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Advances on broadband and resonant chiral metasurfaces

June 2024
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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.

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npj | nanophotonics Review article

https://doi.org/10.1038/s44310-024-00018-5

Advances on broadband and resonant

chiral metasurfaces

Check for updates

Qian-Mei Deng1,2,3,XinLi

1,2,3, Meng-Xia Hu1,2,3, Feng-Jun Li1,2, Xiangping Li1,2 & Zi-Lan Deng1,2

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.

Polarization is a fundamental property of light. Traditionally, people usually

resort to polarization optical elements composed of birefringent crystals1or

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

phase2,3, polarization4,amplitude

5, arbitrary spin-wavefront manipulation6,

and resonant properties provides a versatile platform for constructing

compact and ﬂexible polarization optical elements. By manipulating arbi-

trary phase, amplitude responses based on arbitrary polarization basis

beyond the linear polarization basis7,8, it enables the creation of optical

devices for polarization projection9–11, polarization beam splitting12,13,

polarization measurements14–21, polarization imaging22–25, and various other

functionalities.

For the full manipulation of polarization states, especially for the cir-

cular 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

1Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan University, Guangzhou 510632,

China. 2College of Physics & Optoelectronic Engineering, Jinan University, Guangzhou 510632, China.

These authors contributed equally: Qian-Mei Deng, Xin Li,

Meng-Xia Hu. e-mail: xiangpingli@jnu.edu.cn;zilandeng@jnu.edu.cn

npj Nanophotonics | (2024) 1:20 1

1234567890():,;

listed “Why Life Needs Chirality”as one of the newly released “125 Most

Cutting-Edge ScientiﬁcIssuesintheWorld”. When the concept of chirality

extends to optics, the emerging chiroptical phenomenon usually refer to the

different intensity or phase responses under different circular polariz ed 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 excitations27,28.

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, cate-

gorized 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.

Themirrorsymmetryintheout-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 structures29–33.Wedeﬁne the 2D chir-

optical responses as chiroptical responses that are manifested on the circular

conversion components, which is the common case for 2D chiral

structures34–37.

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 con-

ﬁguration including both the structure and the incident angle of the illu-

mination 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 auto-

matically introduces mirror symmetry breaking to the overall optical con-

ﬁguration 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.

As for speciﬁc applications that require a broad working bandwidth of

metasurfaces, people design broadband chiral metasurfaces which primarily

served as highly efﬁcientcircularpolarizersoraselementsinpolarization-

controlled devices. Recently, to enhance the chiral light and matter inter-

action, 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

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

discussed29,39,54,67,78,79,88,104,115,155.

https://doi.org/10.1038/s44310-024-00018-5 Review article

npj Nanophotonics | (2024) 1:20 2

metasurfaces and their typical applications. Then, we review the resonant

chiral metasurfaces with moderate Q-factorsandchiralboundstatesinthe

continuum (BIC) metasurfaces with ultra-high Q-factors. Finally, we pro-

vide 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 dif-

ference 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 dis-

persion, refers to the phenomenon where a chiral substance has different

refractive indices for LCP and RCP light, making them propagate at dif-

ferent speeds through the medium, therebyrotatingthepolarizationplane.

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 char-

acterize 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 deﬁ-

nition 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 absorbers41–43,

CD typically deﬁned as:

CDA¼ALCP ARCP :ð1Þ

Fig. 2 | Typical 3D chiral nanostructures. a SEM images of nanohelices with

different geometric sizes51.bSchematic unit cell of gyroid along the [111] directions54.

cThree-layer unit cell left-handed chiral woodpile structure consisting of layers of

nanorods that are stacked in the z-direction57.dMolecularorganization in cholesteric

(chiral nematic) liquid crystal phases62.eSchematic of a designed L-shaped curled

metasurface58.fWindow decoration–type nanobarriers, and a deformable spiral103.

gSchematic 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.hLeft: 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.

https://doi.org/10.1038/s44310-024-00018-5 Review article

npj Nanophotonics | (2024) 1:20 3

Here, ALCP and ARCP represent the absorption rates of the chiral

substance to LCP and RCP light, respectively. The concept of CD was

extended to the scattering propertie s of light in terms of its transmission and

reﬂection spectra, the corresponding CD is deﬁned

CDT¼TLCP TRCP ;ð2Þ

CDR¼RLCP RRCP ;ð3Þ

where T

LCP

RCP

LCP

and R

RCP

represent the transmittance and reﬂec-

tance of RCP and LCP components, respectively.

The normalized deﬁnition of CD normalizes the difference b y dividing

it by the sum of the optical responses of LCP and RCP44.Thisdeﬁnition

accounts for the overall optical response strength, providing a ratio that

reﬂects the degree of CD relative to the total optical response. This nor-

malization can make comparisons between samples with vastly different

intensities more meaningful, as it considers both the dichroism and the

overall molecular absorption.

CDk¼KLCP KRCP

KLCP þKRCP

;ð4Þ

whereKcouldbeA,T,orR,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 106times 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 as47,48,

C¼ ω

2c2Imð~

E~

HÞ;ð5Þ

where E and H represent the electric and magnetic ﬁelds, respectively; *

denotes complex conjugation; ωistheangularfrequency;cisthespeedof

light in vacuum. For linearly polarized light, Cis zero; whereas for RCP and

LCP incident light, Ctakes 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 e nhancement 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 2shows 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.The3Dhelixstructurewithsingleormultipleintertwined

segments (shown in Fig. 2a) is one of the most classic chiral structures51,52.

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.

In a theoretical study analyzing dielectric contrasts characteristic of

semiconductor nanostructures, Lee and Chan discovered that spiral struc-

tures exhibit either a complete 3D band gap or a transmission gap for each

state of circularly polarized light individually. This phenomenon is con-

tingent upon whether the spirals are interconnected or spatially separated55.

3D chiral photonic crystals are a typical structure56.AsshowninFig.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 super-

structures, 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 transmission59–61. 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 multi-

functional shape transformations from 2D precursors to 3D architectures,

simplifying the fabrication of complex and unconventional structural geo-

metries. Wang et al. utilized FIB-induced bending to create curled nanos-

tructures 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 con-

ductivity 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.Thefabricationprocessreliesonintricate

layering and the precise positioning of multiple materials, including semi-

conductors and metals, to achieve chiral metamolecules capable of hand-

edness switching. Initially, silicon pads are patterned on a silicon-on-

sapphire wafer using photolithography and reactive ion etching.

https://doi.org/10.1038/s44310-024-00018-5 Review article

npj Nanophotonics | (2024) 1:20 4

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 litho-

graphy techniques, such as ultraviolet lithography or electron beam litho-

graphy, to create structures with ﬁner detail. In contrast,themanufacturing

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 2hdemonstrates

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 chir-

optical responses33,71. This is primarily achieved through near-ﬁeld inter-

actions between layers, structural rotation in space, and the disruption of

symmetry to facilitate 3D chirality. Adam et al. have realized this 3D chir-

optical 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 char-

acterized 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 peri-

odic 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 chir-

optical 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.

Typical2Dchiralstructuresinclude Gammadions, Z-shaped75,

L-shaped76, Split rings77,U-shaped,S-shaped

78, and multiple meta-atom

structures, in which speciﬁc chiroptical responses imposed on circular

conversion components can be ﬂexibly designed. Some of these structures

exhibit C

symmetry, meaning that they obtain the same structure after a

180° rotation. For example, the symmetric fan-shaped (shown in Fig. 3a)79

andS-shaped(showninFig.3b)80 metasurface structures, maintain their

geometric shapes after rotating half a turn around their central axis. C

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. Addi-

tionally, 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 struc-

tures. 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 conﬁgura-

tions. Wang et al. deconstructed the planar chiral Jones matrix into an

amalgamation of two birefringent waveplates (Fig. 3f)4.Theyachievedthis

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. C

Symmetry Structures: aSymmetric fan-shaped

structure metasurface structure. The substrate is

Al2O3and the pillar material is c-Si79.bArray

sample of S-shape chiral nanostructures80.C

Sym-

metry Structures: cScanning electron micrographs

of gold gammadia. Panels show a top view of the

right-handed arrays50.dSEM images of left handed

chiral metasurface105. Non-C

or C

Symmetry

Structures: eSEM images of the fabricated L-shaped

gold nanoantennas76.fMetasurface periodic array

with double rod structure as a unit4.

https://doi.org/10.1038/s44310-024-00018-5 Review article

npj Nanophotonics | (2024) 1:20 5

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 range33,34,81,82.

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 stacking21,84. 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 fre-

quency 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.LCsarecom-

posed 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 response86,87. 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 manu-

factured. 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.

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 des igned 2D all-dielectric chiral metas urface polarizers.

They realize simultaneous broadband and high CD in the optical

communication band (Fig. 4c)88. Utilizing the broadband characteristics

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.bTransmission of LCP and

RCP waves through a stack of rotated metasurfaces by increasing the number of

layers30.cTop: 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.dTop:

TiO2metasurface unit structure and its corresponding transmission spectrum. Bot:

Experimentally captured optical images of the metasurface nanoprinting illumi-

nated with LCP and RCP light35.eTop: 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.

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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 polariz ation states (linear, circular, or ellipt ical) 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. Exper imental 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 trans mission of left- and right-handed circ ularly 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 phe-

nomenon. 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 absorbers41,42,90 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 multi-

spectral 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.bTop: RCP and LCP images

taken of the resolution test chart with the chiral meta-lens for RCP and LCP illu-

mination 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.cTop: 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.

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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 holo-

graphic techniques to store and reconstruct images. These metasurfaces,

through precise control of light’s chirality and phase, are capable of pro-

ducing 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 dif-

ferent 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.

Continuous dielectric nano arcs can manipulate circularly polarized

light over a broad spectrum. The nanoarc shown in Fig. 6c93.isdesignedto

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

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.bLeft: SEM

micrograph of the metasurface composed of chiral meta-atoms for hologram ima-

ging. 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.cSEM 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.dLeft: Target images and partial SEM

images of hologram A (Monkey) and hologram B (Pig). Right: Captured holographic

images at differen t wavelengths of 720 nm, 770 nm, 820 nm under RCP (top row)

and LCP (bottom row) incidence. Scale bar: 10 μm95.eLeft: 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.fLeft:

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.

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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 holo-

graphic 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 dis-

covered to modulate the reﬂectivegeometric phase in a polychromatic and

polarization-determined manner86,98. 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-multi-

plexed holography at visible regions based on a chirality invertible LC line

superstructure. The key mechanism involves the modulation of the geo-

metric 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 character-

istics, as typical 3D metallic spiral structures shown in Fig. 7a29.Withthe

advancement of 3D structural fabrication techniques, M. Decker and col-

leagues developed a dual-layered twisted split-ring-resonator (SRR) pho-

tonic 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

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 cir-

cular dichroism spectra of the 3D structures under LCP and RCP light; bTop:

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.eMeasured 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.hCircular transmission and polar-

ization conversion measured for electromagnetic waves incident at a tilt angle of

α¼30oand the schematic diagrams of structure67.

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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. fabricat ed 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 demon-

strated 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 sub-

jected 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.

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,asdepictedinFig.7f, capable of

switching handedness (chirality) in response to external optical stimulation.

By integrating a photoactive medium into the met amaterial’sstructure,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.

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 enantio-

meric metasurfaces105.bTop: Schematic representation of chiral Fano oligomers

under normal circularly polarized light. Bottom: Scattering cross sections for inci-

dent LCP and RCP light110.cL-shaped gold nanoantenna metasurface, top: L-shaped

gold nanoantenna structural unit, Bottom: The corresponding transmission

spectrum76.dTop: Schematic illustration of silicon-based chiral metasurfaces with

high-Q Fano resonances. Bottom: Polarized transmission Jones matrix116.eTop:

Schematic of a hypersurface consisting of a germanium Z-shaped resonator on a

silicon dioxide substrate. Bottom: Co-polarization and cross-polarization trans-

mission coefﬁcients of metasurfaces78.fTop: Schematic diagram of 2D chiral

metasurface generated by achiral meta-atoms. Bottom: Transmission spectra of

chiral metasurfaces in the visible and near-infrared regions118.

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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 dis-

persed 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 nano-

particles. The nanoparticle surfaces, along with the amino acids and pep-

tides 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 fabri-

cating 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 encom-

passes 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 excita-

tion 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.Throughmeticulously

controlling the orientation of planar metamaterial structures relative to the

incident electromagnetic wave, the authors successfully unveiled pro-

nounced 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.Thisdesign,

asshowninFig.7i, utilizing high-resistance silicon cylinders withembedded

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 elec-

tronic 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, commu-

nications and stealth technology. Many plasmonic76,105–114 and all-

dielectric78,115–120 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 meta-

surface 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 inﬂu-

ence 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 struc-

tural 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.

Although plasmonic metasurfaces contribute signiﬁcantly to generat-

ing 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 techni-

que. 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.Chiralityis

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 geo-

metric 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 wave-

length 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 metasur-

face provided CD enhancement base d 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.

Researchers also have proposed the concept of 2D chiral metasurfaces

characterized by dielectric-type extrinsic chirality119,120. 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 θmdeg of 6350 mdeg, which deﬁned as

θmdeg ¼180000

πarctanðﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

TRCP TLCP

TRCP þTLCP

qÞ. In 2023, Lee et al. proposed an angle-

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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 applicatio ns, these

metasurfaces signiﬁcantly enhance detection accuracy and efﬁciency

due to their high sensitivity to physical or chemical changes and selec-

tivity 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 sensors28,46,121,122and dielectric-based sensors49,123–128.

These chiral sensors have potential applicationsinbiomedicine,environ-

mental 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

thermophotovoltaics129–131.

In nature, the chirality response of chiral molecules is typically weak.

Directly measuring the chirality of chiral molecules through CD spectro-

scopy 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,asshownin

Fig. 10a. Near-ﬁeld chiral enhancement can signiﬁcantly improve the sen-

sitivity 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 detec-

tion 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 pro-

posed 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

nearly30foldsinthevisiblelightband, and successfully distinguished

between L-phenylalanine and D-phenylalanine in experiments. Due to its

unique properties, this conﬁguration enables direct discrimination of phe-

nylalanine enantiomers in the visible frequency range. The refractive index

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.bLeft: 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.cLeft:

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.dLeft: 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.

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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 pro-

nounced extrinsic chirality. Such attributes render them invaluable for

applications demanding meticulous environmental surveillance and com-

prehensive analysis of material properties.

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

capacity120,132–134. 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 MoS

metasurface heterostructures

(Fig. 11a)132. The resonant ﬁeld of the chiral metasurface can couple to the

valley-polarized excitons of the MoS

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

showninFig.11b. They developed a method where perovskite ﬁlms are

spin-coated onto a structure with broken symmetry. This method allows for

asigniﬁcant 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.

Efﬁcient manipulation and control of the polarization state of emitted

light is one of the main goals of modern optics135–138. 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 sub-

wavelength 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 rota-

tion, as depicted in Fig. 11d. This work distinguishes itself by focusing on

optical rotation attributable to intrinsic structural chirality, which is a sig-

niﬁcant shift from the commonly studied CD in such contexts. The struc-

tural 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 gen-

eration of SHG signals at 400 nm. This study contributes to the under-

standing 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

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.bLeft: 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.cLeft:

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.dBilayer

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.

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Z-shaped lithium niobate nanoantennas and supported by a gold substrate

(Fig. 11e)139.ThisdesignispivotalforachievinggiantCDandhighlyefﬁ-

cient 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,asshowninFig.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 non-

linear 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.Theycon-

structed 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 inﬁniteQ-factor.Inopenornon-

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 col-

lapses into a Fano resonance with a ﬁnite radiation Q-factor, known as

Fig. 11 | Resonant chiral metasurfaces for nonlinear optics and chiral light

emission. a Left: Schematic of MoS

-metasurface structure, where CVD-grown

MoS

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 different 132.bLeft:

Schematic of circularly polarized emission via chiral Fano resonance. Right: Pho-

toluminescence (PL) enhancement factors for RCP and LCP components133.cTop:

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.dA chiral plasmonic gold nanohelix-

based metasurface for second harmonic generation at 400 nm for optical rotation

instead of circular dichroism138.eLeft: Unit cell structure of dielectric chiral meta-

surfaces supported by metal substrates. Right: Dependence of SHG efﬁciency ηand

SHG chiral dichroism (SHG-CD) on the central wavelength of pump139.fLeft: Unit

meta-atom structure with C3 (top) and C4 (bottom) chiral plasmonic nanor-

esonator. 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.

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quasi-BIC, which has been used to obtain ultra-high Q in a variety of

photonic systems. It has broad application prospects in sensing141–143,

laser144–147, and nonlinear ﬁelds148–152.

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

showninFig.12a, it outlines the process of manipulating BICs by intro-

ducing rotational symmetry and selective coupling to circular polarization

of light, resulting in sharp resonance s 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 sub-

sequent work primarily focuses on theoretical validation. Subsequently,

scientists experimentally validated this theory using the double-bar struc-

tures 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, loss-

less metasurfaces with 3D optical chirality, with the out-of-plane symmetry

breaking of the metasurface (see Fig. 12b)153.Inadditiontothebreaking

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,asdepictedinFig.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 eigen-

states. This allows the output light to couple into a single circular polar-

ization 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 oppor-

tunities 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).Thisdesignhasledtoan

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 plas-

monic metasurfaces capable of achieving high-Q quasi-BICs with pro-

nounced CD in the mid-infrared spectrum(Fig. 12e)131.Thisconﬁguration

Fig. 12 | Typical 3D chiral BIC metasurfaces. a–e. Left: Schematic diagrams or

scanning electron microscope images of structures exhibiting 3D chiral BIC prop-

erties. Right: Corresponding transmission-reﬂection spectra of the 3D structures

under LCP light and RCP light39,72,131,153.fTop: Schematic of the proposed achiral

metasurface and simulated transmittance spectra of two distinct spin states for

different θat δ=2 mm. Bottom: Dependence of transmittance spectra of the structure

on different parameters154.

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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.

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, intro-

ducing 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 C

symmetry in a

metasurface without geometric chirality154,asshowninFig.12f. The

extrinsic 3D chiroptical responses under the circular preserving compo-

nents 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 explora-

tion into extrinsic 3D chirality through achiral structure provides a foun-

dation for designing systems with tunable chiral optical responses,

broadening the scope of applications in various technological and scien-

tiﬁcﬁelds .

2D chiral BIC metasurfaces

Compared to 3D chiral metasurfaces, planar chiral metasurfaces have the

advantage of being easy to manufacture, cost-effective, and readily integr-

able with other electronic and optical components79,155–161. 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,asshowninFig.13a. Double sided

sickle (DSS-) shaped α-Si inclusions with in-plane inversion C

symmetry

but without in-plane mirror symmetry are employed to construct the BIC

states. By breaking the in-plane inversion symmetry or changing the illu-

mination symmetry, planar chiral quasi-BIC states with strong intrinsic or

extrinsic 2D chiroptical responses are achieved, exceeding CD = 0.99 (in

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=W

-W

(bottom). Right: Predicted Jones matrix spectra

and CD spectra of simulated T

and T

under oblique incidence (top) and

normal incidence (bottom) structural parameters79.bLeft: 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.

cLeft: The side view of the single crystal cell on a planar silicon metasurface under

oblique incidence (top) and normal incidence (bottom). Right: Jones matrix spec-

trum and CD spectrum of the transmittance of the chiral q-BIC metasurface at

oblique incidence (top) and normal incidence (bottom)160.dLeft: Schematic illus-

tration 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.

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npj Nanophotonics | (2024) 1:20 16

Fig. 14 | Nonlinear optics and chiral light emission based on chiral BIC meta-

surfaces. a. Schematic for the reciprocity calculation of chiral emission162.bTop:

Schematic of the nanostructures. Middle: Top view and side view of unit cell with

different parameters. Bottom: The corresponding directions of polarization vector

ﬁelds163.cLeft: Schematic of the chiral metasurface. Right: Nonlinear responses of

the chiral metasurface164.dLeft: 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.eLeft: 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 col-

lection for dL = 300 nm159.

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npj Nanophotonics | (2024) 1:20 17

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 chir-

optical responses with inversion and mirror asymmetry.

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,asshowninFig.13c.

Intrinsic planar chirality can be achieved by adjusting the in-plane sym-

metry at normal incidence, while tunable extrinsic planar chirality can be

achieved by changing the illumination symmetry at oblique incidence.

Furthermore, a hybrid Si-VO

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 chir-

optical 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 opennewpossibilitiesforefﬁcient optical switches,

modulators, and harmonic generation by enhancing the material’snon-

linear 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, infor-

mation 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 (TiO

) 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 c onversion, 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 10−4fortwopeaksinthenear-

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.

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 experimen-

tally demonstrated signiﬁcant enhancement of nonlinear CD159.They

demonstrated that while maintaining high conversion efﬁciency, the non-

linear 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 meta-

surfaces 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 quan-

tities 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 asso-

ciated with a helicity-preserving and helicity-conversion feature, respec-

tively. 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 custo-

mization 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 algorithms166,167,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,markinga

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 meta-

surfaces realize more complex, high-resolution miniaturized display sys-

tems and holograms, marking a signiﬁcant advancement in holographic

technology.

Following the remarkable developments in broadband chiral meta-

surfaces, 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

https://doi.org/10.1038/s44310-024-00018-5 Review article

npj Nanophotonics | (2024) 1:20 18

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, com-

plementing 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 technol-

ogies 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.

Received: 11 February 2024; Accepted: 26 April 2024;

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Acknowledgements

This work was supported by the National Key Research and Development

Program of China (No.2021YFB2802003), National Natural Science

Foundation of China (NSFC) (62075084, 62325503), Guangdong Basic and

Applied Basic Research Foundation (2022B1515020004), Guangzhou

Science and Technology Program (202102020566).

Author contributions

All authors wrote the mainmanuscript text. Qian-mei Deng prepared ﬁgures

1-6. Xin li prepared ﬁgures 7-11. Meng-xia Hu prepared ﬁgures 11-14. All

authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to

Xiangping Li or Zi-Lan Deng.

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