposite spins for upward and downward emissions. e Upward and downward PL spectra for the metasurfaces coated with organic dyes.

Figure 4d demonstrates the x-z cross sections of E_x ± iE_y of the eigenstate in the case of α = 0.09 and δt = 11 nm. The upward and downward leaky waves are RCP (red circular line) and LCP (blue circular line), both represented by E_x − iE_y due to the change in direction²¹. The reversal of radiated circular polarization is expected, given that the structure exhibits opposite chirality when viewed from above and below. Experimentally, a prominent RCP peak is detected in the upward emission, with negligible LCP contribution, resulting in a DCP of 0.83 (upper half of Fig. 4e). Conversely, for downward emission, the LCP component dominates, yielding a DCP of −0.9 (lower half of Fig. 4e). This reversal in circular polarization confirms the absence of net chiral flux in the far field, consistent with the preserved σ_z symmetry. The small difference in absolute DCP between the upward and downward emissions may be attributed to the coated thin dye layer, which degrades the out-of-plane symmetry to some extent.

Discussion

In conclusion, we demonstrated vertical chiral emission in metasurfaces that preserve σ_z symmetry and presented a versatile approach for generating arbitrary radiated polarization in such systems, exemplified by a silicon metasurface design. Circularly polarized emission at the Γ point is not constrained by σ_z symmetry breaking; instead, the polarization state can evolve from linear to elliptical and eventually to circular on the σ_z-symmetric metasurface by introducing the anisotropic continuum as a new degree of freedom. This is experimentally validated by integrating the metasurface with fluorescent organic dyes, achieving PL with a maximum DCP of 0.83 for upward emission and −0.9 for downward emission. The metasurface geometry follows the same in-plane design as our earlier work, which involved an unbalanced refractive index between the substrate and superstrate. In contrast, the present study maintains σ_z symmetry, isolating the role of the anisotropic continuum in generating vertical chiral emission. This distinction clarifies the fundamental difference between substrate-induced and symmetry-preserved chirality. Furthermore, as a general theoretical framework, the present theory is applicable to metasurfaces with arbitrary refractive indices (Supplementary Section 8) and metasurfaces with a substrate, provided that the substrate refractive index is close to that of the superstrate medium (Supplementary Section 9), thereby enhancing its suitability for practical applications. Our findings establish a novel methodology for designing metasurfaces that achieve vertical emission of arbitrary polarization while maintaining σ_z symmetry, paving the way for promising applications in fields such as thermal radiation manipulation, chiral sensing, and polarized photodetection.

Methods

Numerical simulation

All simulations are performed using COMSOL Multiphysics 5.6 “Electromagnetic Waves Frequency Domain” module. The refractive indices of silicon and surrounding materials are taken as n_Si = 3.9 (Supplementary Section 10) and n_PMMA = n_SiO2 = 1.45, respectively. Periodic conditions are applied in the x and y directions of the simulated unit cell, and perfectly matched layers are employed along the z direction. The far-field eigen-polarization is calculated through COMSOL Multiphysics 5.6 with MATLAB. In the calculation of continuum transmission, the incidence port is set as x/y polarization by setting the electric mode field to (1, 0, 0) or (0, 1, 0). Equation (1) is calculated through the polarization-resolved scattering coefficients obtained from ports.

Sample fabrication

A layer of 210 nm silicon is deposited on a 4-inch silica substrate through plasma-enhanced chemical vapor deposition and subsequently cleaved into small 1 cm × 1 cm slices. Each sample is cleaned sequentially in acetone, isopropyl alcohol, and deionized water, and then spin-coated with 270 nm electron-beam resist (PMMA 679.04). The pattern is defined by E-beam exposure (Raith150 EBL) and developed in methyl isobutyl ketone and isopropyl alcohol. A 30 nm Al film is deposited using electron beam evaporation (Denton) as the hard mask onto the patterned resist and is lifted off by ultrasonic cleaning in acetone. The pattern is then transferred to the silicon by dry etching (Multiplex ICP STS) using C₄F₈ and SF₆, and the residual Al is finally removed in the Al etching solution. The fabrication process of the sample is summarized in Supplementary Section 10.

Covering of IR140 and PMMA

The fabricated sample is cleaned first with acetone and isopropyl alcohol, and then baked on a hot plate at 120 °C for 10 min. The clean sample is spin-coated with a drop of 20 μL IR140 dissolved in ethanol in the following steps: starting at 500 rpm for 3 s, followed by 2000 rpm for 30 s, and lastly at 0 rpm for 3 s. The coated sample is left for stand for 5 min to evaporate the ethanol and improve the film uniformity. To form a thick PMMA layer on the metasurface, the PMMA solution is concentrated through solvent evaporation. Then, the PMMA is spin-coated on the metasurface in the following steps: starting at 500 rpm for 3 s, followed by 4000 rpm for 60 s, and lastly at 0 rpm for 3 s. A layer of 50 μm PMMA is formed by repeating this procedure five times.

Optical characterization

In the transmission measurement, the sample is fixed on a rotating sample stage (Supplementary Section 6). A white light source is turned on and the light passes through a polarizer and an aperture before being focused onto the sample by an objective lens of a low numerical aperture, in which case the incident wave can be treated as an approximate plane wave. The incident polarization is switched between x and y by rotating the polarizer. The transmission spectra for different incident angles are collected by rotating the sample stage. In the PL measurement, a 532 nm continuous-wave laser is used to stimulate IR140 organic dyes. The PL is collected by a 20× objective lens, and analyzed by a zero-order quarter-wave plate, a half-wave plate and a linear polarizer. The fast axis of the quarter-wave plate is fixed to 45° with respect to the polarizer. When the fast axis of the half-wave plate is rotated to 0°/45°, RCP/LCP light is selected.

Author contributions

Y.S., Y.J., S.H., and C.-W.Q. conceived the main ideas. Y.S., M.L., and J.C. developed the theoretical framework, with input from D.G. under the supervision of L.W. Y.S. performed the numerical simulations and designed the experiments. Z.H. fabricated the samples. Y.S. and Z.H. carried out the optical characterization measurements. Y.S., M.L., Z.H., and K.S. analyzed the data, and all authors discussed the results. Y.S. wrote the manuscript with comments and revisions from all authors. Y.J., S.H., and C.-W.Q. supervised the overall project.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data needed to evaluate the conclusions in this paper are present in the paper or the Supplementary Information. Additional data related to this paper may be requested from the corresponding authors upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-68728-2.