Chiral Metasurfaces: A Review of the Fundamentals and Research Advances

Abstract

Chirality, the absence of mirror symmetry, is predominant in nature. The chiral nature of the electromagnetic field behaves differently with chiral matter for left circularly polarized and right circularly polarized light. The chiroptical behavior in the sensing of naturally occurring chiral objects is weak, and improving the chiroptical response enhances the chiral sensing platform. This review covers the fundamental concepts of chiral metasurfaces and various types of single- and multi-layered chiral metasurfaces. In addition, we discuss tunable and deep-learning-based chiral metasurfaces. Tunability is achieved by manipulating the meta-atom’s property in response to external stimuli for applications such as optical modulation, chiral photonics, advanced sensing, and adaptive optics. Deep-learning modeling techniques, such as CNNs and GANs, offer efficient learning of the complex relationships in data, enabling the optimization and accurate prediction of chiral metasurface properties. The challenges in the design and fabrication of chiral metasurface include achieving broadband performance and scalability and addressing material limitations. Chiral metasurface performance is evaluated by optical rotation, circular dichroism enhancement, and tunability, which are quantified through the spectroscopic measurement of circular dichroism and optical rotation. Chiral metasurface progress enables applications, including metaholography, metalenses, and chiral sensing. Chiral sensing improves the detection of pharmaceuticals and biomolecules, increasing the sensitivity and accuracy of analytical diagnostics.

1. Introduction

The word “chirality” comes from the Greek word name χϵιρ (kheir), which translates to “hand”. Based on the description of chirality, a device is chiral if it is non-superimposable with its mirror image [1,2,3]. Chiral devices have right- or left-handedness and lack any inversion symmetry or mirror symmetry plane [1,4]. Chiroptical response is the optical response of an object with chirality. Chiral media exhibit unique optical responses to the left circularly polarized (LCP) and right circularly polarized (RCP) light, resulting in a chiroptical response. Specifically, optical activity (OA) is the rotation in polarization of a linearly polarized light when it propagates through a chiral media, and circular dichroism (CD) is the absorption difference between RCP and LCP light. Chiroptical response is important for distinguishing enantiomers in chemistry, physics, and biology [5,6].

Artificially engineered chiral metamaterials have been presented to improve the weak chiroptical signals in chiral media [5]. Among these, metasurfaces, which are 2D artificial composite meta-atoms, stand out because of their advantageous fabrication process, cost, and capabilities. They feature characteristic electromagnetic (EM) wave-manipulating abilities similar to those of three-dimensional (3D) metamaterials and support a wide spectrum of designed optical responses owing to their comparative flexibility in structural fabrication [6,7,8,9,10,11,12,13,14,15,16,17,18]. Thus, metasurfaces that respond differently to LCP and RCP have emerged as recent academic and industrial hotspots [14,15,16,17,18,19,20,21]. Chiral metasurfaces play a crucial role in advancing science and technology by enabling precise control of light–matter interactions, paving the way for innovations in material science, optics, and sensing applications. Chiral metasurfaces have demonstrated significant improvements in chiroptical responses in practical applications such as improved CD spectroscopy for biomolecular analysis, enhanced chiral molecule detection in chemical sensing, and advanced chiral photonics components for telecommunications and quantum information processing [22,23,24].

Various geometries of metasurface have been investigated to optimize the chiroptical response, including nanorods [25,26,27,28,29], Slavic symbols [30,31,32], V-shaped nanoantennas [33,34,35,36,37,38], Y-shaped meta-atoms [39,40,41], split-ring resonators [42,43,44,45,46], helix meta-atoms [47,48,49,50,51], twisted cross meta-atoms [52,53], multi-layered arc meta-atoms [54,55,56,57], and others that support optical resonances in subwavelength meta-atoms. Active chiral metasurfaces using optical, electrical, and mechanical methods have also been introduced to actively modify their chiroptical responses [9,10,11,58,59,60,61,62,63,64,65,66].

Metasurfaces designed to improve the chiral optical response can be broadly categorized into single- [21,67,68] and multi-layered [15,20,69,70,71,72,73] chiral metasurfaces. Single-layer chiral metasurfaces are typically favored for their ease of fabrication, simplicity, and lower optical loss, making them suitable for efficient and compact applications like light structuring and metaholography. However, a multi-layered metasurface provides a greater degree of freedom to manipulate waves and attain the desirable chiroptical response, enabling advanced functions such as enhanced sensing sensitivity and polarization manipulation [20,69,70]. In a layered metasurface, geometrical parameters, including the orientation angle, number of layers, and distance between layers, affect the chiroptical response [69,70]. Thus, the choice between single- and multi-layered chiral metasurface depends on the specific application needs and involves balancing design complexity, fabrication constraints, and the target application’s specific requirements.

Recent studies have shown that chiroptical responses such as CD and OR can be tuned using tunable chiral metasurfaces [7,8,9,10,11,58,61,62,63,64,74,75,76,77]. The active modification of chiroptical responses in chiral metasurfaces is attained by external stimuli like electrical fields, magnetic fields, thermal change, or incident light, which changes the properties of individual atoms [58,66]. These changes can be controlled by integrating responsive materials, such as liquid crystals or phase-change materials, into the chiral metasurface design or by using dynamic modulation methods, such as optical pumping or voltage tuning, to attain real-time manipulation over the chiroptical properties [24,60,75]. The computation time required to characterize the chiroptical responses in numerical studies is substantial; thus, some researchers have introduced deep-learning-enabled chiral metasurfaces to improve the computation time and effectively characterize the chiroptical response of chiral structures [78,79,80,81]. Researchers have been successfully employing deep learning in chiral metasurface design by training neural networks, like generative adversarial networks (GANs) and convolutional neural networks (CNNs) using extensive datasets containing chiral metasurface designs or predict chiroptical responses with high speed and accuracy, facilitating the efficient exploration of intricate parameter spaces and overcoming the limitations of traditional design procedures. The advantages of this method include quicker design refinements, improved performance, and the ability to discover novel chiral metasurface configurations for specific applications [79,80,81].

There are several challenges in the fabrication of chiral metasurfaces that researchers must address for practical applications [82,83]. Achieving broadband performance across a wide range of wavelengths remains a challenge, as it often requires precise control over the shape and size of meta-atoms. Scalability to large areas while maintaining structural integrity is another challenge, and addressing material constraints, especially for nonstandard operating conditions, is crucial for the practical deployment of chiral metasurfaces [83,84,85,86].

Herein, we review the fundamentals and different types of chiral metasurfaces. First, the fundamentals of chiral metasurfaces are introduced. These include the optical response, transmittance analysis for different symmetries of chiral structures, and assessment of CD. Second, we introduce different types of chiral metasurfaces. This section discusses single-layered chiral metasurfaces composed of arrays of 2D meta-atoms [87,88] and multi-layered chiral metasurfaces. Multi-layered chiral metasurfaces allow for a greater degree of freedom in manipulating waves to achieve desirable chiral optical responses. In addition, we review some recently developed tunable chiral metasurfaces [58,61,74,75,76,77,89,90,91] that enable active EM wave control. Finally, we cover some of the deep-learning-enabled designs of chiral metasurfaces that reduce the amount of time required for computer simulations [78,79,80,81].

2. Fundamentals of Chiral Metasurfaces

2.1. Chirality and Chiroptical Response

Chirality is a phenomenon in which a device cannot overlap with its mirror through simple translations or rotations, and it is identified as an enantiomer [92,93]. Charity is obvious in various everyday structures and materials [4,39,40,41]. For example, many organic compounds, such as amino acids and DNA molecules, demonstrate chirality, influencing their biological roles. In optics, chiral molecules can rotate the polarization of light, as seen in some sugar solutions, and chiral structures such as spiral seashells display this property in nature. Recent research on artificial meta-atoms, mainly on the optical response of plasmonic meta-atoms, has aided in the development of chiroptical metamaterials. Metasurfaces can provide new platforms for creating chiral materials with adjustable and strong signals by arranging and shaping nanostructures [94,95,96].

The chiroptical response is the optical response of chiral materials caused by chiral meta-atom differences in the refractive index and extinction coefficient for LCP and RCP light, resulting in CD and OA, respectively [4]. OA and CD are both optical aspects associated with chirality, and they are used in various fields such as material science, chemistry, and biochemistry to study and characterize chiral systems. In material science, OA and CD are used to characterize the secondary structure of proteins and study conformational changes, while in chemistry, they are employed for stereochemical analysis of chiral molecules and assessing reaction kinetics. In biochemistry, OA and CD are valuable for evaluating the stability and folding of biomolecules, including proteins, carbohydrates, and nucleic acids. While OA involves the rotation of linearly polarized light, CD involves the absorption difference of circularly polarized light [39,40,41]. Optical rotation (OR), the specific manifestation of OA, occurs when a linearly polarized light beam passes through a chiral medium. CD is the difference between the transmittances of the LCP and RCP light [97].

2.2. The Optical Response of Chiral Media

The constitutive relationships that typically illustrate the optical response of a broadly chiral material are as follows:

$$\overline{\mathrm{D}}={\mathsf{ϵ}}_0\stackrel{̿}{\mathrm{\epsilon }}\overline{\mathrm{E}}+\frac{\mathrm{i}}{{\mathrm{c}}_0}\stackrel{̿}{\mathrm{\chi }}\overline{\mathrm{H}},$$

(1)

$$\overline{\mathrm{B}}=-\frac{\mathrm{i}}{{\mathrm{c}}_0}\stackrel{̿}{\mathrm{\chi }}\overline{\mathrm{E}}+{\mathsf{\mu }}_0\stackrel{̿}{\mathrm{\mu }}\overline{\mathrm{H}},$$

(2)

where ${\mathsf{ϵ}}_0$ and ${\mathsf{\mu }}_0$ are the permeability and permittivity of the vacuum, respectively. The magnetic field is $\overline{\mathrm{H}}$, the electric displacement is $\overline{\mathrm{D}}$, the electric field is $\overline{\mathrm{E}}$, and the magnetic induction is $\overline{\mathrm{B}}$. Additionally, $\stackrel{̿}{\mathrm{\mu }},\stackrel{̿}{\mathrm{\chi }}$, and $\stackrel{̿}{\mathrm{\epsilon }}$ are the permeability, chirality tensor, and permittivity, respectively. The chirality tensor quantifies the asymmetrical response to EM fields, elucidating how they generate or interact with OR and CD. This tensor is vital for understanding and predicting chiroptical properties, accelerating the design of objects for applications like optical signal control and chiral sensing. For an isotropic material $\stackrel{̿}{\mathrm{\mu }},\stackrel{̿}{\mathrm{\chi }}$, and $\stackrel{̿}{\mathrm{\epsilon }}$ could be simplified to scalars μ, χ, and ε, which supports different responses to magnetic and electric fields for the RCP and LCP incidents, the RCP and LCP light have different refractive indices(n), which are given by

$${\mathrm{n}}_{\pm }=\sqrt{\mathsf{\epsilon }\mathsf{\mu }}\pm \mathrm{\chi }.$$

(3)

According to Equation (3), waves with opposing handedness would accumulate phases differently as they pass through chiral materials. However, the impedances of both circular polarizations are similar:

$$\mathrm{Z}={\mathrm{Z}}_0\sqrt{\frac{\mathsf{\mu }}{\mathsf{\epsilon }}},$$

(4)

where ${\mathrm{Z}}_0$ represents impedance in vacuum.

Impedance represents the resistance an EM wave encounters as it travels through a medium. RCP and LCP waves with the same amplitude were superimposed to form a linearly polarized wave. Equation (3) makes it simple to demonstrate that the different refractive indices (${\mathrm{n}}_{+}$, ${\mathrm{n}}_{-}$) of the LCP and RCP waves cause the linear polarization to rotate by angle φ.

$$\mathsf{\phi }=\frac{({\mathrm{n}}_{+}-{\mathrm{n}}_{-})\mathrm{\pi }\mathrm{d}}{{\mathsf{\lambda }}_0} \mathrm{or} \mathrm{can} \mathrm{also} \mathrm{be} \mathsf{\phi }=\frac{[\mathrm{a}\mathrm{r}\mathrm{g}\left({\mathrm{T}}_{+}\right)-\mathrm{a}\mathrm{r}\mathrm{g}\left({\mathrm{T}}_{-}\right)]}{2},$$

(5)

where ${\mathsf{\lambda }}_0$ represents the wavelength of the light in vacuum, and d represents the chiral medium’s thickness. ${\mathrm{T}}_{+}$ and ${\mathrm{T}}_{-}$ are the coefficient of transmission for the right and left spin states, respectively. OR involves both physical effects and processes.

2.3. Jones Matrix and Circular Dichroism

The incident and scattered field’s amplitude are also associated with the Jones matrices, which simplify the characterization of the optical responses of planar materials [92]. Jones matrices are used in experimental setups to depict the transformation of polarization states as light interacts with planar chiral materials, allowing for the prediction and analysis of optical responses like OR and CD. They provide a comprehensive and mathematically rigorous framework to design and understand chiroptical objects, making it simple to manipulate and control polarized light in different applications like imaging, sensing, and communications [98].

$$\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{r}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{r}}}^{\mathrm{y}}\end{array}\right]=\left[\begin{array}{cc}{\mathrm{r}}_{\mathrm{x}\mathrm{x}}& {\mathrm{r}}_{\mathrm{x}\mathrm{y}}\\ {\mathrm{r}}_{\mathrm{y}\mathrm{x}}& {\mathrm{r}}_{\mathrm{y}\mathrm{y}}\end{array}\right]\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{y}}\end{array}\right]=\mathrm{R}\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{y}}\end{array}\right],$$

(6)

$$\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{t}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{t}}}^{\mathrm{y}}\end{array}\right]=\left[\begin{array}{cc}{\mathrm{t}}_{\mathrm{x}\mathrm{x}}& {\mathrm{t}}_{\mathrm{x}\mathrm{y}}\\ {\mathrm{t}}_{\mathrm{y}\mathrm{x}}& {\mathrm{t}}_{\mathrm{y}\mathrm{y}}\end{array}\right]\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{y}}\end{array}\right]=\mathrm{T}\left[\begin{array}{c}{{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{x}}\\ {{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{y}}\end{array}\right],$$

(7)

where ${{\mathrm{E}}_{\mathrm{i}}}^{\mathrm{x}}$, ${{\mathrm{E}}_{\mathrm{r}}}^{\mathrm{x}}$, and ${{\mathrm{E}}_{\mathrm{t}}}^{\mathrm{x}}$ are the incident, reflected, and transmitted polarized electric fields along the x-direction, correspondingly. T and R are the transmission and reflection matrices for linear polarization, respectively. The electric fields polarized in the y-direction are denoted by symbols identical to the superscripts.

$${{\mathrm{T}}_{\mathrm{circ}}}_{ }=\left[\begin{array}{cc}{\mathrm{t}}_{++}& {\mathrm{t}}_{+-}\\ {\mathrm{t}}_{-+}& {\mathrm{t}}_{--}\end{array}\right]=\frac{1}{2}\left[\begin{array}{cc}{\mathrm{t}}_{\mathrm{x}\mathrm{x}}+{\mathrm{t}}_{\mathrm{y}\mathrm{y}}+\mathrm{i}({\mathrm{t}}_{\mathrm{x}\mathrm{y}}-{\mathrm{t}}_{\mathrm{y}\mathrm{x}})& {\mathrm{t}}_{\mathrm{x}\mathrm{x}}-{\mathrm{t}}_{\mathrm{y}\mathrm{y}}-\mathrm{i}({\mathrm{t}}_{\mathrm{x}\mathrm{y}}+{\mathrm{t}}_{\mathrm{y}\mathrm{x}})\\ {\mathrm{t}}_{\mathrm{x}\mathrm{x}}-{\mathrm{t}}_{\mathrm{y}\mathrm{y}}+\mathrm{i}({\mathrm{t}}_{\mathrm{x}\mathrm{y}}+{\mathrm{t}}_{\mathrm{y}\mathrm{x}})& {\mathrm{t}}_{\mathrm{x}\mathrm{x}}+{\mathrm{t}}_{\mathrm{y}\mathrm{y}}-\mathrm{i}({\mathrm{t}}_{\mathrm{x}\mathrm{y}}-{\mathrm{t}}_{\mathrm{y}\mathrm{x}})\end{array}\right]={\mathrm{M}}^{-1}\mathrm{T}\mathrm{M},$$

(8)

$${{\mathrm{R}}_{\mathrm{circ}}}_{ }=\left[\begin{array}{cc}{\mathrm{r}}_{++}& {\mathrm{r}}_{+-}\\ {\mathrm{r}}_{-+}& {\mathrm{r}}_{--}\end{array}\right]=\frac{1}{2}\left[\begin{array}{cc}{\mathrm{r}}_{\mathrm{x}\mathrm{x}}+{\mathrm{r}}_{\mathrm{y}\mathrm{y}}+\mathrm{i}({\mathrm{r}}_{\mathrm{x}\mathrm{y}}-{\mathrm{r}}_{\mathrm{y}\mathrm{x}})& {\mathrm{r}}_{\mathrm{x}\mathrm{x}}-{\mathrm{r}}_{\mathrm{y}\mathrm{y}}-\mathrm{i}({\mathrm{r}}_{\mathrm{x}\mathrm{y}}+{\mathrm{r}}_{\mathrm{y}\mathrm{x}})\\ {\mathrm{r}}_{\mathrm{x}\mathrm{x}}-{\mathrm{r}}_{\mathrm{y}\mathrm{y}}+\mathrm{i}({\mathrm{r}}_{\mathrm{x}\mathrm{y}}+{\mathrm{r}}_{\mathrm{y}\mathrm{x}})& {\mathrm{r}}_{\mathrm{x}\mathrm{x}}+{\mathrm{r}}_{\mathrm{y}\mathrm{y}}-\mathrm{i}({\mathrm{r}}_{\mathrm{x}\mathrm{y}}-{\mathrm{r}}_{\mathrm{y}\mathrm{x}})\end{array}\right]={\mathrm{M}}^{-1}\mathrm{R}\mathrm{M}.$$

(9)

M = $\frac{1}{\sqrt{2}}\left(\begin{array}{cc}1& 1\\ \mathrm{i}& -\mathrm{i}\end{array}\right)$ was used to switch the basis from Cartesian to circular. The subscript ”−” denotes LCP wave, “+” denotes RCP wave, and likewise “t₊₋” represents LCP incidence and RCP transmission. ${\mathrm{r}}_{+-}$, ${\mathrm{r}}_{--}$, ${\mathrm{r}}_{-+}$, and ${\mathrm{r}}_{++}$ are the reflection coefficients of LCP to RCP, LCP to LCP, RCP to LCP, and RCP to RCP, respectively.

Symmetry considerations can notably influence the optical properties of chiral metasurfaces, potentially reducing or enhancing specific chiral effects [83,99,100,101]. For example, C4 symmetry, signifying four-fold rotational symmetry, leads to orientation-dependent chiral effects, essential for designing tailored chiral metasurfaces in applications such as chiral sensing and polarization manipulation. For periodically arranged metamaterials with four-fold (C4) or three-fold (C3) rotational symmetries, the Jones matrices can be expressed as [56]:

$${\mathrm{T}}_{\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}}=\left(\begin{array}{cc}{\mathrm{t}}_{\mathrm{x}\mathrm{x}}+\mathrm{i}{\mathrm{t}}_{\mathrm{x}\mathrm{y}}& 0\\ 0& {\mathrm{t}}_{\mathrm{x}\mathrm{x}}-\mathrm{i}{\mathrm{t}}_{\mathrm{x}\mathrm{y}}\end{array}\right),\mathrm{a}\mathrm{n}\mathrm{d} {\mathrm{R}}_{\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}}=\left(\begin{array}{cc}{\mathrm{r}}_{\mathrm{x}\mathrm{x}}& 0\\ 0& {\mathrm{r}}_{\mathrm{x}\mathrm{x}}\end{array}\right)={\mathrm{r}}_{\mathrm{x}\mathrm{x}}\mathrm{I},$$

(10)

where I represents the identity matrix.

The transmission coefficient matrix can be used in conjunction with the rotation matrix (Dr) to analyze symmetrical circumstances by calculating a transmission matrix Tr.

$$\mathrm{Tr}={\mathrm{Dr}}^{-1}{\mathrm{T}}^{\mathrm{f}} \mathrm{Dr}=\left[\begin{array}{cc}\cos \mathsf{\theta }& -\sin \mathsf{\theta }\\ \sin \mathsf{\theta }& \cos \mathsf{\theta }\end{array}\right]\left[\begin{array}{cc}{\mathrm{t}}_{\mathrm{x}\mathrm{x}}& {\mathrm{t}}_{\mathrm{x}\mathrm{y}}\\ {\mathrm{t}}_{\mathrm{y}\mathrm{x}}& {\mathrm{t}}_{\mathrm{y}\mathrm{y}}\end{array}\right]\left[\begin{array}{cc}\cos \mathsf{\theta }& \sin \mathsf{\theta }\\ -\sin \mathsf{\theta }& \cos \mathsf{\theta }\end{array}\right].$$

(11)

For chiral structures that possess a second two-fold) symmetry (C2) in relation to either the x-axis or y-axis, the transmission coefficient matrix becomes

$${\mathrm{T}}_{\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}}=\frac{1}{2}\left(\begin{array}{cc}{\mathrm{t}}_{\mathrm{x}\mathrm{x}}+{\mathrm{t}}_{\mathrm{y}\mathrm{y}}+2\mathrm{i}{\mathrm{t}}_{\mathrm{x}\mathrm{y}}& {\mathrm{t}}_{\mathrm{x}\mathrm{x}}-{\mathrm{t}}_{\mathrm{y}\mathrm{y}}\\ {\mathrm{t}}_{\mathrm{x}\mathrm{x}}-{\mathrm{t}}_{\mathrm{y}\mathrm{y}}& {\mathrm{t}}_{\mathrm{x}\mathrm{x}}+{\mathrm{t}}_{\mathrm{y}\mathrm{y}}-2\mathrm{i}{\mathrm{t}}_{\mathrm{x}\mathrm{y}}\end{array}\right).$$

(12)

The factors that influence the intensity difference between LCP and RCP light include the wavelength of the incident light, the chiral sample’s structure, the concentration, the sensitivity of the CD instrument, and the path length through the chiral sample. Following the definition of CD as the difference in absorption of LCP light and RCP light, its intensity was determined as [16]:

$$\mathrm{CD}={\left|{\mathrm{t}}_{++}\right|}^2-{\left|{\mathrm{t}}_{--}\right|}^2.$$

(13)

3. Types of Chiral Metasurfaces

Chiral metasurfaces are artificial structures made of various materials and configurations that are used to improve the chiroptical signal. Various chiral metasurfaces with excellent chiroptical properties have been developed. To explore the mechanisms of increasing the chiroptical response, we divided chiral metasurfaces into single-layered and multi-layered. In addition, we discuss tunable and deep-learning-enabled chiral metasurfaces.

3.1. Single-Layered Chiral Metasurfaces

We discuss the geometrical patterns of single-layered chiral metasurfaces and classify them based on their symmetry. To achieve giant chirality, a series of unit cell meta-atoms, such as gammadions [30,102], Z-shapes [21], L-shapes [103], split rings [92,104], U-shapes [43], S-shapes [93], wind-mill structures [105], and other periodically arranged shapes [33,34,35,36,37,38] have been used. The symmetry type that occurred in the geometrical patterns of chiral metasurface structures is an intriguing feature. Most chiral structures exhibited C4 or C2 symmetry.

Chiral structures do not exhibit in-plane mirror symmetries and are pure chiral structures; however, several have C4 rotational symmetry. The C4 rotational symmetry prevents anisotropic effects that lead to polarization conversion [69,106]. C4 symmetric single-layered chiral metasurfaces have potential applications in areas including circular polarizers, advanced optical components, and chiral sensing devices [82,106,107]. Some of the C4 symmetric single-layered chiral metasurfaces are shown in Figure 1.

Gammadion structures are among the C4 rotationally symmetrical structures mentioned in several references [82,107,108]. The gold-gammadion structure enhanced the chiroptical response in terms of enantiomeric sensing [82]. The orthogonal orientation of the arms with one another and the associated conversion of linear polarization trigger chiral behavior in gammadion chiral structures [107].

Giessen et al. introduced a gold-gammadion chiral metasurface for enantiomeric sensing [82]. The gammadion structure was embedded in air, and chirality was locally improved mainly at the back and front of the arms (Figure 1a,b). However, the chiroptical response was small, and the regions with improved chiroptical responses were at the same location.

An enhanced chiroptical response was achieved in a C4 planar dielectric gammadion nanostructure, in which the radiation patterns in the visible spectrum were surrounded by strong magnetic and electric higher-order multipole responses [107]. In contrast to the typical dipole approximation, which describes the chiral event, an improved chiroptical response was obtained by adjusting the multipole higher-order response, which is important for off-normal scattering. As shown in Figure 1d, three opposite-sign antinodes are observed in the in-plane component, where approximately 87% of the RCP wave is transmitted in zero order [107].

In addition, a C4 THz chiral metasurface that has robust OA under a normal incidence was introduced using a folded meta-atom with a gammadion shape to initiate an effective cross-coupling in between the magnetic and electric fields (Figure 1f) [82,109]. This pseudo-planar metasurface (PPM) comprises a single-layer metasurface with each meta-atom deflected into a 3D configuration. Similar to 3D metamaterials, PPMs exhibit OA that is triggered by the chiral geometry of the meta-atoms, which produces potent and controllable optical chirality [109].

Single-layer chiral metasurfaces can demonstrate improved nonlinear optical responses compared to traditional nonlinear materials because of their engineered nanostructures that allow precise control over local field enhancement and phase matching [108]. This improved nonlinearity has advantages such as reduced power requirements for nonlinear effects, tailored nonlinear responses, and compact device designs, aiding applications in signal processing, frequency conversion, and efficient light generation for nonlinear optical processes. A single-layered chiral metasurface with a significantly improved nonlinear optical response and giant nonlinear CD was suggested [108]. It is the first nonlinear chiral metasurface framework that combines multiple quantum well (MQW) structures with massive second- and third-order nonlinear responses with a C3 Trisceli type and a C4 gammadion type plasmonic chiral nanoresonators for second harmonic generation (SHG) and third harmonic generation (THG), correspondingly (Figure 1g,h). Unlike previous chiral metasurfaces, it employs intersubband transition coupling as well as chiral cavity modes. This method demonstrates excellent THG-CD and SHG-CD value in the mid-IR spectra from a nonlinear chiral metasurface [108].

Figure 1. C4 single-layered chiral metasurfaces. The chiroptical response improvement using a gammadion configuration subjected to incident light of (a) LCP and (b) RCP at a 2.01 μm wavelength. ((a,b) reproduced from [82]). (c) Optical response of a gammadion metasurface composed of gammadion TiO₂ placed on thin film TiO₂ on a substrate made of glass. (d) Simulated CD response (the yellow dotted line) and experimental CD response (the black solid line). (e) I_yz cut plane demonstrating the magnetic field (the colored field plot) Hx and the corresponding distribution of current (the arrows that are black) J_z+J_y in a gammadion metasurface under RCP (right) and LCP (left) light. ((c–e) reproduced from [107]). (f) Pseudo-planar gammadion chiral metasurface, where the gammadion meta-atoms are deposited on a planar substrate with a squared trench and are deflected on the trench. ((f) reproduced from [109]). (g,h) Wavelength-based THG-CD from simulated (black) and measured (red) data of the C4 and C3 of the intersubband polaritonic metasurfaces, respectively. ((g,h) reproduced from [108]). (i) Three-dimensional (3D) view of the subwavelength crescent aperture (SCA) made in the MDM layer. (j) CD responses of the SCA. ((i,j) reproduced from [88]). (k) CD spectra comprising both-handed chiral metasurfaces, that result in an excellent mirror image. ((j,k) reproduced from [83]).

Figure 1. C4 single-layered chiral metasurfaces. The chiroptical response improvement using a gammadion configuration subjected to incident light of (a) LCP and (b) RCP at a 2.01 μm wavelength. ((a,b) reproduced from [82]). (c) Optical response of a gammadion metasurface composed of gammadion TiO₂ placed on thin film TiO₂ on a substrate made of glass. (d) Simulated CD response (the yellow dotted line) and experimental CD response (the black solid line). (e) I_yz cut plane demonstrating the magnetic field (the colored field plot) Hx and the corresponding distribution of current (the arrows that are black) J_z+J_y in a gammadion metasurface under RCP (right) and LCP (left) light. ((c–e) reproduced from [107]). (f) Pseudo-planar gammadion chiral metasurface, where the gammadion meta-atoms are deposited on a planar substrate with a squared trench and are deflected on the trench. ((f) reproduced from [109]). (g,h) Wavelength-based THG-CD from simulated (black) and measured (red) data of the C4 and C3 of the intersubband polaritonic metasurfaces, respectively. ((g,h) reproduced from [108]). (i) Three-dimensional (3D) view of the subwavelength crescent aperture (SCA) made in the MDM layer. (j) CD responses of the SCA. ((i,j) reproduced from [88]). (k) CD spectra comprising both-handed chiral metasurfaces, that result in an excellent mirror image. ((j,k) reproduced from [83]).

A C4 metal–dielectric–metal planar plasmonic chiral metasurface with crescent meta-atoms arranged in a specific orientation also exhibited multiband CD at near-infrared (NIR) wavelengths (Figure 1i) [88]. In addition, the symmetric orientation of the metasurface affected the CD peak. The CD values identified as CD1, CD2, CD3, and CD4 were obtained in the 1565 nm, 999 nm, 878 nm, and 810 nm wavelengths, correspondingly, as shown in Figure 1j [88]. Designing a meta-atom at visible frequencies was a significant challenge [110]. To address the challenge, [111] proposed the use of a chiral-selective plasmonic metasurface consisting of an η shape operating in the visible frequency. This chiral metasurface generates a CD peak of around 0.5 due to the selective absorptive property of the plasmonic chiral metasurface [111].

Furthermore, a C4 symmetric chiral metasurface with nanoslits arranged on a layer of thin gold exhibited a potent controllable chiroptical resonant response from visible to NIR frequencies [83]. This metasurface emphasizes couplings in between surface-propagating and localized forms, allowing for an additional degree of freedom in manipulating the chiroptical response. As shown in Figure 1k, the CD spectra are comparable to the chirality strength provided by more sophisticated 3D structures, which require significantly more complicated fabrication methods.

The structure with C2 symmetry comprised two identical parts. In C2 rotational symmetry, the element remains invariant after a π rotation in the z-axis (Figure 1d). Streptavidin tetramers, hemoglobin tetramers, and dimeric proteins are among the few biological structures with C2 symmetry [3,99,100,101]. Single-layered chiral metasurfaces with C2 rotational symmetry are suitable for enhancing the chiroptical responses of biomolecules with C2 symmetry. Five significant examples of C2 symmetric metasurfaces are presented in Figure 2.

A single-layer dielectric metasurface composed of a C2 symmetric arrangement of 2D meta-atoms, as shown in Figure 2a, produces a chiroptical effect [21]. For LCP illumination at 1550 nm, the meta-atom transmits RCP and reflects LCP waves for an incident RCP wave (Figure 2a). The C2 symmetric unit element shows an asymmetric transmission (AT) of 0.74 in the transmission of an incident LCP wave and an AT of 0.8 in the reflection of the RCP incident light at 1550 nm (Figure 2b) [21]. A variety of phase-dependent phenomena can be realized because these designs can achieve full-phase control. However, the chiroptical response obtained is insufficient for certain applications.

Similarly, Hong et al. introduced a low-loss all-dielectric chiral metasurface that has gigantic CD and AT values near unity [102]. Because of the negligible absorption loss beyond 1.5 μm of germanium (Ge), the illumination light would either be reflected or transmitted, resulting in the CDs for reflection and transmission being roughly opposite (Figure 2c). The massive chirality (approximately 88%) of these and similar Ge metasurfaces far exceeds the limit of prior ultrathin metallic chiral metasurfaces. The dielectric nature of these metasurfaces allows them to reduce the absorption loss and mitigate the fabrication problems associated with metal-based chiral metasurfaces. In addition, these designs offer a platform for controlling wavefronts by modifying them in line with the handedness of circularly polarized light (CPL) to create multipurpose chiral metasurfaces [102].

The spiral chiral metasurfaces (Figure 2e) have fewer sharp corners, allowing for a smooth distribution of fields with enhanced optical chirality [82]. The spiral meta-atom has a rotation of one and a half, whereas the gammadion has only one 90-degree kink within each arm. Thereby, the two polarizations of incidents exhibited distinct behaviors. Compared to C4 gammadion meta-atoms, nanospiral meta-atoms offer stronger optical chirality and more uniform enhancement with a stronger twist. However, because of their discontinuous responses and fabrication challenges, these spiral metasurfaces are impractical for some applications [82].

Despite the rapid advancements in planar and dielectric chiral metasurfaces, there are some limitations, such as absorption and scattering losses, discontinuous responses, and limited chiroptical responses. These limitations can be overcome using chiral metasurfaces with bound states in the continuum (BICs) [112,115,116,117,118,119,120,121,122,123,124]. BICs are EM modes that exist within the radiation continuum but do not radiate energy away [125]. In chiral metasurface, BICs can be harnessed to improve chiroptical response by concentrating and confining EM fields at specific frequencies, leading to stronger interactions with chiral materials [112,118,119,120,121,122]. Tailoring chiral metasurfaces to host BICs at specific wavelengths enhances the sensitivity and efficiency of chiroptical devices such as modulators and sensors [125]. Moreover, BIC non-coplanar symmetry breaking has a direct chirality manipulation potential [112]. Thus, the choice between plasmonic and dielectric materials for single-layer chiral metasurfaces rests on factors such as desired operation wavelength, fabrication feasibility, and material losses [82,102,113,114]. Dielectric materials, with broader spectral ranges, and lower losses are preferred for applications requiring broadband chiroptical responses. Plasmonic materials provide strong field confinement and sensitivity but are limited to specific narrowband applications because of their inherent losses, emphasizing the importance of material selection for optimizing the chiral metasurface’s performance for various scenarios [113,114,126].

A chiral, substantially transparent metasurface was achieved by transforming BICs into quasi-BICs with the highest chiroptical response, which resulted in the highest peak unit height in the CD spectrum [112]. To perturbate the antiparallel dipole BICs in the symmetric dimers of dielectric bars, the design features a slight vertical shift distance d and a corresponding rotation angle θ. The lattice in Figure 2f has a reflection symmetry regarding three forms of mirror planes: σ₁ and σ₂ and the plane of the drawing. When the vertical C2 axes (the point where σ₁ and σ₂ planes intersect) pass exactly through their centers, all the dimers are symmetric (Figure 2f). The chirality of different kinds of resonant chiral metasurfaces that work in the visible and NIR spectral regions can be enhanced using this design. However, this structure is not as simple to fabricate as pure planar chiral metasurfaces.

Similarly, a chiral metasurface with C2 symmetry, which supports BICs, exhibits chiroptical responses with almost perfect circular dichroism (CD = 0.99) [11]. To create the BIC state, the author used a scythe-shaped double-sided-Si inclusion that has C2 inversion symmetry and without mirror symmetry (Figure 2h). Intrinsic chirality was achieved with a CD peak of > 0.99 at W1 = 191 nm, W2 = 231 nm, and δ = 40 nm and a wavelength of 1392 nm by proposing an in-plane simple geometric asymmetry variable of δ = W₂−W₁ [11]. Extrinsic chirality with C2 symmetry of L₁ = L₂, W₁ = W₂, and L₁ ≠ W₁, at an oblique illumination of θ = 8° and φ = 90° resulted in a strong CD with a peak of over 0.95 (Figure 2i) [11]. These metasurfaces are easily fabricated and have a broad spectrum of applications, such as nonlinear filters, chiral lasers, and tunable chiroptical devices.

To solve the low Q-factor resonance of plasmonic chiral metasurfaces, and to overcome the trade-off between the CD and Q-factor in the BIC chiral metasurfaces, plasmonic chiral BIC metasurfaces were proposed [114,127]. Ref. [114] suggested a plasmonic BIC metasurface with a honeycomb-shaped gold hole metasurface to generate strong CD (Figure 2n). The study proved that with the increment of the period of the structure (hole spacing) the CD decreases gradually (Figure 2o). Spin-selective absorption is facilitated by the structure through the excitation of chiral quasi-BIC resonances, as shown in Figure 2o [114]. However, the CD and Q-factor are strongly correlated due to the configuration, which limits the independent manipulation of the CD and Q-factor. To overcome the trade-off, [113] proposed a novel plasmonic chiral BIC metasurface, as shown in Figure 2l. The metasurface is composed of an integrated-resonance unit that has a twisted vertical split-ring resonator (SRR) as well as a wall. This plasmonic chiral BIC metasurface exhibits a strong CD of around 0.67 and high-Q BIC resonance (Figure 2m). The result shows a new CD manipulation degree of freedom in high-Q plasmonic resonances, which makes it applicable for the detection of chiral molecules and quantum information processing [113]. Fabricating high-Q plasmonic chiral metasurfaces poses challenges related to maintaining structural integrity and lowering losses. These challenges have been addressed through advanced nanofabrication methods, materials with lower optical losses, and precise control over the geometry to obtain both strong CD and high-Q resonance [113,114].

3.2. Multi-Layered Chiral Metasurface

Despite significant progress in single-layered chiral metasurfaces, the metasurface design frequently requires significant computational time and intensive heavy optimization techniques for a better chiroptical response. In single=layer chiral metasurfaces, the interactions between light and ultrathin meta-atoms are usually constrained, yielding a reduced efficiency and constrained controllability for practical applications [128,129,130,131,132,133]. 3D metamaterials have been used to solve controllability issues; however, they also have a complex fabrication process and a loss that reduces their efficiency [134]. Multi-layered chiral metasurfaces are more controllable and efficient compared to 3D metamaterials due to their planar geometry, which enables precise control of the chiroptical responses at multiple wavelengths and ease of fabrication [71,72,73]. Multi-layered chiral metasurfaces, on the other hand, comprehensively manipulate the phase whilst keeping a substantial level of conversion efficiency. They can also provide a large chiroptical response, AT, and full-phase manipulation of EM waves [71,72,73,135]. The computational difficulties in multi-layer chiral metasurfaces arise because of the increased interaction and complexity between the layers, leading to higher computational resources and longer simulation times required. These challenges were addressed by offering design flexibility to fine-tune the chiroptical responses while reducing the computational demands by layer decoupling, making them manageable for optimization and simulations [134,136,137].

The presence of near-field coupling and interference within the layers in a multi-layered chiral metasurface plays a crucial role in substantially redistributing energy and facilitates interaction with the meta-atoms. The key factors considered when customizing the multi-layered chiral metasurfaces for a broadband chiroptical response include adjusting the magnetoelectric couplings in between layers to control the overall response, carefully spacing layers to obtain phase matching along a broad wavelength range and optimizing the inter-layer interactions to enhance the chirality [84,85,136,137]. The number of layers is also carefully selected to balance the bandwidth and complexity, ensuring that the chiral metasurface demonstrates the required chiroptical response [70,85]. Thus, by customizing the magnetoelectric couplings, inter-layer interactions, spacing within layers, the number of layers, and orientation angle, a broadband chiroptical response can be achieved [70,134,136,137]. Some multi-layered chiral metasurfaces that use the orientation angle and the number of layers to enhance the chiroptical response are presented in Figure 3.

It has been demonstrated that by adding a twist to the lattice orientation of the metasurface inclusions, a chiral metasurface with an exotic effect analogous to 3D metamaterials can be achieved [84]. The bandwidth of the multi-layered chiral metasurface was affected by the rotation angle (Figure 3i). In addition, the CD value increased rapidly when more layers were added in a cascade, and in the case of seven layers, the CD virtually encompassed the full visible spectrum (Figure 3a,b). These twisted arrays of chiral metasurfaces were fabricated by conventional lithographic techniques without the need for sophisticated inclusion shapes.

It was also proved that the CD value can be enhanced with the addition of layers in a gradient C-shaped SRR array (Figure 3e) [85]. In contrast to the single-layered chiral metasurfaces (Figure 3d), the four-layer chiral metasurfaces (Figure 3f) show a higher transmission difference [85]. A high conversion coefficient was achieved over a relatively wide bandwidth with a fully controlled phase, and the CPL wave output purity was improved.

Moreover, in a chiral gold ultrathin nanowire film designed using the Langmuir–Schaefer method, the chirality could be monitored using the orientation angle and the number of layers [86]. The maximum CD response of the two-layered gold nanowire assembly was obtained at 45° (Figure 3h), and no CD response was obtained at inter-layer angles of 0° or 90°. In addition, at a fixed 45° inter-layer angle, adding a layer to the two layers almost doubles the CD response (Figure 3i). This multi-layered chiral metasurface was fabricated using a bottom-up assembly, as opposed to the top-down method used to fabricate the previously described chiral metasurfaces, which makes it suitable for a range of different applications, including chiral recognition, separation, and catalysis. Such multi-layered chiral metasurfaces have been used in chiral recognition and separation by optimizing their chiroptical responses to selectively detect and separate enantiomers in biomolecules and pharmaceuticals [86]. In catalysis, they have been used to improve the chiral selectivity in chemical reactions by controlling the phase and polarization of incident light, leading to enhanced reaction outcomes in asymmetric synthesis. Additionally, it exhibited the best performance of the bottom-up-fabricated chiral nanostructures, with a significant CD response and wavelength coverage spanning the visible and NIR ranges.

In a twisted multi-layered chiral metasurface composed of helically organized oligomers, an ultrasharp CD can be obtained, which can be used in a chiral device that operates in the NIR region (Figure 3j) [70]. In these chiral metasurfaces, the chiroptical response originates from the coupling of three layers and was examined by evaluating the CD response and electrical field [70]. CD progressively grew, peaked, and finally declined as the angle raised from 0° to 45°. The highest CD was obtained in the NIR region at an angle of 28° and inter-layer distance of 100 nm (Figure 3k,l). Unlike previously reported multi-layered chiral metasurfaces, the coupling of oligomers and the interactions between the layers were clearly examined.

In addition, multi-layered chiral metasurfaces have been proposed to obtain a dual-band AT of linearly and circularly polarized waves [127,138,139]. A bilayer chiral metasurface that can produce dual AT for linearly polarized light and CPL in NIR was suggested by [135] (Figure 3m). The metasurface gives a maximum AT of around 0.65 and 0.6 for the linearly polarized and circularly polarized waves, respectively (Figure 3n). Moreover, [16] suggested a novel chiral folded metasurface consisting of antisymmetric SRRs that have spin-selective transmission and pronounced controllable intrinsic chirality in the NIR (Figure 3o). The design consists of breaking the mirror symmetry by folding the components of the metasurface at a certain angle, as shown in Figure 3o. A CD value of 0.7 was obtained due to the intrinsic chirality of the chiral metasurface (Figure 3p).

Sequential deposition and lithography techniques, such as electron-beam lithography, are often used to fabricate multi-layered chiral metasurface [70,84,85,86,134,136,137]. These techniques are generally more complex compared to the fabrication of single-layered chiral metasurfaces due to the need for precise control and alignment of multi-layers. However, they offer greater versatility in tailoring the chiroptical response, and they are essential for achieving advanced functionalities along a broad range of wavelengths. Sharp CD peaks in multi-layered metasurfaces have various potential applications, including thermal-radiative detection, chiral-optic spectroscopy, and NIR telecommunication systems [84,127,134,136,137,138,139].

3.3. Tunable Chiral Metasurfaces

In addition to extensive research on enhancing the optical properties of chiral metasurfaces, tunability has emerged as the most recent advancement in this field of study. Tunable chiral metasurfaces offer improved approaches for the theoretical investigation of distinctive EM and optical features. Some tunable chiral metasurfaces are shown in Figure 4, Figure 5 and Figure 6.

The CD and OR can be tuned by creating building blocks from materials that undergo a phase shift [61]. Phase-change materials (PCMs), which have EM properties that change dramatically when heated, are widely used as thermally modulated materials. Recently, a phase-change material, vanadium dioxide (VO₂), was utilized to create electro-optic switches, thermal switches, and all-absorber metasurfaces. The use of phase-change materials such as VO₂ allows for the tuning of chiroptical response in the metasurface by exploiting their reversible phase transition between metallic and insulating states in response to external stimuli such as temperature or light [61,91]. This property, when integrated into the chiral metasurface, enables dynamic control of chiroptical response, making them suitable for applications such as reconfigurable lenses, adaptive optics, and modulators, where real-time tuning of optical properties is critical for optimizing device performance in varying conditions or applications [140,141,142].

In a study by Mandal et al. [91], a novel chiral plasmonic structure with a dagger-like shape composed of metal and metal–VO₂ alloys was proposed, as shown in Figure 4a. Using an exogenous thermal stimulus, the hybrid structure was tuned, displaying switching in the CD spectrum with minimal chirality in the OFF state (corresponding to the metallic phase of VO₂) and conversely maximum chirality in the ON state (corresponding to the semiconducting phase of VO₂). The phase of the VO₂ phase-change material switches from the state of semiconducting to metallic (Figure 4b). Fascinatingly, external temperature control of the hybrid system allows for the dynamic modulation of the VO₂ phase, effectively turning the device into a CD switch (Figure 4c).

Using the metal-to-insulator shift characteristics of VO₂, thermal switching of perfect absorption and AT effects can be attained in the NIR [140]. When the VO₂ is in the metallic state, the metasurface device behaves as a chiral-selective plasmonic absorber that can result in a CD signal of around 0.7. When the state of VO₂ is in insulation, the proposed metasurface exhibits a dual-band AT effect [140].

Similarly, incorporating VO₂ into a metasurface device enabled the achievement of a switching effect between absorption and AT, attributed to the VO₂ phase-changing property [141]. When the VO₂ behaves as a metal, the metasurface becomes a selective absorber for linearly polarized light, and by changing the VO₂ phase, it achieves a broadband AT effect. The chiroptical response of this metasurface device can also be realized with the AT effect [141]. In addition, a switchable and tunable terahertz metasurface that results in the AT effect and dual polarization conversions can be achieved by integrating VO₂ with graphene [142]. Hence, using VO₂, a phase-change material known for its conductivity change in response to thermal, electrical, and optical stimuli, it becomes possible to tune a chiral metasurface [91,140,141,142].

Meanwhile, recent studies have shown that the CD and OR can be tuned by electrical doping [58,66]. Most of these devices use graphene owing to its high electrical conductivity, continuous chemical resistance, and wide electro-optical spectrum [59,84,85]. The Fermi energy level of the graphene could be tuned, and its optical properties can be altered by introducing an external voltage to the graphene layer [58]. Because the doping level can be controlled at a fine scale, the electrical doping with graphene enables precise tuning of the chiroptical response with high precision. The level of tunability is determined by factors such as the number of graphene layers, the applied voltage, and the design of the chiral metasurface [84,85]. Higher voltage or adding graphene layers can result in better tunability, allowing for tailored chiroptical effects [58].

Furthermore, it has been demonstrated that by adjusting the Fermi energies of various bilayer split-ring (BSR) components, CD signals can be dynamically modified [58]. The BSR consists of two layers of rings labeled TOP₁, TOP₂, BOTTOM₁, and BOTTOM₂. When the rings are composed of graphene, BSRs with various Fermi energies for their formation can produce a CD effect. The tunability of the chiroptical response of BSR can be obtained by changing the gate voltage of the various parts of the BSR, given that the graphene’s dielectric characteristics are contingent on its gate voltage. The CD of BSRs for V_bottom1 = 0.5 eV and V_bottom2 = 0.9 eV, and V_bottom1 = 0.9 eV and V_bottom2 = 0.5 eV are shown in Figure 4e. The chiroptical effect occurs at the resonant spectra. After exchanging the gate voltage at the wavelength related to the positive CD spectrum, a consequent negative CD response was observed. Whenever the gate voltage was changed, BSR transformed into its enantiomer. This condition demonstrates that by adjusting the Fermi level of graphene the chiroptical response of the BSR can be changed instead of restructuring the geometry [58].

Gate-controlled transmission difference and optical activity in gated graphene-integrated chiral metasurfaces were also demonstrated [24]. The coefficients of transmission for the LCP (T_LCP) and RCP (T_RCP) through the graphene-conjugated double Z-shaped metasurface at four different gate voltages are shown in Figure 4i. When voltage was applied to the gate, the T_RCP was strongly modulated because of the significant transition arising from the shift in the optical conductivity of graphene; however, the T_LCP was insensitive to the applied voltage (Figure 4f) [24]. Increasing the applied voltage significantly reduced the T_RCP while maintaining the T_LCP at the 1.1 THz resonance frequency (Figure 4j). This method of polarization control can be used in telecommunications and imaging devices as well as in ultrasensitive sensors to determine the chiroptical response and formation of biomolecules [24].

Optically tunable chiral metasurfaces are also available [60,143], most of which use semiconductors for the optical regulation, because the semiconductor’s conductivity changes significantly with the optical pumping at energies exceeding the band gap. Multi-layered graphene (MLG) with conjugated four-”petal” resonators was integrated to demonstrate that THz metasurfaces can offer dynamically tunable chiroptical responses utilizing optical pumping [62]. The metasurface was a sandwich structure with an MLG and metallic resonators arrayed alongside the dielectric structure (Figure 5a). Optical pumping with a 980 nm continuous wave laser was used to control the conductivity of the MLG resonator. As the linearly polarized wave traverses via the chiral metasurface, the linearly polarized wave changes into an elliptically polarized wave. As shown in Figure 5b, when the power of optical pumping increases, the amplitude of the cross-polarized spectra increases at frequencies exceeding 0.65 THz. The transmitted wave is converted into CPL near the frequencies of the observable CD, and with an increase in the optical pumping power, the resonant frequency of the ellipticity angle is shifted (Figure 5c). This proves that the ellipticity angle can be tuned very well by changing the optical pump.

Recent studies have shown that chiral metasurfaces can be magnetically tuned [66]. The magnetic tuning of silver/cobalt (Ag/Co) composite chiral nanohole array (CCNA) [66] is shown in Figure 5d. CCNA contains different compositions of Ag and Co, and it breaks the mirror symmetry in-plane as well as in the light propagation direction, which results in a high chiroptical response. The Co component of the CCNAs enables a magnetically tunable chiroptical response, and Ag further increases the chiroptical response tunability. A magnetic field of B = 0 T, B = −1.7 T, and B = +1.7 T was used to switch the chiroptical response of CCNA (Figure 5e,f). Circular differential transmission (CDT) changes were the greatest in the total sample of Co, and the amount of change decreased as the concentration of Co decreased, reaching zero in the whole sample of Ag CCNA. These magnetically tunable chiral metasurfaces are suitable for sensing and imaging applications. Magnetically tunable chiral metasurfaces, like Ag/Co composite nanohole arrays, have advantages, including the precise control of the chiroptical responses, which optimizes sensitivity in sensing applications [66]. However, they also have disadvantages, which may involve the demand for external magnetic fields, which limit the portability and create complexity, as well as the material constraints, like the need for particular magnetic materials such as cobalt [66].

Liquid crystals (LCs) have also been used to reconfigure chiral metasurfaces. LCs are used to reconfigure metasurfaces by changing the orientation angle of the LC molecule, which affects the chiroptical response. LC molecules are anisotropic, where their optical properties depend on the orientation of their long axes [74]. The alignment of the LC molecule on the chiral metasurface can be controlled by introducing an external electric field or temperature gradient. This change in alignment leads to changes in chiroptical responses such as OR and CD, enabling dynamic control and modulation in various applications, including beam steering, chiral sensors, and displays. In a study performed by Yin et al. [74], a chiral metasurface, which is reconfigurable by a sandwiched structure of metal–insulator–metal, was numerically demonstrated (Figure 6a). The sign and magnitude of CD spectra were tuned by varying the CPL illumination angle or by incorporating the chiral metasurface with nematic LCs. The effective refractive index of the LC layer was changed by varying the orientation of the LC molecule using external voltage, allowing for the tunability of the chiral plasmonic metasurface (Figure 6a). The computed CD response along the LC molecule’s tilt angle is shown in Figure 6c. Voltage applied from outside can change the angle of tilt of the molecules from 0° (”ON” state) to 90° (“OFF” state) (Figure 6c). When the angle changes from 90° to 0°, the magnitude and sign of CD alter (Figure 6c). The CD spectra and reflections for the LCP and RCP light are shown in Figure 6d,e, correspondingly. The achieved CD still exceeded 50% in the “OFF” state because of the nonuniform refractive index (Figure 6d).

Owing to the CPL oblique illumination, the light propagation time across different sections of the chiral metasurface meta-atom varies. The phase difference between LCP and LCP stimulated resonant modes causes strong mode interference. Extrinsic chirality is obtained from the difference in absorption of LCP and RCP incident light [74]. As demonstrated in Figure 6b, the CD response decreased as the illumination angle increased, and at 45°, the CD spectra almost disappeared [74]. Such metasurfaces have many applications in spin-orbit communications, chirality sensing, and polarimetric imaging.

Mechanically reconfigurable metasurfaces can also be used to continuously and reversibly tune chiroptical responses [9,59]. Tunability is achieved through structural deformation in mechanically reconfigurable chiral metasurfaces, such as those using graphene Miura-origami (G-Mori). The geometry and periodicity of the chiral nanostructures are altered by physically stretching or bending the metasurface, which changes the chiroptical response of the chiral metasurface [59]. A mechanically reconfigurable chiral metasurface with high CD was demonstrated by combining U-shaped split-ring resonators (USRRs) with G-Mori [59]. Planar metasurfaces with four ordered counterclockwise or clockwise USRRs were drawn on a graphene film under a flexible substrate (Figure 6f–j). The CD response and linear to the CPL conversion of the chiral metasurface polarization were dynamically tuned by regulating the Fermi energy level and folding angle of the G-Mori structure. The planar metasurface (θ = 0°) did not exhibit intrinsic chirality, but when the planar metasurface was folded mechanically into a 3D arrangement in the absence of mirror symmetry, an intrinsic chiroptical response was generated, as demonstrated in Figure 6k. The amplitudes of the CD spectra increase considerably as θ changes from 0° to 60°, reaching the maximum at 60°, which is attributed to a steadily improving impedance match among the metasurface and free space, as well as improved geometrical chirality (Figure 6k). The metasurface exhibited a prominent chiroptical response with an almost blocked LCP (t_LL = 0.014) and transmitted RCP (t_RR = 0.966), as shown in Figure 6l. The enhanced interproximal coupling effect between neighboring USRRs was responsible for the high intrinsic CD of 0.93, which can be tuned by reverse and continuously regulating the Fermi energy level and folding angle of G-Mori. Because of their capability to control EM waves, reconfigurable chiral metasurfaces are excellent candidates for reconfigurable circular polarizers, dynamic beam shaping, ultrasensitive sensors, reconfigurable chiral sensing system, and polarization modulators.

3.4. Deep-Learning-Enabled Design of Chiral Metasurface

Recently, a modeling procedure based on deep learning (DL) was introduced to shorten the time required for characterization while maintaining accuracy [78,79,80,110,144,145]. Available computer simulation methods such as the finite-element method (FEM) are accurate but require considerable computational time. The CD value of the chiral metasurface depends on different parameters, as previously discussed, and the value of each parameter also determines the chiroptical response. Thus, it is vital to explore new uses of effective algorithms for effective and timely computation in computer modeling. However, due to the complexity of neural network models, DL-based chiral metasurface design often demands substantial computational resources, such as powerful TPUs or GPUs for efficient inference and training. Furthermore, access to massive datasets and software frameworks such as TensorFlow or PyTorch are required for effective implementation [78,79,80,81]. Two significant examples of the DL-enabled design of chiral metasurfaces are presented in Figure 7.

To analyze the CD signal of the chiral metasurface, a CNN model called CDCNN was presented [79]. The CD signal of the metasurface was considered as a regression problem. In addition, DL for CD spectral prediction was used rather than solving Maxwell’s equations by training with data from COMSOL Multiphysics simulations, as illustrated in Figure 7a. The dataset used to train the CDCNN model consists of the images of the metasurface and their CD spectra with 41 data points in the 400-1000 nm wavelength range. It began with 200+ basic L-shaped nanohole structures and was later enlarged to 1000+ structures through mirroring and rotations for better deep-learning performance [79]. The purpose of the convolution layer was to extract the structural features of the metasurface data associated with the CD response. As shown in Figure 7b, the CDCNN employs several sequential conventional layers. Activation functions like ELU and ReLU are important in CDCNN models because they introduce nonlinearity into the neural network. They allow the neural network to recognize complicated correlations in data and make precise predictions. In this study, ELU outperformed ReLU because it handles negative inputs more effectively [79]. CD spectra data can contain negative values, and utilizing ReLU as an activation function would result in the loss of essential information by forcing all negative CD values to zero during training. ELU, on the other hand, enables a smooth transition between negative and positive inputs, preserving the data’s characteristics and resulting in more accurate CD spectra predictions [79]. The predicted CD response with the activation functions of rectified linear unit (ReLU) and exponential linear unit (ELU) fits real value, with ELU fitting better than ReLU, as depicted in Figure 7c,d. Compared with traditional finite-element approaches, it is a thousand times faster to properly map chiral metasurfaces and forecast their optical responses [79].

DL-based design of an anisotropic metasurface with a full-phase attribute in the ultrawide band was also proposed in [81], and metasurface patterns were produced using GANs. The design procedure includes a forward deep neural network (DNN) predictor that is trained from the dataset created with different geometry data and the associated reflective spectra, and it predicts the phase responses and amplitude of a meta-atom structure with extremely high accuracy in seconds and a GAN trained on geometric data for an inverse design, as shown in Figure 7e. The dataset of geometric data and associated reflective spectra for the GAN-based inverse design was generated by assembling the binarized pattern images of the meta-atom and their respective reflective spectra produced by forward EM simulations. The dataset was used to train the forward DNN predictor to successfully forecast the phase and amplitude response of the meta-atom [81]. The GANs network, which underwent training using this geometric data, was used for the purpose of inverse design when the input reflective spectra were provided, generating candidate patterns that matched the target spectra through Euclidean distance calculations. However, generating the dataset was not easy due to the resource-intensive nature of traditional simulations and the need to successfully train GANs for accurate design generation while avoiding concerns such as insufficient gradients for the generator [81]. The inverse design outcomes of the patterns created in the intended phases are displayed in Figure 7f. The network received an anisotropic reflection of the phase data from the modified handwritten MNIST digital sample test (Figure 7f) and sector sample test (Figure 7g). The solid curve represents the anisotropic reflection phases of the sample test, while the circle represents the response of the created pattern. The comparison results indicate that the test samples and generated patterns agree very well. This result demonstrates that using a GAN to inverse-design anisotropic chiral metasurfaces with full-phase characteristics can be used for a range of potential applications, including metalenses, reflect arrays, and stealth surfaces [81]. DL-based design in metalenses enables precise control of light manipulation, leading to high-performance, compact imaging systems. Reflect arrays benefit from optimized designs for beam forming and steering designs, which improve communication and radar systems, while DL-based stealth surfaces enable enhanced radar-absorbing materials, which improve stealth technologies by decreasing radar signatures [78,79,80,81].

4. Conclusions and Outlook

Chiral metasurfaces with CD, AT, OR, and other EM properties have been used to improve the chiroptical response. In this review, the fundamentals of chiral metasurfaces and their different types are summarized. First, we discuss single- and multi-layered chiral metasurfaces that can optimize the chiroptical response. Single-layer chiral metasurfaces are relatively easy to fabricate, whereas multi-layer chiral metasurfaces are the finest for providing a greater degree of freedom to manipulate waves and achieve a desirable chiral optical response. Furthermore, it is possible to control the chiroptical response of layered chiral metasurfaces using different geometrical parameters, including the orientation angle, the number of layers, and the distance between layers [69,70]. Fabrication of multi-layered chiral metasurfaces poses challenges in attaining precise alignment, identical layer thickness, scalability, and material compatibility [84,85,86]. To address these issues, researchers can examine in detail advanced fabrication techniques and improved materials for compatibility. More efforts should be made to optimize procedures for scalability and to incorporate quality control measures to improve practical applications. Third, we review recently developed tunable chiral metasurfaces. These metasurfaces facilitate active control of EM waves. Electrically, optically, magnetically, and mechanically tunable chiral metasurfaces have also been developed. Although these metasurfaces are rapidly evolving, the majority of current tunable chiral metasurfaces have been achieved through numerical simulations and theoretical analyses, and only a few have been experimentally validated. Experimental techniques for evaluating the performance of tunable chiral metasurfaces consist of spectroscopic measurements of OR and CD augmentation under various conditions, such as changes in external stimuli. These measurements offer important information about the tunability of chiral metasurfaces. However, the existing validation limitations include evaluating tunable chiral metasurfaces over a wide range of wavelengths and the need for precise equipment to measure the chiroptical response. Furthermore, most of today’s tunable chiral metasurfaces are composed of metal–dielectric metasurfaces, which limits their efficiency. Finally, DL-based approaches for chiral metasurface modeling approach have been examined, and it was found that it effectively reduced the numerical modeling computation time while maintaining accuracy. DL-based design of chiral metasurfaces enables optimization of optical parameters, including but not limited to amplitude distributions, phase profiles, and spectral responses. DL enables this optimization by effectively investigating complex design spaces and learning intricate correlations between desired optical outcomes and structural parameters. It speeds up the design process by iteratively refining metasurface topologies to attain specific optical capabilities, resulting in tailored responses and improved performances in applications such as holography, metalenses, and sensing [78,79,80,81]. In chiral metasurface design, specific DL architectures such as CNNs and algorithms such as GANs have been successfully employed. They offer advantages over the traditional methods by learning complicated correlations in data effectively, leading to faster optimization and precise prediction of chiral metasurface features [79,81].

In general, research on chiral metasurfaces focuses on geometrically chiral meta-atoms (i.e., intrinsic chirality) [74,83,107,146,147] as well as achiral and homogenous meta-atoms (i.e., extrinsic chirality) [148,149,150,151,152]. The chirality of an intrinsic chiral metasurface is mostly affected by the match between the meta-atom size and the wavelength spectra of the illumination light, whereas in an extrinsic chiral metasurface, it is primarily affected by the angle of the incident light. Furthermore, chiral objects can interact with light differently because achiral light can locally be chiral near or across meta-atoms. The role of achiral fields and systems in the enantioselective interaction between light and matter indicates that old ideas of chiral systems should be widened. Subwavelength chiral particles can also be utilized as meta-atoms to effectively develop continuous chiral media [153,154,155,156,157,158,159,160,161,162]. Chiral particles offer novel and improved characteristics and are candidates for use in optical materials and chiral sensing. Recent advances in chiral particles include plasmonic chirality by interacting with nanocrystals and biomolecules [159,160,161]; biosensing using plasmon-enhanced EM fields [157,158]; chiral nanocrystal structures with well-controlled geometries [154,155,156]; understanding the clustering behavior of biomolecules using an optimal viewing angle [162]; and encoding chirality into nanoparticles using chiral molecules including peptides, DNA, and proteins with chiral conformation [163,164,165,166,167,168,169]. The use of chiral metasurfaces spans a multitude of disciplines, thereby promising profound advancements in the times ahead. Chiral metasurface finds utility in the realm of metaholograms enabling multiplexing polarization sensitivity using an additional degree of freedom [170,171,172]. Optical spin isolation, switchable holography, and amplitude modulation for LCP and RCP illumination light are among the potential applications of chiroptical metaholograms [170,171,172]. Furthermore, the use of chiral metasurface extends to metalenses, facilitating the focusing of two images simultaneously. Chiral metasurfaces demonstrate the potential for multifunctional metalenses, including those with multi-focal length metalenses [81,170,173,174,175]. Moreover, chiral metasurfaces can be applied for imaging [24,66,74], chiral metamirrors [149], and various other functionalities.

As described, due to high speed and accuracy and applicability in inverse design, the DL-enabled design of chiral metasurface has a lot of potential. We envision that the DL concept can be used to optimize and develop chiral metasurfaces using the required optical parameters. This offers a novel outlook for understanding and facilitating chiroptical phenomena, as well as further advancing the applications of chiroptical effects, mainly in chiral sensing [173,174,176,177,178,179,180], spintronics [181], and stereochemistry [182]. Chiral metasurfaces can provide precise spin-polarized electron control in spintronics, which is critical for data processing and storage. They also show promise for sensitive detection and manipulation of chiral molecules, which is important in chemistry and pharmaceuticals. However, issues such as broad spectrum sensing, scalability, efficient light-spin coupling, chemical compatibility, and enhanced detection limits must be addressed so that these applications reach their full potential [173,174,177,178,179,180,181,182].