Advances on broadband and resonant chiral metasurfaces

Abstract

Chirality describes mirror symmetry breaking in geometric structures or certain physical quantities. The interaction between chiral structure and chiral light provides a rich collection of means for studying the chirality of substances. Recently, optical chiral metasurfaces have emerged as planar or quasi-planar photonic devices composed of subwavelength chiral unit cells, offering distinct appealing optical responses to circularly polarized light with opposite handedness. The chiroptical effects in optical metasurfaces can be manifested in the absorption, scattering, and even emission spectra under the circular polarization bases. A broadband chiroptical effect is highly desired for many passive chiral applications such as pure circular polarizers, chiral imaging, and chiral holography, in which cases the resonances should be avoided. On the other hand, resonant chiroptical responses are particularly needed in many situations requiring strong chiral field enhancement such as chiral sensing and chiral emission. This article reviews the latest research on both broadband and resonant chiral metasurfaces. First, we discuss the basic principle of different types of chiroptical effects including 3D/2D optical chirality and intrinsic/extrinsic optical chirality. Then we review typical means for broadband chiral metasurfaces, and related chiral photonic devices including broadband circular polarizers, chiral imaging and chiral holography. Then, we discuss the interaction between chiral light and matter enhanced by resonant chiral metasurfaces, especially for the chiral bound states in the continuum metasurfaces with ultra-high quality factors, which are particularly important for chiral molecule sensing, and chiral light sources. In the final section, the review concludes with an outlook on future directions in chiral photonics.

Introduction

Polarization is a fundamental property of light. Traditionally, people usually resort to polarization optical elements composed of birefringent crystals¹ or polarization-sensitive gratings to manipulate the phase and amplitude responses under an orthogonal linear polarization basis. These traditional polarization control components often have a huge thickness on the many wavelengths scale and polarization controllability is extremely inflexible, that is, usually limited to linear polarizations. The overall optical system based on such conventional polarization optical elements is therefore typically cumbersome and of fewer degrees of freedom.

The emerging metasurface with the capability of controlling light’s phase^2,3, polarization⁴, amplitude⁵, arbitrary spin-wavefront manipulation⁶, and resonant properties provides a versatile platform for constructing compact and flexible polarization optical elements. By manipulating arbitrary phase, amplitude responses based on arbitrary polarization basis beyond the linear polarization basis^7,8, it enables the creation of optical devices for polarization projection^9,10,11, polarization beam splitting^12,13, polarization measurements^{14,15,16,17,18,19,20,21}, polarization imaging^22,23,24,25, and various other functionalities.

For the full manipulation of polarization states, especially for the circular polarization generation and filtering, chiral photonic structures are typically required to produce the chiroptical effect that manifests the intensity or phase response differences between the right-handed circular polarization (RCP) and the left-handed circular polarization (LCP), which are referred to as the circular dichroism (CD) and optical activity (OA) effect, respectively. In general, we define an object as chiral when it cannot be coincident with its own mirror image through translation and rotation operations²⁶. Chirality is a very important property of an object not just in physics but also in chemistry and life science, therefore “Science” magazine listed “Why Life Needs Chirality” as one of the newly released “125 Most Cutting-Edge Scientific Issues in the World”. When the concept of chirality extends to optics, the emerging chiroptical phenomenon usually refer to the different intensity or phase responses under different circular polarized light excitations, because the electric field vector trajectory of circular polarized light along the propagation path naturally forms a chiral helix that cannot be superimposed by its anti-handedness counterpart. chiroptical effects are commonly used to study the chirality of matter, which is mainly based on the chiral interaction between light and matter.

By employing chiral nanostructures as the unit-cell elements of the metasurface, one can construct a kind of chiral metasurface that could be designed to manifest the chiroptical effect in terms of the absorption, transmission, and reflection spectra for LCP/RCP light excitations^27,28.

Intuitively, Strong chiroptical response requires photonic structures with chiral geometries. There are typically two types of chiral photonic structures, namely, the 2D chiral structure and 3D chiral structures, categorized by their breaking degrees of mirror symmetry. The 3D chiral structure has no mirror symmetries in the entire 3D space and therefore is a chiral object in the strict sense. On the other hand, the 2D chiral structure has no mirror symmetry on the in-plane direction of the metasurface, nor is there symmetry along the perpendicular direction to the metasurface plane. The mirror symmetry in the out-of-plane direction makes the 2D chiral structure not strictly a chiral object. However, the chiroptical effect still may happen in such 2D chiral structures if we investigate the intensity/phase differences between LCP and RCP incident light on the circular conversion components (that is LCP - > RCP, and RCP- > LCP), which is in contrast to the chiroptical effect in the strict 3D chiral structure manifested on the circular preserving components (that is LCP- > LCP, and RCP- > RCP). It is because, for the 2D chiral structure, preserved mirror symmetry in the propagation direction theoretically restricts the propagation behavior of one circular polarized component along the positive direction should be exactly the same as that of the opposite circular component along the reversed negative direction. According to the time-reversal symmetry, the circular preserving components should always be the same, while the circular conversion components can be different with unlimited contrast. Based on the above analysis, we define the 3D chiroptical responses as the chiroptical responses that are manifested on the circular preserving components, which is the common case for 3D chiral structures^{29,30,31,32,33}. We define the 2D chiroptical responses as chiroptical responses that are manifested on the circular conversion components, which is the common case for 2D chiral structures^34,35,36,37.

According to the design strategy, 2D/3D chiroptical response can be further divided into intrinsic chirality and extrinsic chirality responses (Fig. 1). The intrinsic and extrinsic aspects of chirality are mainly defined to distinguish whether the illumination light is normal incident or not (Fig. 1). Because the chiroptical effect is completely determined by the geometric chirality of the photonic structure, the chirality of the overall optical configuration including both the structure and the incident angle of the illumination light should be considered. If the incident angle is 0° (normal incidence), the chiral property of the overall optical configuration is in line with the intrinsic geometry chirality of the structure itself, therefore, we call this type of optical chirality as intrinsic chirality. Depending on whether the chiroptical responses are defined on the circular conversion or circular preserving components under the normal incidence precondition, we categorize them as intrinsic optical 2D chirality and intrinsic optical 3D chirality. On the other hand, if the illumination light is oblique incident on the metasurface, the optical chirality cannot be defined by the geometric chirality of the structure solely³⁸. Because the oblique incidence automatically introduces mirror symmetry breaking to the overall optical configuration no matter whether the structure has mirror symmetry or not. As this chirality is introduced externally by the optical excitation configuration, rather than the intrinsic structure itself, we call it extrinsic optical chirality. In this way, we can typically see some achiral structures may have strong extrinsic optical chirality, because the chiroptical effect is induced by the overall chiral optical configuration, not just by the structure chirality.

Fig. 1

Summary of the chiral metasurfaces in broadband, weak resonance, and BIC resonance regime. In all those bandwidth regimes, different types of optical chiral responses including 3D intrinsic chirality, 3D extrinsic chirality, 2D intrinsic chirality, and 2D extrinsic chirality are thoroughly discussed^{29,39,54,67,78,79,88,104,115,155}.

As for specific applications that require a broad working bandwidth of metasurfaces, people design broadband chiral metasurfaces which primarily served as highly efficient circular polarizers or as elements in polarization-controlled devices. Recently, to enhance the chiral light and matter interaction, chiral resonant metasurfaces have been proposed, which are highly demanded for many active chiral applications.

Here we review recent research on chiral metasurfaces with remarkable chiral light-matter interactions. We first introduce the fundamentals of chiral optical responses, followed by a discussion on broadband chiral metasurfaces and their typical applications. Then, we review the resonant chiral metasurfaces with moderate Q-factors and chiral bound states in the continuum (BIC) metasurfaces with ultra-high Q-factors. Finally, we provide a summary with an outlook on future directions in chiral photonics.

Fundamentals of chiral optical responses

Chiroptical effects typically include optical activity (OA)/optical rotation (OR) and circular dichroism (CD). OA/OR quantifies the phase delay difference that is imposed on circularly polarized light of different handedness³⁹, which typically originates from the circular birefringence phenomenon. Circular birefringence is also known as optical rotatory dispersion, refers to the phenomenon where a chiral substance has different refractive indices for LCP and RCP light, making them propagate at different speeds through the medium, thereby rotating the polarization plane.

On the other hand, CD describes the amplitude differences that are imposed on LCP and RCP components, which commonly happens when light passes through chiral absorbing materials. CD takes up the majority of the chiroptical phenomenon and is the most common indicator to characterize the chirality of materials⁴⁰. Therefore, in the current review article, we mainly discuss the CD-type chiroptical effects. The quantity of CD can be defined either in an absolute way or a normalized way. The absolute definition of CD is a straightforward difference between the absolute value of the responses imposed on LCP and RCP components. It directly reflects the disparity in absorption or scattering. For chiral metasurface absorbers^41,42,43, CD typically defined as:

$${{CD}}_{A}={A}_{{LCP}}-{A}_{{RCP}}.$$

(1)

Here, ${A}_{{LCP}}$ and ${A}_{{RCP}}$ represent the absorption rates of the chiral substance to LCP and RCP light, respectively. The concept of CD was extended to the scattering properties of light in terms of its transmission and reflection spectra, the corresponding CD is defined

$${{CD}}_{T}={T}_{{LCP}}-{T}_{{RCP}},$$

(2)

$${{{CD}}_{R}=R}_{{LCP}}-{R}_{{RCP}},$$

(3)

where T_LCP T_RCP, R_LCP and R_RCP represent the transmittance and reflectance of RCP and LCP components, respectively.

The normalized definition of CD normalizes the difference by dividing it by the sum of the optical responses of LCP and RCP⁴⁴. This definition accounts for the overall optical response strength, providing a ratio that reflects the degree of CD relative to the total optical response. This normalization can make comparisons between samples with vastly different intensities more meaningful, as it considers both the dichroism and the overall molecular absorption.

$${{CD}}_{k}=\frac{{K}_{{LCP}}-{K}_{{RCP}}}{{K}_{{LCP}}+{K}_{{RCP}}},$$

(4)

where K could be A, T, or R, representing the absorption, transmission, or reflection, respectively. The normalized definition of CD is widely applied in chiral metasurfaces, and it is important to note which definition is used when comparing similar work. As the normalized CD spectra are a typical signature to differentiate different stereoisomers of chiral molecules, it is widely used to identify and analyze chiral compounds⁴⁵.

When the size of a chiral structure is comparable to that of circularly polarized light, the structure can exhibit strong chirality. In contrast, the size of typical organic molecules is much smaller than the wavelength of light, resulting in a weak chiroptical response. Metasurface structures can create localized chiral optical fields with an effective wavelength much smaller than that in free space, thus the interaction between chiral molecules and the localized optical fields at the metasurface remains strong. Hendry et al. exploited the hyperchiral electromagnetic field generated by the optical excitation of plasmonic planar chiral metamaterials to demonstrate unprecedented sensitivity to chiral supramolecular structures. This work reports that the effective refractive index difference between chiral samples exposed to left- and right-handed suprachiral fields up to ${10}^{6}$ times greater than that observed in conventional optical polarization measurements⁴⁶. This significant enhancement allowed for the characterization of picogram quantities of adsorbed molecules. To characterize the magnitude of optical field chirality, the concept of “optical chirality density” is used, which is defined as^47,48,

$$C=-\frac{\omega }{2{c}^{2}}{\rm{Im}}(\vec{{E}^{* }}{{\cdot }}\vec{H}),$$

(5)

where E and H represent the electric and magnetic fields, respectively; * denotes complex conjugation; $\omega$ is the angular frequency; c is the speed of light in vacuum. For linearly polarized light, C is zero; whereas for RCP and LCP incident light, C takes non-zero values with opposite signs. Near the chiral metasurface, it is often possible to achieve large absolute values of C. By meticulously adjusting the structural parameters of the metasurface, Dionne et al. achieved a local enhancement of optical chirality density up to 138 folds⁴⁹. From the optical chirality density spectrum, the trend of chirality density varying with wavelength in specific spatial regions can be observed. It enables the rapid identification of light frequencies with strong chiroptical responses and the determination of the polarization handedness at those locations, which is crucial for enhancing the performance and efficiency of devices. Specifically, the co-occurrence of electric dipole modes and magnetic dipole modes at specific frequencies and spatial regions can be promoted. This synergy, particularly near the first Kerker condition, leads to maximized field strengths while maintaining a π/2 phase lag between the electric and magnetic fields of circularly polarized light. Such conditions are paramount for inducing strong chiral interactions, crucial for applications in chiral sensing²⁸, separation⁴⁹, and beyond.

Figure 2 shows typical 3D chiral nanostructures which have no mirror symmetry in the entire 3D space. Compared to 2D chiral metasurfaces⁵⁰, 3D ones²⁹ generally provide more design freedoms, but their fabrication is more challenging. The 3D helix structure with single or multiple intertwined segments (shown in Fig. 2a) is one of the most classic chiral structures^51,52. The chiroptical responses of the 3D helical structures can be modulated by adjusting their dimensions, pitch, and material parameters. Due to the fabrication precision limitation, helical structures have a relatively large unit cell size, therefore they are widely used for chiral modulation in the mid-infrared band or longer wavelengths. Plasmonic spiral gyrators (PSG) ⁵³ are considered as an excellent candidate for achieving chirality at visible wavelengths, as their composed triply-periodic metallic nanostructures have fine chiral structural features. Oh et al. analyzed the tri-helical metamaterial (Fig. 2b) model to elucidate the chiral behavior of nanoplasmonic gyroid metamaterials⁵⁴. They theoretically expounded upon the chiral properties of metallic gyroid structures and quantified their chirality at visible frequencies.

Fig. 2: Typical 3D chiral nanostructures.

a SEM images of nanohelices with different geometric sizes⁵¹. b Schematic unit cell of gyroid along the [111] directions⁵⁴. c Three-layer unit cell left-handed chiral woodpile structure consisting of layers of nanorods that are stacked in the z-direction⁵⁷. d Molecular organization in cholesteric (chiral nematic) liquid crystal phases⁶². e Schematic of a designed L-shaped curled metasurface⁵⁸. f Window decoration–type nanobarriers, and a deformable spiral¹⁰³. g Schematic images of a two-layer active chiral structure. The purple, blue, and yellow colors represent the gold structures at different layers, and the two silicon pads are shown in green¹⁶⁹. h Left: A unit cell of the 2D chiral metamaterial formed by four interlocked chiral-SRRs. Right: A photo of the fabricated chiral metamaterial⁷⁰. i. Bilayer structure supporting 3D chirality. The structure comprises a structured silicon slab surrounded by silica environment⁷². j. SEM image of 432 helicoid III nanoparticles evolved from an octahedral seed⁷⁴.

In a theoretical study analyzing dielectric contrasts characteristic of semiconductor nanostructures, Lee and Chan discovered that spiral structures exhibit either a complete 3D band gap or a transmission gap for each state of circularly polarized light individually. This phenomenon is contingent upon whether the spirals are interconnected or spatially separated⁵⁵. 3D chiral photonic crystals are a typical structure⁵⁶. As shown in Fig. 2c⁵⁷, they consist of layers of nanorods stacked along the z-direction, forming a 3D chiral photonic crystal structure, also known as a helical woodpile structure. In this type of structure, the polarization band gap can be adjusted through the lateral periodicity within the woodpile structure.

In the realm of materials science, self-organized soft helical superstructures, specifically cholesteric liquid crystals (LCs), stand as exemplary models for exploring insights into the properties influenced by morphology and orientation within supramolecular dynamic helical architectures. (Fig. 2d)⁵⁸. In the terahertz frequency range, the liquid crystal chiral metasurface offers an effective method for dynamically manipulating the spin state conversion and optical chiral transmission^59,60,61. Ji et al. have demonstrated flexible and dynamic control over terahertz spin state conversion and optical chirality by integrating asymmetric metasurfaces with anisotropic liquid crystal layers⁶². The introduction of the liquid crystal layer is crucial as it breaks this mirror symmetry and introduces both spin conversion and spin-preserving chirality, making the device actively controllable.

Stress-induced 3D chiral fractal metasurfaces are a type of structure constructed by applying stress to create complex 3D shapes⁶³. The nano-kirigami method can create complex 3D twisted structures with 3D intrinsic chirality and enhanced optical properties⁶⁴. Kirigami enables multifunctional shape transformations from 2D precursors to 3D architectures, simplifying the fabrication of complex and unconventional structural geometries. Wang et al. utilized FIB-induced bending to create curled nanostructures standing along horizontal rectangular apertures, breaking the mirror symmetry of the structure. This method allows for spin-selective transmission with high efficiency (shown in Fig. 2e). This category also includes metasurfaces formed from stress-induced 3D archimedean spirals, exhibiting enhanced and stabilized broadband chiroptical responses due to their chiral fractal morphology (shown in Fig. 2f)⁶⁵.

Beyond this category, many scholars specifically study photoactive chiral metamaterials. These materials feature the ability to switch their chiral properties dynamically using photoexcitation. Utilizing the variable conductivity of silicon that depends on light intensity, Zhang et al. engineered adjustable optical chirality in the terahertz range by altering the electro-magnetic coupling (Fig. 2g)⁶⁶. The fabrication process relies on intricate layering and the precise positioning of multiple materials, including semiconductors and metals, to achieve chiral metamolecules capable of handedness switching. Initially, silicon pads are patterned on a silicon-on-sapphire wafer using photolithography and reactive ion etching. Subsequently, gold pads are added on top of the Al-coated silicon pads through a process of photolithography, electron beam evaporation, and lift-off. The next steps include coating with SU-8, a photoresist, and creating holes through photolithography and dry etching, which are then filled with gold through electroplating to form pillars. Gold bridges connecting these pillars are fabricated in the final photolithography and lift-off step, after which SU-8 and Al are removed, completing the metamaterial structure. Unlike mechanically tunable planar metamaterials, this approach integrates photosensitive materials that can dynamically modulate the chirality of the metamaterial through external optical stimulation. The fabrication of chiral metasurfaces operating in the terahertz range requires higher precision due to the shorter wavelength, often necessitating the use of advanced lithography techniques, such as ultraviolet lithography or electron beam lithography, to create structures with finer detail. In contrast, the manufacturing process for devices operating in the GHz range are even much easier, which typically relies on the commercial printed circuit technique⁶⁷.

Stereo split-ring structures ^68,69 are a commonly used structure in chiral metamaterials. These stereo split-ring resonators are tiny metallic structures, typically shaped like multiple ring-shaped structures arranged in a 3D configuration. Figure 2h demonstrates coupled split-ring resonators within a cubic lattice unit cell, achieving interaction between electric and magnetic dipole resonance modes⁷⁰. This interaction facilitates strong CD, OA, and a negative refractive index at specific chiral resonance frequencies.

The bilayer metasurface is a classic approach for realizing 3D chiroptical responses^33,71. This is primarily achieved through near-field interactions between layers, structural rotation in space, and the disruption of symmetry to facilitate 3D chirality. Adam et al. have realized this 3D chiroptical response by designing a bilayer photonic crystal slab structure (Fig. 2i)⁷². They designed a periodic bilayer structure consisting of two atoms in each layer and precisely control the length, width and rotation angle of these double atoms in each layer to realize chiroptical responses. This kind of layered metasurfaces leverage the intrinsic properties of each layer and their interlayer interactions to manipulate light in innovative ways. A pivotal aspect of layered metasurfaces is their capacity for modeling both through comprehensive numerical methods and theoretically via transmission line theory⁷³. Within the broader category of layered metasurfaces, Moiré metasurfaces represent a distinct and intriguing subset. They are characterized by the relative arrangement of their constituent layers through either rotation or displacement, leading to the formation of Moiré patterns. These patterns are not merely aesthetic but play a crucial role in the emergence of novel optical phenomena. The interaction between the periodic patterns of the overlaid layers in Moiré metasurfaces can result in enhanced control over light-matter interactions, offering new pathways for the engineering of chiroptical responses.

The above-mentioned are all physics-based methods to obtain chiroptical responses. There is also a more special type of molecularly oriented chiral nanostructures. This category includes the “432 helicoid III” chiral gold nanoparticles described by Lee et al.⁷⁴ (shown in Fig. 2j). These are characterized by their synthesis through molecular interactions, specifically using amino acids or peptides to direct the growth of chiral, helical, or otherwise asymmetric nanostructures. The process results in unique 3D twisted chiral elements that exhibit strong chiral optical properties due to their intricate shapes and interaction with light. By incorporating the molecularly directed chiral nanostructures category, this classification acknowledges the unique and customizable nature of chiral structures that can be achieved through molecular-scale interaction and manipulation, expanding the possibilities for designing and utilizing chirality in nanoscale materials.

Typical 2D chiral structures include Gammadions, Z-shaped⁷⁵, L-shaped⁷⁶, Split rings⁷⁷, U-shaped, S-shaped⁷⁸, and multiple meta-atom structures, in which specific chiroptical responses imposed on circular conversion components can be flexibly designed. Some of these structures exhibit C₂ symmetry, meaning that they obtain the same structure after a 180° rotation. For example, the symmetric fan-shaped (shown in Fig. 3a)⁷⁹ and S-shaped (shown in Fig. 3b)⁸⁰ metasurface structures, maintain their geometric shapes after rotating half a turn around their central axis. C₄ symmetry structures return to their original configuration after a 90° rotation. This would include designs like Gammadions (Fig. 3c) or windmill structures (Fig. 3d), where the design repeats every quarter turn. Additionally, there is a class of structures that do not exhibit any rotational symmetry but achieve chirality through unique geometric arrangements, such as L-shaped (Fig. 3e.)⁷⁶, split-ring structures⁷⁷, and U-shaped structures. The selection and combination of geometric patterns with specific symmetry can fine-tune the optical response of the surface. Beyond the traditional patterns mentioned above, the design of chiral structures can be also designed through combinations of dual-rod or multi-rod configurations. Wang et al. deconstructed the planar chiral Jones matrix into an amalgamation of two birefringent waveplates (Fig. 3f)⁴. They achieved this by varying the dimensions and rotational angles of two distinct rectangular silicon pillars within each unit cell, thereby crafting structures that align with the individual Jones matrices of the respective birefringent waveplates. This approach allows for a nuanced modulation of chiral optical properties. The choice and design of these geometric patterns are crucial for achieving the desired circularly polarized light response, optical rotation, and other chiral optical effects.

Fig. 3: Typical 2D chiral nanostructures.

C₂ Symmetry Structures: a Symmetric fan-shaped structure metasurface structure. The substrate is ${{\rm{Al}}}_{2}{{\rm{O}}}_{3}$ and the pillar material is c-Si⁷⁹. b Array sample of S-shape chiral nanostructures⁸⁰. C₄ Symmetry Structures: c Scanning electron micrographs of gold gammadia. Panels show a top view of the right-handed arrays⁵⁰. d SEM images of left handed chiral metasurface¹⁰⁵. Non-C₂ or C₄ Symmetry Structures: e SEM images of the fabricated L-shaped gold nanoantennas⁷⁶. f Metasurface periodic array with double rod structure as a unit⁴.

Broadband chiral metasurfaces

Typical structures for broadband chiroptical responses

People always desire broadband chiroptical responses that can effectively manipulate the chiral properties of light over a wide frequency range^33,34,81,82. 3D chiral metasurfaces can achieve chiral operations over a broad frequency range, making them suitable for sophisticated optical applications such as optical communication, imaging, and topological optics. 3D broadband chiral metasurfaces mainly exhibit the characteristics of spiral lines⁸³ and multi-layer stacking^21,84. Planar broadband chiral metasurfaces are typically composed of planar layers of materials, such as metals or dielectrics, to achieve control over electromagnetic waves. Planar broadband chiral metasurfaces can achieve broadband control within a certain frequency range, allowing manipulation of electromagnetic waves at multiple frequency points.

In addition to the above-mentioned metal and dielectric materials to achieve broadband chirality, the use of LCs to achieve broadband chirality is considered an advanced means to achieve optical chirality⁸⁵. LCs are composed of a group of LC molecules that are typically ordered in a specific manner. The ordered arrangement of LC molecules serves as the foundation for the unique optical broadband chiroptical response exhibited by liquid crystals. Through self-assembly techniques, LCs can generate unique 3D broadband chiroptical response^86,87. The broadband chiroptical responses are required in typical passive chiral applications, such as circular polarizers, chiral imaging, and chiral holography, as discussed in detail in the following subsections.

Broadband chiral circular polarizers

Conventional ways to filter circularly polarized light rely on the cascading of waveplate and linear polarizers, with increased complexity and size of the optical system. On the other hand, the unique response of chiral structures to circular polarization can be utilized to achieve the generation or filtering of circularly polarized light in a compact system. Through the design of 3D chiral structures, polarization-preserving circular polarizers can be manufactured. Gansel et al. utilized photonic metamaterials with metallic helical structures to achieve broadband circular polarization (Fig. 4a). Within a certain range, the increased height of the upward-growing metallic helices enhances the structure’s plasmon resonance, which aids in improving the polarization conversion efficiency and achieving a broadband circularly polarized chiroptical response²⁹. Based on this helical growth concept, Zhao and others have developed twisted optical metamaterials as planarized ultra-thin broadband circular polarizers (Fig. 4b)³⁰. These metamaterials consist of elongated dielectric nanoplates. By adjusting the geometric shape of the nanoplates, they achieve more compact and broadband polarization control.

Fig. 4: Broadband chiral circular polarizers.

a Normal-incidence measured and calculated transmittance spectra (no analyzer behind sample) of a metallic spiral metasurface are shown in the left and right columns²⁹. b Transmission of LCP and RCP waves through a stack of rotated metasurfaces by increasing the number of layers³⁰. c Top: Experimental transmission spectra of high extinction ratio structure under LCP and RCP illumination. Bot: Experimental images of the transmission of the pattern under the illumination of LCP and RCP at different wavelengths⁸⁸. d Top: ${{TiO}}_{2}$ metasurface unit structure and its corresponding transmission spectrum. Bot: Experimentally captured optical images of the metasurface nanoprinting illuminated with LCP and RCP light³⁵. e Top: Simulated co-pol and cross-pol reflectance for circularly polarized incidence. Bot: H-shaped unit cell and H-shaped unit cell printed on 1.2 mm thin grounded dielectric substrate⁸⁹.

Compared to 3D chiral structures, planar 2D structures have the advantage of easier fabrication and better stability. Continuous patterns (Fig. 4c)⁸⁸, four-rod structures (Fig. 4d)³⁵, and H-shape structures (Fig. 4e)⁸⁹ are three typical types of broadband 2D chiral unit structures. Wang et al. have designed 2D all-dielectric chiral metasurface polarizers. They realize simultaneous broadband and high CD in the optical communication band (Fig. 4c)⁸⁸. Utilizing the broadband characteristics of metasurfaces, when illuminated by different wavelengths, RCP/LCP light incident on the metasurface will exhibit different transmittance, resulting in changes in the grayscale of the image. In this scenario, the chiral metasurface acts as a broadband circular polarizer. Xu et al. proposed a new class of metasurface polarization devices where two arbitrary and independent amplitude profiles can be imposed on a pair of orthogonal polarization states (linear, circular, or elliptical) by a single metasurface, conceptually shown in Fig. 4d. Incident light of different helicities entering the metasurface will result in different amplitude modulations, thereby producing different patterns³⁵. Shukoor et al. have focused on broadband linear and circular polarizers for applications in radar cross-section (RCS) reduction⁸⁹. Experimental results indicate that these polarizers maintain excellent performance across a wide range of incident angles and frequencies. The previously mentioned work on broadband circular polarizer in metasurfaces is based on the structural asymmetrical transmission of left- and right-handed circularly polarized light on lossless chiral metasurfaces, where one handedness component completely passes through while the other one is reflected. The circular polarizer can also be realized by employing the chiral absorption phenomenon. For example, Yang et al. proposed chiral metasurface absorbers that operate in the near-infrared wavelength range⁴¹. It allows one circular polarization state (e.g., LCP) of incident light to transmit through the metasurface, while the other circular polarization state (RCP) is completely absorbed by the metasurface and converted into heat. This type of broadband metasurface absorbers^41,42,90 not only achieves the fundamental functionality of circular polarizers but also extends to applications in thermal energy harvesting and filters.

Broadband chiral imaging

In chiral metasurface imaging technology, the broadband chiroptical response is preferred to enhance the contrast and resolution of images. With this respect, Capasso et al. proposed a multispectral chiral lens (MCHL), which integrates the functions of polarization and dispersive optical elements into a single ultra-thin device (Fig. 5a)⁹¹. It overcomes the limitations of bulk optical devices and provides chiral and spectral information across the entire visible spectrum without the need for additional optical components. Xu et al. propose and demonstrate that a Fourier transform setup incorporating an all-dielectric metasurface can perform a 2D spatial differentiation operation and thus achieve isotropic edge detection (Fig. 5b)⁹². Groever et al. have proposed a highly efficient chiral metasurface lens. Compared to traditional geometric phase-based designs, it can focus circularly polarized light with an efficiency of up to 70%, demonstrating high polarization contrast and significant imaging performance (Fig. 5c).

Fig. 5: Chiral metasurface imaging technology.

a Top: Imaging with the multispectral chiral lens (MCHL). The MCHL forms three images of the beetle. Mid: Circular dichroism from two different parts of the beetle as a function of wavelength. Bot: Image of the fabricated multispectral chiral lens⁹¹. b Top: RCP and LCP images taken of the resolution test chart with the chiral meta-lens for RCP and LCP illumination at 500 nm. Mid: The first row is a traditional brightfield image captured with LCP incidence. The second row is the case of RCP incidence. From left to right, the illumination wavelengths are 480 nm,530 nm, 630 nm. Bot: SEM images of titanium dioxide nanopillar array⁹². c Top: RCP and LCP images taken of the 1951 USAF resolution test chart with the chiral meta-lens for RCP and LCP illumination at 500 nm. Mid: The polarization contrast for RCP - LCP from 470 nm to 650 nm. Bot: Top-side view of the SEM micrograph picture at the edge of the metasurface lens¹⁷⁰.

Broadband chiral holography

Chiral holography utilizes the circular polarization selective properties of structures to create complex optical patterns or 3D light fields. These metasurfaces can be used in a variety of applications, such as security labels, data storage, and the generation of complex light fields, employing holographic techniques to store and reconstruct images. These metasurfaces, through precise control of light’s chirality and phase, are capable of producing high-resolution and dynamic 3D images. Luo et al. proposed a monolayer metasurface that can simultaneously realize circular asymmetric transmission and wavefront shaping based on asymmetric spin-orbit interactions. The rotation angles of the four rods in one unit cell can be controlled to realize 2D chiroptical response in the mid-infrared wavelength range. Under the illumination of LCP/RCP, the metasurface can form different goldfish holographic patterns in the transmission field/reflection field respectively (Fig. 6a)³⁴. Chen et al. also utilize geometric Pancharatnam-Berry (PB) phase control to achieve complex wavefront manipulations (Fig. 6b)³⁷. It allows the metasurface to manipulate the phase of circularly polarized light across a broad spectrum efficiently. The study demonstrates the capability of these structures to produce spin-dependent holographic images in the near-infrared band, laying the groundwork for broadband holographic applications.

Fig. 6: Chiral holography technology.

a Left: Metasurface four-rod unit structure. Metasurface simulation and experimental transmission and reflection spectra. right: The measured holographic images generated by the hologram³⁴. b Left: SEM micrograph of the metasurface composed of chiral meta-atoms for hologram imaging. Scale bar: 500 nm. Measured diffraction efficiency at different wavelengths. Right: Measured hologram imaging with RCP/LCP incidence at the wavelength of 980 nm. Scale bar: 20 μm³⁷. c SEM image of the nanoarc hologram. Scale bar is 3 µm. Simulated and experimental results of absolute efficiency and conversion efficiency over a broadband spectrum. Imaging images of left and right circularly polarized light respectively irradiating nanoarcs⁹³. d Left: Target images and partial SEM images of hologram A (Monkey) and hologram B (Pig). Right: Captured holographic images at different wavelengths of 720 nm, 770 nm, 820 nm under RCP (top row) and LCP (bottom row) incidence. Scale bar: 10 μm⁹⁵. e Left: The combined structure of liquid crystal and metasurface. Right: The simulated meta-holograms for: LCP and RCP illuminations on the designed metasurface reproduce specific holographic information at the working wavelength of 488, 532, and 633 nm, respectively⁹⁶. f Left: Components and images of the dynamic cholesteric liquid crystal hologram. Right: Reflected diffraction images at 633,580,550 nm light irradiation time under front(first row) and back illumination(second row)⁹⁹.

Continuous dielectric nano arcs can manipulate circularly polarized light over a broad spectrum. The nanoarc shown in Fig. 6c⁹³. is designed to support different electromagnetic resonance modes, providing continuous phase gradients for efficient wavefront manipulation.

Besides the dielectric nanostructure, metal nanoantennas can also manipulate light by exciting collective electronic oscillations on metal surfaces⁹⁴. Chen et al. proposed chiral geometric metasurfaces based on intrinsically chiral plasmonic stepped nanoapertures (Fig. 6d)⁹⁵ and experimentally achieved the generation of mixed-order Poincaré sphere beams. In addition to the traditional single-layer metasurface to achieve chiral holography.

Metasurfaces can also be integrated with electrically tunable liquid crystals to quickly switch optical responses in real time. Ultimately, the resulting metadisplay shows different phase information under different external stimuli. Naeem et al. combined LCs with chiral metasurfaces to achieve dynamic optical response and rapid reconfiguration of holographic images across a broad spectral range (Fig. 6e)⁹⁶. Based on liquid crystal molecules, spectrally tunable, polarization direction-dependent holograms can be created⁹⁷. Lately, planar Cholesteric LCs has been discovered to modulate the reflective geometric phase in a polychromatic and polarization-determined manner^86,98. Chen et al. achieve dynamic optical response and rapid reconfiguration of holographic images across a broad spectral range (Fig. 6f)⁹⁹. They propose the light-activated hybrid-multiplexed holography at visible regions based on a chirality invertible LC line superstructure. The key mechanism involves the modulation of the geometric phase across the metasurface, facilitated by the chiral reversibility and light-activated characteristics of liquid crystals. Specifically, this process is based on the ability of liquid crystal molecules to change their chirality from left-handed to right-handed under external light activation, thereby altering the phase delay of light traversing the medium. This change in chirality enables dynamic modulation of the geometric phase since the geometric phase is directly related to the orientation of liquid crystal molecules. This modulation of the geometric phase by liquid crystals is achieved through two main aspects: firstly, the light-induced chirality change in liquid crystal molecules alters their reflection or transmission properties of incident light, thus adjusting the geometric phase at a microscopic level. Secondly, through the manipulation of the spatial distribution and orientation of molecules within the liquid crystal layer, the geometric phase can be precisely regulated on a macroscopic level, thereby enabling accurate control over holographic images. Broadband chiral metasurfaces, by offering efficient and controllable chiroptical responses over a wider frequency range, provide a powerful new tool for modern optoelectronic technology.

Resonant chiral metasurfaces

3D Chiral resonant metasurfaces

In the early research period, 3D structures were first employed to achieve resonant optical chiroptical effects. As shown in Fig. 7a. These structures inherently possess chiral features, such as helical or spiral-like characteristics, as typical 3D metallic spiral structures shown in Fig. 7a²⁹. With the advancement of 3D structural fabrication techniques, M. Decker and colleagues developed a dual-layered twisted split-ring-resonator (SRR) photonic metamaterial¹⁰⁰, significantly enhancing OA by arranging SRRs in a unique lateral pattern to eliminate linear birefringence, as shown in Fig. 7b. The fabrication involved a meticulous dual-layer assembly using advanced EBL, crucial for achieving the desired chiral behavior and optical properties in this novel, two-layered metamaterial structure. Then, Vignolini et al. report a novel 3D optical metamaterial synthesized using a self-assembly method that manipulates block copolymers to form a continuous metal phase within a polymer scaffold¹⁰¹(Fig. 7c). The resulting material features a unique gyroid structure in nanoscale. It significantly modified the optical properties of the metamaterial, demonstrating anisotropic plasmon modes and optical chirality. This work underscores the potential of self-assembly-related nanofabrication techniques to create complex 3D metamaterials with tailored optical properties. In 2012, Hentschel et al. fabricated a double-layered array of chiral gold nano-discs¹⁰² using multiple EBL techniques, as illustrated in Fig. 7d. The chiroptical response of these structures could be tuned by adjusting size parameters and configurations, allowing for the tailored design and control of their chiral properties. This work demonstrated that plasmonic near-field coupling is a necessary condition for generating resonant chiroptical responses. Additionally, the investigation of these oligomeric structures provided guidance for the design and analysis of further plasmonic chiroptical responses. Furthermore, in 2018, Liu et al. employed a straightforward fabrication process: initially, a gold foil was designed and cut based on a mechanical model. Subsequently, it was subjected to gallium ion beam irradiation. Under the influence of pressure and stress, the gold foil underwent stretching and rotation, transforming into 3D nanostructures. This approach enabled versatile shape transformations from 2D to 3D structures. In comparison to traditional multilayer stacking techniques, it significantly simplified the fabrication process and enriched the diversity of structures. By utilizing the 3D pinwheel arrays structure, as depicted in Fig. 7e, resonant 3D chirality was achieved¹⁰³.

Fig. 7: 3D structures with chiral resonances.

(a²⁹, c¹⁰¹, d¹⁰², f⁶⁶, g⁷⁴, i¹⁰⁴) Left: Schematic diagrams or scanning electron microscope images of structures exhibiting 3D chiral properties, Right: Corresponding transmission-reflection spectra or circular dichroism spectra of the 3D structures under LCP and RCP light; b Top: Illustration of metamaterial’s chiral unit cell composed of gold SRRs. Bottom: Measured normal-incidence intensity transmittance spectra for LCP and RCP light incident onto the sample¹⁰⁰. e Measured CD in transmission versus wavelength for 2D left-handed (LH), 3D LH, and 3D right-handed (RH) pinwheels, respectively, and SEM image of LH 3D pinwheel arrays¹⁰³. h Circular transmission and polarization conversion measured for electromagnetic waves incident at a tilt angle of $\alpha ={30}^{o}$ and the schematic diagrams of structure⁶⁷.

With the development of chiral structures and the inherent properties of metasurfaces, there is a growing interest in achieving precise control over dynamic metasurfaces. This shift in focus is driven by the recognition that metasurfaces, in their conventional static form, have inherent limitations in enabling independent dynamic control. In 2012, Shuang Zhang et al. demonstrated a chiral metamaterial⁶⁶, as depicted in Fig. 7f, capable of switching handedness (chirality) in response to external optical stimulation. By integrating a photoactive medium into the metamaterial’s structure, they achieved reversible control over the material’s chirality without needing any structural reconfiguration, verified through numerical simulations and experiments. This method marks a significant advancement in the dynamic control of chiral electromagnetic properties, specifically in terahertz frequencies.

Many nanoscale structures are prepared through chemical methods, differing from the typical ordered arrangement of chiral metasurfaces on fixed substrates. These structures exhibit random orientations when dispersed in a solution but still display significant optical chirality. As illustrated in Fig. 7g, a method for synthesizing chiral gold nanoparticles was proposed by Lee et al. in 2018⁷⁴. This approach utilizes amino acids and peptides to control the optical chirality and surface plasmon resonance of the nanoparticles. The nanoparticle surfaces, along with the amino acids and peptides themselves, possess chirality, leading to enantioselective interactions at the interfaces that induce twisting and rotation of the nanoparticles. Even gold nanoparticles grown in solution exhibit strong optical chirality, as confirmed through computational analysis and macroscopic color changes. This work introduces new methods and insights for designing and fabricating 3D chiral micro/nanostructures.

All the mentioned structures above exhibit resonant 3D intrinsic chirality, which means that they display 3D chiroptical responses under normal incident light. However, in recent years, researchers have also explored a new realm of 3D extrinsic chirality, which involves excitations induced by external conditions or non-normal incident light to achieve polarization-preserving 3D chirality responses. This expansion encompasses chirality responses induced under non-uniform light fields, multi-mode light fields, different incident angles, and other conditions. The generation of these extrinsic chirality responses no longer relies on the intrinsic shape of the structure solely, but also depends on external excitation conditions and the non-uniformity of light. For example, as depicted in Fig. 7h, e. Plum et al. demonstrate the strong OA effects induced by 3D extrinsic chirality in non-chiral metamaterials⁶⁷. Through meticulously controlling the orientation of planar metamaterial structures relative to the incident electromagnetic wave, the authors successfully unveiled pronounced circular birefringence and dichroism, marking a significant advancement in the study and application of chiral resonant phenomena. And in 2022, Jin Peng et al. presented a novel all-dielectric terahertz metasurface characterized by remarkable extrinsic chirality¹⁰⁴. This design, as shown in Fig. 7i, utilizing high-resistance silicon cylinders with embedded rectangular slots, demonstrates a pronounced chirality when terahertz waves are incident obliquely. This metasurface exhibits extrinsic chirality, which emerges from its structural arrangement under specific illumination condition with oblique incidence angles, rather than from the inherent geometric chirality of the structure. The study underscores the significance of structural design in inducing extrinsic chirality and opens new avenues for advanced electromagnetic and optical applications.

2D chiral resonant metasurfaces

Planar 2D chiral metasurfaces have attracted attention for their ability to achieve precise control of light waves on extremely thin layer. In addition, due to their simple fabrication, low cost, easy integration with other electronic and optical components, planar metasurfaces provide the possibility to realize smaller, more efficient and integrated optical devices. They can change the polarization state, direction and phase of light waves, providing innovative solutions in areas such as optical imaging, sensors, communications and stealth technology. Many plasmonic^{76,105,106,107,108,109,110,111,112,113,114} and all-dielectric^{78,115,116,117,118,119,120} resonant planar metasurfaces have been reported in the previous literature, and these planar resonant chiral metasurfaces are based on internally or externally introduced chiral effects.

Among them, Wang et al. proposed a plasmon-based 2D chiral metasurface consisting of nanoarrays milled on a thin gold layer on sapphire, which exhibits distinct chiral optical resonance responses in the visible to near-infrared frequency range, emphasizing localization (Fig. 8a)¹⁰⁵. The coupling between modes and propagation modes provides additional degrees of freedom for the design of planar resonant chiral metasurfaces. The influence of surface lattice clusters introduced by propagation modes can be precisely controlled by tuning the lattice period and nanogap length. The enhancement of light-matter interactions in metal nanostructures can also be achieved through Fano resonance. Zu et al. crafted flat heptamer formations and analyzed the chiroptical response by adjusting the rotation angle of the elliptical nanorods and the distance between them, as depicted in Fig. 8b¹¹⁰. The chiroptical response reaches a maximum value of 30% when the structural asymmetry reaches its maximum. It is found that the chiral spectral properties are obviously dependent on the Fano resonance intensity and the related near-field optical distribution, and the Fano resonance is proved by the coupling of the magnetic quadrupole mode and the electric dipole mode through the multipole mode expansion theory. In 2017, Ye et al. prepared periodic “L” shaped gold nanoarrays (Fig. 8c)⁷⁶. When the LCP light is incident, the reflected light is still LCP light, that is, the spin of the LCP light can be reflected and retained, while when the RCP light is incident, the transmitted light mainly changes to LCP, and the experimental verification shows that better performance can be achieved at 1.5 µm.

Fig. 8: 2D resonant chiral metasurfaces with intrinsic chirality.

a Top: Schematic of a metasurface constructed from an array of nanoslits in a gold layer on a sapphire substrate. Bottom: SEM images and CD spectra of left- and right-handed enantiomeric metasurfaces¹⁰⁵. b Top: Schematic representation of chiral Fano oligomers under normal circularly polarized light. Bottom: Scattering cross sections for incident LCP and RCP light¹¹⁰. c L-shaped gold nanoantenna metasurface, top: L-shaped gold nanoantenna structural unit, Bottom: The corresponding transmission spectrum⁷⁶. d Top: Schematic illustration of silicon-based chiral metasurfaces with high-Q Fano resonances. Bottom: Polarized transmission Jones matrix¹¹⁶. e Top: Schematic of a hypersurface consisting of a germanium Z-shaped resonator on a silicon dioxide substrate. Bottom: Co-polarization and cross-polarization transmission coefficients of metasurfaces⁷⁸. f Top: Schematic diagram of 2D chiral metasurface generated by achiral meta-atoms. Bottom: Transmission spectra of chiral metasurfaces in the visible and near-infrared regions¹¹⁸.

Although plasmonic metasurfaces contribute significantly to generating optical responses, their practical applications face limitations due to ohmic losses in the visible and near-infrared bands and the limitation of exciting only electric dipole resonances. Consequently, the field of meta-optics has shifted towards adopting all-dielectric approaches to overcome these constraints. In 2014, Wu et al. proposed and demonstrated a Fano resonant all-dielectric metasurface based on a CMOS-compatible technique. It was fabricated on a silicon-on-insulator (SOI) wafer using standard CMOS-compatible semiconductor manufacturing techniques (Fig. 8d)¹¹⁶. The metasurface can exhibit a remarkable degree of 2D chirality, the transmittance and reflectance of LCP and RCP are very different, and the optical resonant Q-factor reached 4100. Ma et al. proposed a planar chiral all-dielectric metasurface composed of an array of high-index germanium Z-shaped resonators (Fig. 8e)⁷⁸, which has huge CD and a transmission asymmetry of more than 0.8, with negligible losses and no bianisotropy or violation of reciprocity. These resonators break the endoscopic face scale and induce cross-polarization conversion. In addition, at the transmission peaks of one-handedness, the transmitted light is effectively converted into the opposite circular polarization state. In 2023, Gryb et al. proposed a geometrically simplest 2D chiral metasurface platform consisting of non-chiral dielectric rods arranged in a square lattice (Fig. 8f)¹¹⁸. Chirality is created by rotating individual atoms, making their arrangement chiral and resulting in a chiral reaction that is stronger or comparable to that of more complex designs. Resonances of different arrangements are robust to geometric changes and behave similarly in experiments and simulations.

Previous literature discusses 2D resonant chiral metasurfaces based on intrinsic chirality. In addition, extrinsic 2D chiral resonant metasurfaces using specific external conditions have also been reported in some literature. In 2016, Cao et al. designed a symmetric metasurface with a gold-based circular hole designed to introduce surface plasmon polariton (SPP) modes at non-normal incident waves, leading to 2D extrinsic chirality (Fig. 9a)¹¹³. The extrinsic chirality of obliquely incident light was induced at terahertz wavelengths. The additionally proposed concept can be easily extended to higher frequency regions for applications from visible to infrared wavelength bands. Leon et al. proposed a diffractive metasurface that consists of a gold-based split-ring resonator with a refractive index matching that of glass encapsulated on a glass substrate (Fig. 9b)¹¹⁴. Near-field diffractive optics demonstrated an enhanced exogenous chiroptical response. The metasurface provided CD enhancement based on an oblique incidence angle close to the normal incidence angle and exhibits a spectral response that is extremely sensitive to the illumination angle. In addition, the proposed exogenous chiral metasurface behaves as an ultrathin CP spectral filter in the near-infrared band with a tuning range of 200 nm.

Fig. 9: 2D resonant chiral metasurfaces with extrinsic chirality.

a Left: Schematic of the MDM trilayer perforated with a rectangular array of circular holes suspended in air. Right: Circular polarization conversion difference (CPCD) for different values of rotation angle φ¹¹³. b Left: Schematic of a metasurface consisting of gold SRRs arranged in a square lattice geometry. Right: Metasurface transmission spectra simulated using circularly polarized illumination at incident angles θ = 3.5°¹¹⁴. c Left: Schematic of a left-handed perovskite chiral metamolecule on quartz substrate. Right: Simulated circular dichroism spectra of RPCM and LPCM at incident angle ϕ = 5.74°¹¹⁵. d Left: Schematic diagram of a chiral metasurface combining achiral dye molecules with chiral mirrors. Right: Far-field simulation results for an incident angle of 50°¹²⁰.

Researchers also have proposed the concept of 2D chiral metasurfaces characterized by dielectric-type extrinsic chirality^119,120. Long et al. employed planar nanostructures to achieve fully dielectric chalcogenide metasurface exhibiting substantial superstructural chirality (Fig. 9c)¹¹⁵. They established a direct spectral correlation between near-field and far-field chirality and adjusted the electric and magnetic multipole moments of the resonant chiral metamolecules, resulting in a significant anisotropy factor of 0.49 and chiral response of ${\theta }_{{mdeg}}$ of 6350 mdeg, which defined as ${\theta }_{{mdeg}}=\frac{180000}{\pi }\arctan (\sqrt{\frac{{T}_{{RCP}}-{T}_{{LCP}}}{{T}_{{RCP}}+{T}_{{LCP}}}})$. In 2023, Lee et al. proposed an angle-sensitive chiral metasurface for gyrotropic switching using a Mie resonator that supports a magnetic dipole with a large field enhancement (Fig. 9d)¹²⁰. The preferential interaction between the chiral supersymmetric mirror and CPL can result in different magnetic field intensification of the incident light of LCP and RCP, achieve selective suppression of zero-order reflectance of RCP at 40° and LCP at 50°, and excite the CP emission with opposite rotation within 10°.

Applications of resonant chiral metasurfaces

Resonant chiral metasurfaces, as advanced platform for efficient chiral light and matter interaction, play a pivotal role in the fields of sensing, nonlinear optics, and chiral light emission. In sensing applications, these metasurfaces significantly enhance detection accuracy and efficiency due to their high sensitivity to physical or chemical changes and selectivity towards specific molecules, particularly in biological detection and environmental monitoring. In the realm of nonlinear optics, they expand possibilities for optical switch and modulator manufacturing and optical signal processing through enhanced material nonlinearity and the realization of novel optical phenomena. As for chiral light emission, resonant chiral metasurfaces with high Q factor can provide strong feedback for chiral light sources, but also allow precise control over the emission, including polarization states and direction, which is crucial for optical communication and advanced imaging technologies. Overall, the unique optical characteristics and tunability of these metasurfaces demonstrate their immense application potential and scientific value in these three domains.

Chiral sensing and detection

Several sensors based on resonant chiral metasurfaces have been developed, such as plasmonic sensors^{28,46,121,122}and dielectric-based sensors^{49,123,124,125,126,127,128}. These chiral sensors have potential applications in biomedicine, environmental monitoring, and chemical sensing. In particular, chiral plasmonic metasurfaces working in the infrared regime play a crucial role in chiral thermal switches, selective molecular sensing, and thermophotovoltaics^129,130,131.

In nature, the chirality response of chiral molecules is typically weak. Directly measuring the chirality of chiral molecules through CD spectroscopy requires a large quantity of chiral molecules, making it challenging to achieve high-sensitivity detection. Resonant chiral metasurfaces, on the other hand, can effectively enhance the interaction between light and chiral molecules, offering an efficient approach for trace-level detection of chiral molecules. In 2017, Zhao et al. designed and fabricated a chiral sensing platform based on a chiral metasurface composed of double-layered twisted gold nanorods with strong plasmonic chiroptical responses²⁸, as shown in Fig. 10a. Near-field chiral enhancement can significantly improve the sensitivity for detecting chiral molecules. The system’s CD spectra are measured in the visible and infrared regions. Experimental results demonstrated that this platform could enhance the detection sensitivity of chiral molecules to the level of 10^-21 moles. Solomon et al. introduced an achiral metasurface comprised of high-index dielectric disks, explicitly designed for the detection and differentiation of chiral molecules, as well as for their separation, as depicted in Fig. 10b⁴⁹. Through carefully engineered geometric parameters, these achiral structures achieve a chiroptical response by enhancing the local electromagnetic fields, leading to a 138-fold increase in optical chirality and a 15-fold magnification of Kuhn’s dissymmetry factor g. This mechanism of chiroptical response allows for the precise sensing of individual chiral molecules and indirectly permits the assessment of the chirality of mixtures containing diverse chiral molecules. The significant local enhancements facilitate the interaction with chiral molecules, thus addressing the dual aspects of chiral detection and estimation. García-Guirado et al. also proposed a chiral plasmonic sensor composed of a racemic mixture of γ-diketones without intrinsic CD but with high optical chirality and near-field electric field enhancement¹²¹. In 2019, Garcia-Guirado et al. designed a chiral superstructure surface (Fig. 10c) of silicon nanocrystals to distinguish chiral molecules¹²⁵. They studied the effects of electric dipole resonance and magnetic dipole resonance detuning on CD, improving the CD response by nearly 30 folds in the visible light band, and successfully distinguished between L-phenylalanine and D-phenylalanine in experiments. Due to its unique properties, this configuration enables direct discrimination of phenylalanine enantiomers in the visible frequency range. The refractive index sensing capability of the metasurface (see Fig. 10d), as detailed by Jin Peng et al. (2022)¹⁰⁴, is attributed to its responsive resonant peaks under varying environmental conditions. This work presents metasurfaces, especially those exhibiting chiral properties, as highly sensitive to refractive index variations at distinct resonant frequencies, leading to measurable shifts in their transmission spectra. This sensitivity underscores the potential of chiral metasurfaces in precise refractive index detection and monitoring applications. Notably, these metasurfaces demonstrate a unique capability for the sensing and discrimination of chiral substances, due to their pronounced extrinsic chirality. Such attributes render them invaluable for applications demanding meticulous environmental surveillance and comprehensive analysis of material properties.

Fig. 10: Resonant chiral metasurfaces for chiral sensing and detection.

a Sensing performance of the metasurface: with incidence of circularly polarized waves. The variation of C1 and C2 with the refractive index and the peak values variation of C1 and C2²⁸. b Left: Schematic of silicon nanodisk metasurface illuminated by CPL with enantiospecific absorption. Right: Spatial maximum in C around the outside of a silicon metasurface with varying disk radii and incident wavelengths⁴⁹. c Left: Scanning electron micrograph of Si sensor cross-section. Right: Experimental CD spectrum of the coated sensor, the red and blue curves correspond to the L and D enantiomers of the phenylalanine coating on the sensor, respectively¹²⁵. d Bilayer gold metasurfaces for sensing propanediol. Top: A schematic representation for chiral molecule detection. Bottom: A SEM image depicting the structure of a metasurface and the circular dichroism (CD) spectra¹⁰⁴.

Nonlinear chiral optics and chiral light emission

Over the past few decades, many studies have been conducted on nonlinear optics to improve its functionality and expand its information capacity^{120,132,133,134}. So far, several nonlinear chiral metasurfaces designed with plasmonic and dielectric materials have been reported. Li et al. utilized metal-dielectric-metal plasmonic chiral structures to manipulate valley polarized photoluminescence (PL) in MoS₂ metasurface heterostructures (Fig. 11a)¹³². The resonant field of the chiral metasurface can couple to the valley-polarized excitons of the MoS₂ metasurface. It modulates the PL under opposite-helical excitation, allowing the observation of the degree of valley polarization (DVP). Valley contrast PL in chiral heterostructures is also observed when illuminated with linearly polarized light. In 2021, Lim et al. demonstrated that circular polarization emission can be strongly enhanced at the narrow mode position of the chiral Fano resonance¹³³, as shown in Fig. 11b. They developed a method where perovskite films are spin-coated onto a structure with broken symmetry. This method allows for a significant increase in degree of circular polarization (DCP) without necessitating the direct patterning of the perovskite layer. The study demonstrates that a DCP greater than 0.5 can be achieved through this process, utilizing the narrow mode position of chiral Fano resonances. In 2023, Yoon Ho Lee et al. explored the fabrication and theoretical aspects of chiral plasmonic nanostructures for altering the PL of quantum dots (QDs), as illustrated in Fig. 11c¹³⁴. The team employed mechanical force and metal deposition to impart extrinsic chirality to nanostructured substrates, creating a chiroptical environment that influences the PL characteristics of QDs. Theoretically, the asymmetry in these structures arises from varying oblique angles of incident light, which modifies the light-matter interaction and leads to enhanced chiroptical properties.

Fig. 11: Resonant chiral metasurfaces for nonlinear optics and chiral light emission.

a Left: Schematic of MoS₂-metasurface structure, where CVD-grown MoS₂ monolayer is placed into the SiO2 layer and sandwiched between chiral metasurface and Au film. Right: The PL intensity of the molybdenum sulfide monolayer at different excitation points (green, blue, red, black) is different¹³². b Left: Schematic of circularly polarized emission via chiral Fano resonance. Right: Photoluminescence (PL) enhancement factors for RCP and LCP components¹³³. c Top: Schematic images of hierarchical LH- and RH-chiral plasmonic patterns. Bottom: Polarization-sensitive PL spectra and schematic images of GR- and GB-QDs on the LH-patterned chiral plasmonic structures¹³⁴. d A chiral plasmonic gold nanohelix-based metasurface for second harmonic generation at 400 nm for optical rotation instead of circular dichroism¹³⁸. e Left: Unit cell structure of dielectric chiral metasurfaces supported by metal substrates. Right: Dependence of SHG efficiency η and SHG chiral dichroism (SHG-CD) on the central wavelength of pump¹³⁹. f Left: Unit meta-atom structure with C3 (top) and C4 (bottom) chiral plasmonic nanoresonator. Mid: Top view scanning electron microscopy images of the fabricated C3 and C4 meta-atom arrays. Right: Wavelength-dependent CD_SHG and CD_THG from the measured (red. and simulated (black) data¹³⁶.

Efficient manipulation and control of the polarization state of emitted light is one of the main goals of modern optics^{135,136,137,138}. Due to the enhanced interaction between light and matter, chiral optics based on resonant metasurfaces has been explored to show control of circularly polarized light emission. It has been demonstrated that chirality can be included in metasurfaces without destroying the time rehearsal symmetry of subwavelength structures. In the study conducted by Collins et al., the team demonstrated the use of chiral plasmonic nanostructures, specifically gold nano-helices, to induce second-harmonic generation (SHG) optical rotation, as depicted in Fig. 11d. This work distinguishes itself by focusing on optical rotation attributable to intrinsic structural chirality, which is a significant shift from the commonly studied CD in such contexts. The structural chirality, necessary for the SHG process, arises due to the anisotropic design of the nanostructures¹³⁸. In their experiment, they illuminated the chiral nanostructures with 800 nm pulsed light, which resulted in the generation of SHG signals at 400 nm. This study contributes to the understanding and application of chiral nanostructures in manipulating light properties for advanced optical applications. In 2020, Kwang-Hyon Kim et al. introduced a dielectric chiral metasurface, which is constructed using Z-shaped lithium niobate nanoantennas and supported by a gold substrate (Fig. 11e)¹³⁹. This design is pivotal for achieving giant CD and highly efficient SHG at shorter ultraviolet (UV) wavelengths. When subjected to a peak pump intensity of 5 GW per square centimeter, the metasurface demonstrated an SHG efficiency of 0.001% in the blue UV spectrum, and the SHG-CD value reached 1.8. In the meantime, Kim et al. reported a polarized reflective chiral metasurface for a spin-dependent nonlinear optical response¹³⁶, as shown in Fig. 11f. This metasurface uses Trisceli type (C3 rotational symmetry) and Gammadion type (C4 rotational symmetry) chiral nanoresonators. Those metasurfaces can be designed to produce SHG and third harmonic generation (THG) based on the incident spin of CP light. The optimized hybrid metasurface generates significantly high nonlinear harmonic signals and huge nonlinear CDs.

Chiral BIC metasurfaces

Despite rapid progresses in resonant chiral metasurfaces, there are still some limitations in the visible and near-infrared bands, such as absorption and scattering losses, discontinuous response, and limited chiral optical response. Due to the characteristics of significantly enhancing the optical response and sensitivity of metasurfaces, scientists have recently focused on introducing BIC into chiral metasurfaces. With its high Q factor and strong local field intensity, the selectivity and controllability of metasurfaces can be improved.

BIC was first proposed by von Neumann and Wigner in the field of quantum mechanics using mathematical methods in 1929¹⁴⁰. They constructed an artificial quantum potential to support BIC, that is, an electronic state whose energy falls above a continuous threshold. Traditionally, light is confined to a closed or Hermitian system, prohibited from entering the radiation channel and having an infinite Q-factor. In open or non-Hermitian systems, light waves are spectrally coupled to a continuum of radiating states, producing resonant modes with finite Q-factors. BIC is a special state that is in the continuum spectrum of radiation states and coexists with extended waves, but it is still completely restricted and does not have any radiation. In practical applications, due to finite range of the structure, material absorption and other external perturbations, BIC collapses into a Fano resonance with a finite radiation Q-factor, known as quasi-BIC, which has been used to obtain ultra-high Q in a variety of photonic systems. It has broad application prospects in sensing^141,142,143, laser^{144,145,146,147}, and nonlinear fields^{148,149,150,151,152}.

BIC significantly improves the performance of resonant nanophotonic devices, including low-threshold lasing, sensing, unidirectional emission, and nonlinear optics. Recently, it also has been employed to enhance both the Q-factor and CD of resonant chiral metasurfaces.

3D Chiral BIC metasurfaces

The introduction of BICs boosted the interest for the development of 3D chiral metasurfaces, which can offer improved performance and overcome the challenges associated with their design and functionality. As a result, a multitude of research endeavors have arisen concerning 3D chiral BIC metasurfaces. In 2020, M. V. Gorkunov et al. developed a chiral metasurface that utilizes the physics of BIC to achieve maximum optical chirality³⁹. As shown in Fig. 12a, it outlines the process of manipulating BICs by introducing rotational symmetry and selective coupling to circular polarization of light, resulting in sharp resonances in the CD spectrum. It emphasizes the role of symmetry breaking and critical coupling in enhancing the chiroptical response and demonstrates the concept with numerical simulations based on pairs of dielectric bars. In contrast to the previous discussion, the subsequent work primarily focuses on theoretical validation. Subsequently, scientists experimentally validated this theory using the double-bar structures with a height difference, as shown in Fig. 12b. In 2023, Lucca Kühner et al. introduce an innovative approach to fabricate 3D dielectric metasurfaces by precisely controlling the height differences between the double bars. This work notably leverages the concept of photonic BICs to maximize optical chirality, a property crucial for developing efficient, lossless metasurfaces with 3D optical chirality, with the out-of-plane symmetry breaking of the metasurface (see Fig. 12b)¹⁵³. In addition to the breaking symmetry by introducing the height difference discussed earlier, in 2021, A. Overvig et al. proposes a pair of tightly stacked nanobars with twisted angles in the vertical direction⁷², as depicted in Fig. 12c. By setting vertically oriented elliptical cylinders with different tilt angles in the bottom and top layers, it becomes possible to support distinct circular polarization eigenstates. This allows the output light to couple into a single circular polarization channel with nearly 100% efficiency. Furthermore, by introducing geometric phase, it becomes feasible to control the wavefront of the intrinsic circularly polarized light without affecting the orthogonal linearly polarized light⁷¹, thereby enables resonate beam steering effect, offering new opportunities for active nano-photonics, quantum optics, and nonlinear optics. And then, in 2023, Y. Chen et al. introduce a slant-perturbation metasurface structure¹³¹ to achieve intrinsic 3D chiroptical response by breaking both in-plane and out-of-plane symmetries (Fig. 12d). This design has led to an experimental observation of a strong CD of 0.93 and a Q-factor exceeding 2663 at visible frequencies. In addition to the pursuit of high Q values and strong CD, scientists also aspire to achieve independent control over these two parameters. In 2023, Y. Tang et al. present a novel design of 3D plasmonic metasurfaces capable of achieving high-Q quasi-BICs with pronounced CD in the mid-infrared spectrum(Fig. 12e)¹³¹. This configuration consists of a twisted vertical split-ring resonator (VSRR) paired with a wall, it allows for the independent tuning of two important optical properties: the Q-factor and CD by adjusting the height of the wall and the twisted angle of the VSRR. With a Q-factor around 938 and a CD of approximately 0.67, these metasurfaces offer potential applications in areas that require robust chiroptical effects and strong chiral light-matter interactions.

Fig. 12: Typical 3D chiral BIC metasurfaces.

a–e. Left: Schematic diagrams or scanning electron microscope images of structures exhibiting 3D chiral BIC properties. Right: Corresponding transmission-reflection spectra of the 3D structures under LCP light and RCP light^{39,72,131,153}. f Top: Schematic of the proposed achiral metasurface and simulated transmittance spectra of two distinct spin states for different $\theta$ at $\delta$=2 mm. Bottom: Dependence of transmittance spectra of the structure on different parameters¹⁵⁴.

However, in addition to the 3D intrinsic chiral BIC metasurfaces described above, researchers have also discovered that 3D extrinsic chiral BIC metasurfaces can be achieved by rotating the entire structure, introducing an angle between the light propagation direction and the normal direction of the metasurface plane. In late 2021, J. Wu et al. delved into the transformation of BICs into quasi-BICs by breaking the C₂ symmetry in a metasurface without geometric chirality¹⁵⁴, as shown in Fig. 12f. The extrinsic 3D chiroptical responses under the circular preserving components were achieved by breaking the out-of-plane mirror symmetry through tilting the plane of the metasurface relative to the incoming waves. This alteration results in a significant difference in transmission between RCP and LCP illuminations. This work also discussed how varying the tilted angle of the metasurface and introducing other structural parameters can effectively control and manipulate the extrinsic 3D chirality. This exploration into extrinsic 3D chirality through achiral structure provides a foundation for designing systems with tunable chiral optical responses, broadening the scope of applications in various technological and scientific fields .

2D chiral BIC metasurfaces

Compared to 3D chiral metasurfaces, planar chiral metasurfaces have the advantage of being easy to manufacture, cost-effective, and readily integrable with other electronic and optical components^{79,155,156,157,158,159,160,161}. Recently, the physics of BIC have also been employed in 2D chiral metasurfaces.

In 2022, Shi et al. proposed a chiral metasurface design supporting BIC and experimentally demonstrated the 2D chiroptical BIC responses at optical frequencies for the first time⁷⁹, as shown in Fig. 13a. Double sided sickle (DSS-) shaped α-Si inclusions with in-plane inversion C₂ symmetry but without in-plane mirror symmetry are employed to construct the BIC states. By breaking the in-plane inversion symmetry or changing the illumination symmetry, planar chiral quasi-BIC states with strong intrinsic or extrinsic 2D chiroptical responses are achieved, exceeding CD = 0.99 (in simulations) and CD = 0.93 (in experiments). There were also some other theoretical designs of 2D chiral BIC metasurfaces. For example, as shown in Fig. 13b, an asymmetric cross structure was proposed by Kim et al. to construct the BIC-based chiral metasurface with high-Q 2D chiral optical resonances (Fig. 13b)¹⁵⁶. The proposed cross-shaped metasurface breaks the in-plane mirror symmetry, providing near-unity CD and tunable chiroptical responses with inversion and mirror asymmetry.

Fig. 13: Typical 2D chiral BIC metasurfaces.

a Left: Schematic diagram of the symmetry-breaking process for converting BIC into plane chirality quasi-BIC, photo symmetry breaking with varying incidence angles θ (top) and in-plane geometric symmetry breaking with δ = W₂-W₁ (bottom). Right: Predicted Jones matrix spectra and CD spectra of simulated T_ll, T_rr, T_rl and T_lr under oblique incidence (top) and normal incidence (bottom) structural parameters⁷⁹. b Left: Unit cell structure of a metasurface consisting of crossed silicon atoms having both broken plane inversion and mirror image symmetry (top) and the magnitudes CD of circular dichroism (bottom). Right: The transmission T (top) and reflection spectra R (bottom)¹⁵⁶. c Left: The side view of the single crystal cell on a planar silicon metasurface under oblique incidence (top) and normal incidence (bottom). Right: Jones matrix spectrum and CD spectrum of the transmittance of the chiral q-BIC metasurface at oblique incidence (top) and normal incidence (bottom)¹⁶⁰. d Left: Schematic illustration of the designed chiral metasurface on a silver substrate. Right: Scattering energy from the five main multipoles of the chiral metasurface under LCP (top) and RCP (bottom) illumination at θ = 10° and Φ = 0°¹⁵⁵.

In addition, Liu et al. proposed that a planar chiral silicon metasurface controlled by BIC can be used to reveal steerable chiral optical responses containing intrinsic and extrinsic planar chirality¹⁶⁰, as shown in Fig. 13c. Intrinsic planar chirality can be achieved by adjusting the in-plane symmetry at normal incidence, while tunable extrinsic planar chirality can be achieved by changing the illumination symmetry at oblique incidence. Furthermore, a hybrid Si-VO₂ metasurface based on chiral coupling mode theory is proposed to achieve loss-controlled chiral optical response. Active tuning of temperature-dependent dissipative losses is demonstrated to achieve the desired quasi-BIC sustained optical chirality. By breaking the rotational symmetry and the up-down mirror symmetry, Li et al. have theoretically reported an absorbing extrinsic 2D chiral BIC metasurface consisting of two pairs of parallel and staggered rectangular silicon rods (Fig. 13d)¹⁵⁵. Under oblique incidence, BIC exhibits strong extrinsic 2D chiroptical responses when transforming into quasi-BIC. With single-port critical coupling, the planar metasurface can selectively and perfectly absorb one type of circularly polarized light, but non-resonantly reflect the other type, with a CD close to 0.812.

Applications of chiral BIC metasurfaces

The application of chiral BIC metasurfaces in the fields of chiral sensing, nonlinear optics, and chiral light emission demonstrates their multifaceted potential and significance. In chiral sensing, these metasurfaces, with their high Q-factors and strong localized electromagnetic fields, can significantly enhance the sensitivities, making them particularly suitable for precise detection of biomolecules and chemical substances. In nonlinear optics, chiral BIC metasurfaces open new possibilities for efficient optical switches, modulators, and harmonic generation by enhancing the material’s nonlinear response. In the realm of chiral light emission, they serve as efficient sources of chiral light, offering precisely controlled emission characteristics, and paving the way for new applications in optical communication, information encryption, and advanced imaging technologies. In 2023, Yeonsoo Lim et al. discuss the experimental realization of chiral quasi-BIC in the visible spectral range and demonstrates maximally chiral emission using perovskite metasurfaces. The study highlights how chiral nanophotonic structures, specifically those utilizing chiral quasi-BICs, can significantly enhance chiroptical responses beyond what natural materials offer. Through carefully designed experiments, the authors achieved an extremely high DCP in the PL emission, indicating successful and maximal chiral light emission(Fig. 14a)¹⁶². They employed organic-inorganic hybrid perovskite films and controlled etching depths on a patterned substrate to induce out-of-plane symmetry breaking, leading to the desired chiral optical properties. In 2023, X. Zhang et al. propose a type of chiral metasurface composed of slanted titanium dioxide (TiO₂) double-bar structure on an indium tin oxide-coated substrate, which are designed to break both the in-plane and out-of-plane symmetries for strong chiral emission¹⁶³ (Fig. 14b). Their strategy has the ability to simultaneously control and modify the spectral radiation patterns and spin angular momentum of photoluminescence. It enables chiral lasing without any spin injection, promising substantial improvements over conventional methods, offering more efficient, high-quality radiation with perfect polarization conversion, and holding potential applications in active nanophotonics and quantum optics. In 2022, Q. Liu et al. discuss the development of a novel dual-band chiral nonlinear metasurface¹⁶⁴ (Fig. 14c). It is capable of generating strong THG with conversion efficiency reaching the order of 10⁻⁴ for two peaks in the near-infrared region, with a THC CD reaching near-unitary, demonstrating the efficacy of the metasurface in differentiating circular polarized light. Those results make it a promising candidate for applications in areas such as chiral sensing, optical communications, and advanced photonic devices with its efficient light manipulation capabilities at the nanoscale.

Fig. 14: Nonlinear optics and chiral light emission based on chiral BIC metasurfaces.

a. Schematic for the reciprocity calculation of chiral emission¹⁶². b Top: Schematic of the nanostructures. Middle: Top view and side view of unit cell with different parameters. Bottom: The corresponding directions of polarization vector fields¹⁶³. c Left: Schematic of the chiral metasurface. Right: Nonlinear responses of the chiral metasurface¹⁶⁴. d Left: DSS structured chiral metasurface with 2D chiral effect in transmission. Right: Simulated transmission spectra of T_lcp and T_rcp as well as the linear CD spectra (top) and THG intensity under different circularly polarized light incidence and the corresponding nonlinear CD (bottom) of the chiral metasurface⁷⁹. e Left: Schematic of a silicon metasurface with broken-symmetry L-shaped meta-atoms. Right: Measured forward TH signal (top) and TH CD spectrum (bottom) for co-polarized RCP (blue) and LCP (red. Excitation and collection for dL = 300 nm¹⁵⁹.

The DSS-shaped planar chiral BIC metasurface proposed by Shi et al. can also achieve the maximum nonlinear CD⁷⁹. They optimized the q-BIC metasurface to produce circular eigen-polarization states. A significant near-field enhancement contrast between RCP and LCP incident light (400:1) was achieved, which is the key for realizing nonlinear chiral emission (Fig. 14d). They conducted theoretical simulations and experimental measurements of the THG intensity under RCP/LCP pumping, clearly producing a high nonlinear CD contrast in THG emission of up to 0.93 (theoretically) and 0.81 (experimentally). Most recently, Koshelev et al. fabricated a set of chiral nonlinear metasurfaces composed of L-shaped silicon nanoparticles with in-plane asymmetry (Fig. 14e) and experimentally demonstrated significant enhancement of nonlinear CD¹⁵⁹. They demonstrated that while maintaining high conversion efficiency, the nonlinear CD can gradually change from a value of 0.918 ± 0.049 to -0.771 ± 0.004 for samples with different asymmetry parameters. It is revealed that the origin of the nonlinear chirality is due to the nonlinear nonreciprocity, and the dependence of nonlinear chirality on nonlinearity and microscopic symmetry is deduced.

Summary and perspective

In summary, we have reviewed the overall progress of optical chiral metasurfaces with either broadband chiroptical effect or resonant chiroptical responses. We begin with discussing the basic concepts, classification, and typical nanostructures with chiroptical effects. Main characteristic quantities that are used to describe the chiroptical strength, including circular dichroism and optical chiral density are analyzed. We analyzed both 3D and 2D chiral structures and their corresponding chiroptical responses associated with a helicity-preserving and helicity-conversion feature, respectively. We also discuss intrinsic chirality and extrinsic chirality which are excited by normal incidence and oblique incidence, respectively, both of which could be combined with 3D and 2D chiroptical responses. Then, we reviewed broadband chiral metasurfaces with the capability of chiral manipulation over a range of wavelengths, which are extensively applied in circular polarizers, imaging, and holography. After that, we reviewed resonant chiral metasurfaces, in which the optical chirality is accompanied by resonance enhanced absorption, scattering, and localized near-fields.

Chiral metasurfaces have experienced significant development in the past few years, showing rapidly-developing trends. In the domain of design strategy, a complete physical modal theory of the interactions between light and matter at the nanoscale is vital for the precise prediction and customization of the optical responses in chiral metasurfaces¹⁶⁵. In recent years, substantial efforts have been invested in integrating the foundational principles of chiral metasurface with inverse design algorithms^166,167, which led to the emergence of algorithms that facilitate the rapid design of chiral metasurfaces. Additionally, the adoption of machine learning algorithms for the construction of chiral metasurfaces has also gained traction¹⁶⁸, marking a new trend in the field.

The development of broadband chiral metasurfaces enable effective control of light across a broad wavelength range, thereby enhancing the performance of polarizers in various optical systems. In imaging, they are expected to introduce new high-precision applications and multi-channel composite imaging. Additionally, in holography, broadband chiral metasurfaces realize more complex, high-resolution miniaturized display systems and holograms, marking a significant advancement in holographic technology.

Following the remarkable developments in broadband chiral metasurfaces, the evolution of resonant chiral metasurfaces is set to further elevate optical technologies. These metasurfaces, with adjustable chiral properties, promise to refine light manipulation, enhance environmental sensing, and introduce novel security features. They bring a new level of precision in controlling specific wavelengths, leading to more effective optical filtering. In the field of environmental monitoring, their advanced detection capabilities promise greater accuracy in identifying subtle variations across physical, chemical, and biological spectrums. For security applications, their potential in crafting intricate optical signatures offers a new dimension in anti-counterfeiting technologies. This advancement in resonant chiral metasurfaces represents a significant step forward, complementing and extending the capabilities established by broadband chiral metasurfaces.

The uniqueness of BIC metasurfaces lies in their ability to combine high quality factors with strong localized electromagnetic fields, thereby enabling efficient optical control and enhanced light-matter interactions. In the future, we can expect these metasurfaces to find widespread applications in areas such as high-sensitivity sensors, nonlinear optics, chiral light emission, as well as novel optoelectronics and optical communication devices. Additionally, their potential applications in cutting-edge technologies like biomedical imaging, quantum computing, and information security are also worthy of attention. With advances in material science and nanofabrication technology, research on chiral BIC metasurfaces is set to deepen, promising more innovative applications and technological breakthroughs.

Overall, the future trajectory of chiral metasurfaces encompasses the development of metasurfaces with capabilities for real-time tunability and reconfigurability, the emission of high-purity chiral light, and the high-precision detection of chiral molecules.