Enhancement Methods for Chiral Optical Signals by Tailoring Optical Fields and Nanostructures

Hanqing Cai; Liangliang Gu; Haifeng Hu; Qiwen Zhan

doi:10.1016/j.eng.2024.12.022

Engineering ›› 2025, Vol. 45 ›› Issue (2) :28 -47. DOI: 10.1016/j.eng.2024.12.022

Research Subwavelength Optics—Review

review-article

Enhancement Methods for Chiral Optical Signals by Tailoring Optical Fields and Nanostructures

Hanqing Cai ^a
, Liangliang Gu ^a
, Haifeng Hu ^a^,^b^,^c^,^*
, Qiwen Zhan ^a^,^b^,^c

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Abstract

The unique property of chirality is widely used in various fields. In the past few decades, a great deal of research has been conducted on the interactions between light and matter, resulting in significant technical advancements in the precise manipulation of light field wavefronts. In this review, which focuses on current chiral optics research, we introduce the fundamental theory of chirality and highlight the latest achievements in enhancing chiral signals through artificial nano-manufacturing technology, with a particular focus on mechanisms such as light scattering and Mie resonance used to amplify chiral signals. By providing an overview of enhanced chiral signals, this review aims to provide researchers with an in-depth understanding of chiral phenomena and its versatile applications in various domains.

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Keywords

Mie scattering / Optical chirality / Circular dichroism / Orbital angular momentum / Bound states in the continuum / Nonlinear optics

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Hanqing Cai, Liangliang Gu, Haifeng Hu, Qiwen Zhan. Enhancement Methods for Chiral Optical Signals by Tailoring Optical Fields and Nanostructures. Engineering, 2025, 45(2): 28-47 DOI:10.1016/j.eng.2024.12.022

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1. Introduction

Chirality is pervasive, spanning multiple scientific disciplines, including chemistry, biology, physics, and optics. The term “chirality,” which derives from the Greek word “cheir,” meaning hand, hinting at the fundamental property of chirality: namely, enantiomers in chiral phenomena cannot be superimposed through simple translation or right- and left-handed rotation [1]. In chemistry and biology, chirality primarily manifests in enantiomers at the molecular level [2], [3], [4], such as proteins, sugars, and amino acids. These molecules possess chiral characteristics, with stereoisomers sharing the same chemical composition but being unable to be superimposed without mirror transformations. The chirality of molecules not only affects their modes of interaction but also holds significant implications for their biological activity. For example, the chirality of certain drug molecules determines whether they are pharmacologically active or toxic, which underscores chirality as a crucial factor in drug design and synthesis [5]. This has spurred comprehensive research on the use of molecular optical properties for molecular structure analysis.

The geometric chirality of an object is a qualitative property that can be directly determined from its geometric shape, whereas chiroptical effects can be quantitatively measured. Some commonly used chiroptical measurement methods include optical rotatory dispersion (ORD), which measures the wavelength-dependent rotation of plane-polarized light passing through a chiral medium, and circular dichroism (CD), which quantifies the difference in absorption between left-circularly polarized (LCP) and right-circularly polarized (RCP) lights [6]. These methods based on chiroptic effects have become widely used for analyzing chiral molecules [7]. However, chiral optical signals are often weak, which making their measurement challenging [8]. Thus, amplifying and enhancing chiral optical signals are crucial for accurately distinguishing between enantiomers in practical applications.

In recent years, concerted efforts have been made to design and fabricate nanostructures with strong optical resonances. The capability to enhance chiral optical signals in plasmonic and dielectric nanostructures has driven significant advancements in this field [9], [10], [11], [12]. Exploring novel structures to achieve stronger chirality has become a burgeoning area of research [13], [14], [15], [16], encompassing various designs such as cross intersection structures [17], [18], [19], [20], [21], [22], helical structures [23], [24], [25], [26], [27], [28], gammadion nanostructures [29], [30], [31], [32], [33], optical nano-antennas [34], [35], split-ring metamaterials [36], [37], [38], [39], [40], [41], twisted arcs structures [42], [43], [44], [45], [46], and nano disks [47], [48], [49]. Oligomers comprising nanoparticles have been shown to produce strong, tunable optical chirality in the near field [50], [51], [52], [53], [54].

To promote the understanding of how to enhance chirality in light–matter interaction, we outline some representative methods for chiral enhancement in this review. In Section 2, we present an overview of the fundamental theories underlying chiral materials and optics, employing established formulas to elucidate the mechanisms of chiral signal enhancement. In Section 3, we provide a comprehensive review of recent advancements in chirality enhancement, organized into five distinct subsections. In Section 3.1, we provide a general definition of superchiral materials, starting with analytical expressions to facilitate understanding and then discuss the enhancement of chirality through the manipulation of superchiral light. In Section 3.2, we present methods for enhancing chiral responses via optical resonance, with a specific focus on plasmonic effects and Mie resonances. The interaction of matter with chiral light, which is generated through the utilization of orbital angular momentum (OAM), can also enhance optical chiral optical signals, as discussed in Section 3.3. In Section 3.4, we review research related to metasurfaces, mainly introducing principles of chirality enhancement based on a bound states in the continuum (BICs) and achiral metasurface structures. In Section 3.5, we examine the latest advancements in the field of nonlinear chirality enhancement, highlighting key contributions and innovative approaches, including high harmonic generation (HHG), hyper-Rayleigh scattering (HRS), and Raman scattering (RS). In Section 4, we summarize the key findings in the field of chiral signal enhancement, based on the literature reviewed in this paper, and presents a prospect outlook on emerging trends as potential areas for future investigation.

2. The fundamental theory of chiral molecules optics and chiral optics

For a chiral molecule with a size much smaller than the incident wavelength, its optical response can be described as follows [6], [55], [56]:

(1)

p = μ_{E} E - i G B

(2)

m = μ_{B} B + i G E

where p and m are the electric dipole (ED) and magnetic dipole (MD) moments in the light field (E, B), respectively. E and B represent the time-dependent electric and magnetic fields, respectively. i is the imaginary unit. The magnitudes of these moments are related to the electric polarizability μ_E and magnetic polarizability μ_B_, where μ_E and μ_B can be macroscopically related to the permittivity and permeability, respectively. The optical chiral response of a molecule stems from the coupling terms between the ED and MD related to the G coefficient. In general, the three parameters are second-rank complex tensors. For simplicity, we assume that the optical response of chiral molecules is isotropic, which reduces the complex polarization tensors to complex scalars (μ_E, μ_B, G). To assess the chiral response, we can examine the difference in absorption under a pair of monochromatic light fields with opposite chiralities, that is, (E, B) and (−E, B) [57]. The light absorptions A⁺ and A⁻ are expressed in Eq. (3) [58], [59].

(3)

A^{\pm} = \frac{ω}{2} (μ_{E}^{″} {|E|}^{2} + μ_{B}^{″} {|B|}^{2}) \pm G^{″} ω Im (E^{*} \cdot B)

where

μ_{E}^{″}

μ_{B}^{″}

, and

G^{″}

are the imaginary parts of

μ_{E}

μ_{B}

, and

G

, respectively. Im represents the imaginary part; ω is the angular frequency of light. A typical chiral light field is circularly polarized light (CPL). The traditional CD method consists of measuring the dissymmetry factor g_CPL = 2(A⁺ − A⁻)/(A⁺ + A⁻) under CPL illumination. In this case, A⁺ and A⁻ represent the absorptions under left-handed and right-handed CPL, respectively. In addition to CPL, other light fields with chiral properties can be used to probe the chirality of molecules. By introducing carefully designed light fields, the g-factor may exceed that observed under CPL (i.e., g_CPL). The property of this particular type of light field, capable of augmenting the g-factor, is termed “superchirality.” The intensity of a CD signal is proportional to the optical chirality density (also known as Lipkin’s 00-zilch) of the light field, which is defined as follows [60]:

(4)

C = (ε_{0} E \cdot \nabla \times E + μ_{0}^{- 1} B \cdot \nabla \times B) / 2

where ε₀ and μ₀ are the permittivity and permeability in vacuum, respectively; C is the optical chirality density. Hence, by manipulating the optical chirality density of the local field near a molecule, the chiral response can be enhanced. Over the past decade, significant efforts have been directed towards designing light fields to increase the magnitude of C. Notably, this strategy is applicable to molecules whose size is excessively smaller than the wavelength of light, in which case the scattering problem can be addressed using the Rayleigh approximation. Beyond this regime, the scattering process of large chiral objects must be addressed by considering the constitutive relations of chiral media. The constitutive equations for bi-isotropic media are expressed as follows [61], [62]:

(5)

D = ε E + (χ - i κ) \sqrt{μ_{0} ε_{0}} H

(6)

B = μ H + (χ + i κ) \sqrt{μ_{0} ε_{0}} E

where D and H are the electric displacement field and the magnetic induction intensity, respectively; and ε and μ are the permittivity and permeability in the chiral material, respectively. In bi-isotropic media, the absence of mirror symmetry results in the coupling of electric and magnetic fields. Two parameters χ and κ are related to the optical chiral response of materials. κ is the Pasture parameter, which measures the degree of handedness of the materials, and χ is the Tellegen parameter, which is used to characterize the non-reciprocal electromagnetic coupling relationship [63]. Here, we confine our discussion on chiral media within the region of Pasture media (κ ≠ 0, χ = 0).

Substituting the above equation into the source-free Maxwell’s equations, we obtain the Helmholtz equation for chiral materials:

(7)

\nabla^{2} E - 2 i ω μ (χ - i κ) \sqrt{μ_{0} ε_{0}} (\nabla \times E) + ω^{2} μ ε E = 0

Subsequently, the expression for the wavenumber k can be obtained as follows:

(8)

k^{2} = {(\frac{ω^{2} μ ε - k^{2}}{- 2 i ω μ (χ - i κ) μ_{0} ε_{0}})}^{2}

The above equation has two eigen solutions in the form of CPL waves, with the wave numbers given by Eqs. (9), (10):

(9)

k_{+} = i ω μ (χ - i κ) μ_{0} ε_{0} + ω \sqrt{μ ε - μ^{2} κ^{2} μ_{0} ε_{0}}

(10)

k_{-} = - i ω μ (χ - i κ) μ_{0} ε_{0} + ω \sqrt{μ ε - μ^{2} κ^{2} μ_{0} ε_{0}}

For a plane wave in homogeneous chiral media, the eigenstates should consist of left-handed and right-handed CPL waves with different wavenumbers, k₊ and k₋. Chirality parameters χ and κ can induce electromagnetic coupling, through their modulation of the refractive indices. The refractive indices of the two CPL waves can be expressed as

n_{\pm} = \sqrt{ε μ} \pm κ

, implying that LCP and RCP waves experience different refractive indices in chiral media. In this case, the imaginary part of κ induces a differential absorption for the CPL with opposite handedness after passing through the chiral material, giving rise to CD. If A⁺ and A⁻ are the respective absorption rates for RCP and LCP excitations, then the conventional CD signal is defined as follows [6]:

(11)

g = \frac{2 (A^{+} - A^{-})}{A^{+} + A^{-}}

The theoretical upper limit of CD, g = 2 (e.g., when A⁺ ≠ 0 and A^– = 0), indicates that the object can interact with CPL of the same handedness [64]. This maximum CD can be explained by the perfect match between the geometric chirality of the designed structure and the CPL field.

Using Eq. (3) for the absorption of chiral light by chiral molecules and the concept of optical chirality density expressed in Eq. (4), the chiral optical signals from subwavelength particles can be accurately determined. However, for particles with sizes comparable to the wavelength of light, optical chirality density cannot be used for evaluating the intensity of the chiral signals, because high-order multipole moments play a significant role in the optical response. Employing full vectorial wave methods becomes necessary to rigorously solve scattering problems. For particles with complex structures, numerical techniques such as finite-difference time-domain, method of moments, and finite-element methods should be used.

3. Methods for chiral signal enhancement

3.1. Chiral signal enhancement via optical field manipulation

The use of CPL to distinguish enantiomers has found widespread application in numerous fields. However, most molecules are smaller in scale than the wavelength of CPL, thus causing the CPL to undergo only a barely perceptible twist over distances on the order of molecular dimensions. This weak twisting effect results in CD signals that are typically weak and challenging to measure. Increasing the chiral optical response of individual molecules is an effective approach to enhancing CD spectroscopy. The term “superchirality” has been introduced to describe the light field that can produce greater g-factors of chiral molecules than those obtained under CPL illumination. The g-factor under CPL illumination is represented by g_CPL. Enhancement of the CD signal compared with g_CPL can be expressed as shown in Eq. (12) [59]:

(12)

g / g_{CPL} = \frac{cC}{2 U_{e} ω}

where c is the speed of light and U_e is the local electric energy density. For a homogeneous light field, g/g_CPL cannot exceed 1. However, in the local regions of the inhomogeneous field (e.g., a vortex field, or light field with a twisted wavefront), superchirality can be achieved (i.e., g/g_CPL >1). According to Eq. (11), two effective methods can be used to enhance chiral asymmetry: reducing the electric energy density or increasing optical chirality density. To this end, Tang and Cohen [65] proposed to employ a standing wave chiral field, as depicted in Fig. 1(a). This field is constructed from two counterpropagating CPL plane waves with opposite handedness, equal frequencies, and slightly different intensities. We let E₁ denote the electric field amplitude of left-handed CPL propagating from right to left, and let E₂ represent the electric field amplitude of right-handed CPL propagating from left to right, with E₁ being slightly larger than E₂. More specifically, E₁ and E₂ position the chiral molecules at the node of the electric field energy density. Because the dissymmetry factor g is inversely proportional to E₁ − E₂, superchirality can be achieved, resulting in an increase in g compared with g_CPL. In theory, constructing a superchiral standing wave is relatively straightforward. When left-handed CPL is reflected by a mirror, it generates right-handed CPL with a slightly lower amplitude, propagating in the opposite direction. The interference between the two light beams enables the construction of a local superchiral standing wave. The mirror reflectivity is R = (E₂/ E₁)². The maximum g-factor at the node of the standing wave is shown in Eq. (13):

(13)

g_{\max} = g_{CPL} \frac{1 + \sqrt{R}}{1 - \sqrt{R}}

However, in this configuration, immobilizing chiral molecules within the superchiral region (g/g_CPL>1), with a thickness of 0.032λ (where λ is wavelength), is technically challenging [59]. The introduction of the superchiral field [66] has spawned a great deal of research. Hendry et al. [67] proposed a model of nanoslit pairs wherein C was increased by approximately two orders of magnitude compared with the CPL. Subsequently, a new method for constructing superchiral fields was proposed in which an interference field with a homogeneous distribution of |E|² is constructed by multiple plane waves [68]. This method creates lattices of the superchiral field [69], [70]. Aside from using the interference of plane waves, a localized superchiral region can also be generated in a tightly focused field. Building upon this concept, Hu et al. [71] reported a localized superchiral field, which can elongate the superchiral spot in the z-direction by ten wavelengths. They designed the optical system shown in Fig. 1(b) to generate a superchiral optical needle by tightly focusing twisted radially polarized (RP) beams on a dielectric interface.

Although chiral plasmonic structures have been widely used for the chiroptical absorption detection of CPL [72], [73], [74], achiral structures have also been demonstrated to achieve superchirality under certain mechanisms [75]. Vázquez-Guardado and Chanda [76] proposed an achiral cavity-coupled plasmonic system for the generation of a single-handed superchiral near-field, as shown in Fig. 1(c). In 2020, Chen et al. [77] achieved superchiral fields on high-Q metasurfaces, thus enhancing the reflected CD signal of chiral samples by 59 times (Fig. 1(d)). They utilized a metal–dielectric hybrid metasurface with achiral unit cells to achieve strong coupling between the surface lattice modes and toroidal dipoles, where a hybrid mode formed BICs in both the x and y polarization directions. This study optimized the positioning of superchiral fields, enabling simultaneous measurements of CD spectra and molar concentrations on a single metasurface platform. Subsequently, Barkaoui et al. [78] proposed a method based on symmetry-protected BICs to construct a superchiral field on non-chiral metasurfaces. By adjusting structural parameters to couple transverse electric (TE)- and transverse magnetic (TM)-like BICs at the same location, they achieved superchiral fields with enhancement factors of up to 10⁴. This coupling, unlike the merging of BICs in momentum space, does not induce changes to topological charges. This coupling can exhibit full-vector polarization. Zhang et al. [79] experimentally demonstrated this coupling, which is referred to as “vector BIC.” This approach is beneficial for the ultra-sensitive detection of molecular chirality.

Although plasmonic nanostructures are considered as an effective solution for enhancing chiral response, superchiral optical fields are typically confined to spatial regions known as “hotspots.” This is because surface plasmon polaritons (SPPs) modes are inherently TM modes, which results in the propagation of SPP modes without chirality. To address this issue, Pellegrini et al. [80] recently proposed a novel chiral sensing platform based on the combination of a one-dimensional photonic crystal (1DPC) with a carefully engineered anisotropic surface defect, as shown in Fig. 2(a). This chiral sensing platform supports chiral surface waves (CSWs) generated by the coherent superposition of TE and TM surface modes. Fig. 2(b) [80] shows the optical chirality obtained by surface modes at a distance of 5 nm above the 1DPC surface with a coupling phase φ_c. This indicates that the platform can operate across a wide spectral range from visible to ultraviolet wavelengths. The CD signal for a 5 nm flavin mononucleotide (FMN) film with a concentration of approximately 100 ng·cm⁻² is depicted in Fig. 2(c) [80]. The CD signal obtained from the CSWs is 150 times higher than that obtained through plane waves (CD_CSW/CD_ref ≈ 150). Fig. 2(d) [80] shows that the sensor offers CD signal enhancement exceeding 2 orders of magnitude. This CSW sensor provides homogeneous and switchable chiral optical fields over arbitrarily large areas and broad spectral ranges. Movever, the 1DPC platform features a simple geometry, which facilitates fabrication. Compared with CPL, the superchiral light is remarkably effective in enhancing the chiroptical response, enabling the detection of a few picograms of chiral material [81]. This leveraging of superchiral electromagnetic fields introduces a novel approach in the realm of biospectroscopy/biosensing that can offer insights into their structures [82].

3.2. Chiral signal enhancement via photonic resonance

Metasurfaces have been applied to ultrasensitive optical biosensors for near-field enhancement based on the plasmonic effect. This approach can enhance spectroscopic signals by several orders of magnitude, such as in surface-enhanced infrared absorption [34] and surface-enhanced RS [83]. Similarly, in chiroptical spectroscopy, nanostructures can be designed to generate enhanced chiral optical fields in localized regions, which can improve the detection of signals from chiral molecules. Previous experimental and theoretical studies on plasmon-enhanced CD spectra indicated that nanostructures can increase asymmetric enhancement factors up to 10⁵ [23], [59], [65], [84], [85], [86], [87], [88], [89]. Subwavelength-sized Rayleigh particles and chiral molecules typically exhibit extremely weak chiral responses, primarily owing to the dominance of their internal ED responses. Recently, research has been conducted on plasmonic nanostructures, such as nanocubes [90], nanoparticle helices [91], and plasmonic molecule array [92], [93], assembled by weak scattering systems, with optical chirality far exceeding that of individual particles and molecules.

Plasmon-enhanced CD shows great potential in detecting the chirality of a monolayer or single molecule. A study in 2016 explored the electromagnetic interactions between chiral media and plasmonic structures [94]. The researchers simulated three structures: gap antennas (Fig. 3(a) [94]), nanorod arrays (Fig. 3(b) [94]), and chiral dimers (Fig. 3(c) [94]). Their results showed that achiral plasmonic gap antennas showed significantly superior CD-enhancement factor compared to those of their chiral counterparts. A subsequent related study utilized achiral metal hole arrays to enhance nanoscale molecular chirality [95]. Owing to plasmonic near-field enhancement, achiral cylindrical hole arrays can achieve a greater CD signal than chiral hole arrays. This finding further indicates that complex chiral shapes do not provide significant advantages in near-field chiral sensing. A detailed discussion of achiral structures is provided in Section 3.4.

Subsequently, Poulikakos et al. [96] introduced the chiral antenna parameters of chiral flux efficiency and chiral antenna aperture, which can quantify the generation and dissipation of chiral light. The researchers used a two-dimensional (2D) chiral coupled nanorod antenna to verify their concept both theoretically and experimentally. A recent study by Both et al. [97] further elucidated the interaction between plasmonic nanoparticles and chiral molecules (Fig. 4). They proposed a general theory describing chiral interactions as perturbations of resonant modes (also known as resonant states or quasi-normal modes). The entire chiral light-matter interaction can be divided into five contributions (Fig. 4(c) [97]): a nonresonant interaction, changes in the excitation and emission efficiencies of the modes, modal resonance shifts, and intermodal crosstalk. These contributions are proportional to the chirality of the optical field and material. This theory will contribute to the design of plasmonic nanostructures for chiral sensing applications.

The optical chirality density is related to both the electric and magnetic fields. Thus, boosting electric and magnetic resonances is a useful approach to enhancing chirality response. Although metallic and dielectric nanoparticles can individually provide electric and magnetic resonances, their resonance peaks in the spectrum often do not match, thus weakening the chirality response instead. Mohammadi et al. [98] proposed a dual nano-resonator composed of metallic and dielectric nanoparticles, shown in Fig. 5. This structure achieves strong coupled and decoupled electromagnetic resonances, fulfilling optimal conditions for maximizing optical chirality. Subsequently, a metal-dielectric metasurface was proposed, achieving a 300-fold enhancement in local optical chirality and thereby increasing the CD signals by 20 times.

As a chiral particle’s size increases to the wavelength scale (i.e., Mie particles), the particle’s scattering characteristics cannot be adequately described by only Rayleigh scattering. The contribution of higher-order multipole moments to the chiral response becomes significant and cannot be neglected [59], [99]. Hence, solely relying on the optical chirality density, C, is insufficient to accurately analyze the chiral interaction between Mie particles and the light field. In such instances, a rigorous electromagnetic model should be used to calculate the chiral scattering of Mie particles.

The electromagnetic scattering properties of optical nanomaterials have long aroused considerable interest across various fields. However, owing to the inherent limitations of optical materials, harnessing magnetic effects within the visible or infrared wavelength ranges has proved to be difficult. Although magnetic effects have been attained at the microwave and terahertz frequencies, extending these capabilities to the infrared and visible light frequency ranges remains a formidable challenge [100]. Silicon (Si) rod arrays can function as true metamaterials with a left-handed dispersion branch in the visible to mid-infrared range for moderate refractive indices [101]. Motivated by these results and building on Mie theory, García-Etxarri et al. [102] found that the scattering of silicon nanoparticles can be accurately described by dipolar electric and magnetic fields, with quadrupolar and higher-order contributions being negligible. The electric and magnetic resonances in Si nanoparticles can be utilized to enhance the nanoparticles’ quantum efficiency [103]. In recent years, the anapole states in such particles have been intensively studied [104]. The simplest anapole mode arises from destructive interference between the ED and the toroidal moment. In 2015, the radiationless anapole mode was first observed in the visible light spectrum [105]. Internally exciting the multipole magnetic Mie resonances and electric Mie resonances internally in low-loss, high-refractive-index particles is an effective method for discriminating between and separating enantiomers. Consequently, enhancing the weak optical signals of small chiral molecules has been a recent area of focus.

In 2017, Ho et al. [106] demonstrated that high-index dielectric nanoparticles can enhance enantiomeric excesses by up to seven times beyond CPL. The researchers used Mie theory to calculate the local electric and magnetic fields near a silicon nanosphere with a radius of 436 nm in a host medium with a refractive index of 1. The extinction spectrum is depicted in Fig. 6(a) [106], whereas Figs. 6(b) and (c) [106] respectively illustrates the spatial maxima of the CD enhancement (C/C_CPL) and the enhanced dissymmetry factor (g/g_CPL) at each wavelength. Compared with CPL in free space, magnetic multipolar Mie resonances supported by the submicrometer silicon spheres were found to increase the dissymmetry factor by seven-fold. In additon, the CD signal was enhanced by 170-fold. These findings pave the way for more efficient all-optical chiral resolution techniques.

Designing the light field globally is another effective method for enhancing the CD signal for a Mie particle. As discussed above, the local property of the optical chirality density is inadequate for predicting the chiral response of a Mie particle, whose high-order multipoles contribute a notable proportion in the scattering field. Therefore, the relationship between the chiral optical signal and global features of the optical field (e.g., the OAM) should be studied. The scattering CD of large chiral structures can be enhanced to g = 1.2 by increasing the topological charge of the incident light [107]. In 2016, Fernandez-Corbaton et al. [64] reported that upper bounds on chirality exist and are attained when objects meet certain additional conditions. They studied the electromagnetic-chirality properties of the double-turn silver helix (red solid line in Fig. 7(a)). Numerical analysis indicated that even in the presence of losses, the chirality response of this helix structure can reach 92% of the maximum electromagnetic chirality, at least within a narrow frequency band. Two conditions are required to generate a large electromagnetic chirality: a significant contrast between the interaction cross-sections of two helicities (blue dashed line in Fig. 7(a) [64]) and minimal duality breaking (black dashed line in Fig. 7(a) [64]).

Regarding the CD signal of chiral Mie spheres, Hu et al. [108] showed that under optimized light fields, the upper limit of CD can be achieved at specific frequencies (i.e., g = 2), owing to the excitation of anapole states. To calculate the CD scattering from Mie spheres, the researchers proposed a multi-scattering model (Fig. 7(b) [108]) based on the Lorentz–Mie theory. In this calculation, two peaks can be observed at the frequencies of 241.46 and 294.46 THz. When an incident field consisting of vector spherical harmonics (VSH) with the order of (1, 1) illuminates the particle, the CD values can reach the maximum of g = 2.

Mie theory is an analytical method used to solve light-scattering problems. Although this method is only applicable to certain types of particles, it is highly efficient and provides a clear explanation of the physical mechanisms involved in the scattering processes for particles. For this reason, Mie theory offers significant advantages compared to other numerical methods [109]. Numerous related studies have used this theory to solve scattering problems [110], [111], [112], [113], [114]. In many practical applications, the particles are supported by a substrate. For the analytical method to be applicable, the model of the scattering process of particles should be modified by considering a semi-infinite substrate [115], [116], [117], [118], [119], [120].

3.3. Chiral signal enhancement via OAM beams

The angular momentum of a photon consists of two components: the spin angular momentum (SAM) and OAM. Both types are related to the chirality of the light field and can be used to probe the chirality of objects. The current principle used for measuring the chirality of materials/structures is based on their differential response to photons with an SAM of +1 and −1, as observed in techniques such as CD and ORD measurements. When chiral objects interact with CPL, they exhibit optical activity induced by the SAM of photons. The chirality of CPL is manifested in the helical rotation of the electric and magnetic field vectors during propagation. The direction of rotation, which is either counterclockwise or clockwise, determines the quantum number of the SAM with σ = ±1, where the values of σ correspond to counterclockwise and clockwise. In the traditional CD measurement system, photoelastic modulators can be employed to switch the two SAM states of light in the visible and infrared frequency range. Novel devices have been proposed to improve the performance of circular polarization modulation. In the terahertz region, using a kirigami polarization modulator to modulate circular polarization enables the detection of optical chirality in structures ranging from 10 to 20 μm [121].

Beams with OAM exhibit azimuthal phase dependence in the transverse plane with the form exp(ilφ), where the index l, called the topological charge, which determines the OAM of the beam, and φ is the azimuthal angle in cylindrical coordinates. In general, the contributions of spin (polarization) and orbital components to the total angular momentum cannot be considered separately [122]. However, they can be manipulated independently under the paraxial approximation [123]. Various OAM vortex beams can be experimentally realized in optics [124], [125]. OAM beams have also been widely employed to manipulate and trap microscopic particles [126], [127]. In addition to polarized light, which has an intrinsic connection with the SAM of photons, photons can carry OAM, which is related to the helicity of the spatial phase distribution [124], [125], [126], [127], [128], [129]. Over the past few decades, the interaction between OAM photons and chiral molecules has been investigated [130], [131], [132], [133]. Early studies [134], [135], [136] considered paraxial vortex beams, of which electric and magnetic fields are transverse to the propagation direction, and did not reveal chiral selectivity of chiral molecules in response to OAM of the vortex beams. However, a recent study revealed that optical OAM in the non-paraxial regime can actually enhance optical chirality density [137]. Forbes and Jones [138], [139] demonstrated that longitudinal fields (with respect to the propagation direction) can provide chiral selectivity for chiral molecules. The OAM of light is an alternative method for detecting the optical chirality of microscopic objects [140]. A recent study extensively investigated optical chirality at the nanoscale using structured light beams [141]. As defined above, differential absorption by chiral molecules under light with opposite SAM is known as CD. Similarly, helical dichroism (HD) involves the differential absorption of light with opposite helicities. While some references define HD as orbital dichroism and vortex dichroism (VD). These three terms generally refer to the same phenomenon. Light beams carrying OAM exhibit distinct chirality owing to their helical phase structure. This implies that even if the source light beam is unpolarized, it can still exhibit optical chirality. The contribution of optical vortices carrying a phase factor exp(ilφ) to the optical helical density is entirely independent of the polarization state [142]. Chiral absorption can be induced by using a linearly polarized Laguerre–Gaussian (LG) light beam, which originates from the helical phase effect rather than from CPL [143]. Similar to CPL, when the topological charge l > 0, the beam shows left-handed chirality; when l < 0, it exhibits right-handed chirality. In addition, because l can take any integer value, (e.g., 1, 2, 3), OAM beams offer better scalability than CPL.

In 2016, Brullot et al. [144] demonstrated that optical OAM can be utilized to distinguish enantiomers, relying on the contributions of different chiral electric quadrupole (EQ) fields. This approach can result in a weak VD (or HD) of approximately 0.6%. In 2019, Woźniak et al. [145] investigated the interaction between OAM beams and individual chiral dipole scatterers. To ensure that the incident photons carried only OAM and no SAM, the study employed linearly polarized LG beams with a topological charge l = ±1 as the incident light. This work experimentally validated the feasibility of utilizing photon OAM for chiral measurements. Rouxel et al. [146] confirmed that OAM light beams can be used to detect the HD of disordered samples. They studied the HD of a chiral metal–organic dicationic molecular complex. For OAM beams with topological charges of 1 and 3, the asymmetry ratios of the HD ranged from 1% to 5%. Subsequently, chiral nanostructures have been shown to exhibit giant HD under excitation by linearly polarized OAM light [147]. In addition, increasing the OAM of photons can generate larger HD [148]. Three-dimensional (3D) metal metamaterials with opposite chiralities—specifically, copper micro-screws—were fabricated as shown in Fig. 8 [148]. The experimental results for the reflectance for right-handed microhelix matched well with the simulation results shown in Figs. 8(a) and (b) [148], within the range of topological charges from –40 to 40, Similarly, the experimental and simulation results were also in agreement for the left-handed microhelix in Figs. 8(c) and (d). The curves indicate significant differences in reflectance for beams with opposite topological charges. HD was observed on both left- and right-handed copper microhelice structures, with the maximum HD reached ∼50%, as shown in Figs. 8 (e) and (f) [148]. The emergence of such giant HD can be attributed to an optimal match between the topological charge of the OAM light and the chiral nanostructures.

Raman spectroscopy is based on the inelastic scattering, which is caused by the interaction between photons and the low-frequency vibrational modes in a molecule. Therefore, Raman optical activity (ROA), which represents the subtle differences between the Raman signals under left- and right-handed CPL, can be used to measure the chirality of molecules and provide information on their stereoisomeric structures. Combining structured light with ROA can lead to the development of novel methods for chirality measurement. Recently, circularly polarized LG light has been recognized as a promising chiroptical spectroscopic technique [149]. The spin–orbit interaction of OAM light with interfaces enables ROA to be observed in chiral media. The scattering phenomenon induced by circularly polarized OAM light exciting chiral molecules is referred to as circular-vortex differential scattering (CVDS). In 2022, a study used vortex beams with both SAM and OAM as probes, employing ROA to distinguish the contributions of chirality and helicity to RS in liquid crystals [150].

Optical toroidal vortices have received significant attention in recent years [151], [152], [153]. Chirality has also been observed in toroidal systems [154], [155], [156]. In a very recent study, Chen et al. [157] disrupted the symmetry of toroidal spatiotemporal optical vortices (STOVs) to produce what they call the distorted vortices “photonic conchs,” as shown in Fig. 9. These optical vortices possess geometric chirality in free space and encompass an OAM associated with all the dimensions of space–time. The chirality of “photonic conchs” can be controlled by adjusting the spiral parameters and topological charges.

3.4. Chiral signal enhancement via metasurfaces

3D nanostructures, such as helices and multilayer architectures, can exhibit strong optical activity, as has been extensively illustrated above. The development of nanofabrication techniques enabled to produce 3D structures, such as chiral metasurfaces. Recently, 2D (planar) structures have been shown to exhibit intrinsic chirality under normal incidence [33], [158], [159], [160] and extrinsic chirality under oblique incidence [161], [162], [163], [164]. A 2D metasurfaces are easy to manufacture and integrate on-chip [165], [166], [167]. Numerous types of metamaterials and metasurfaces exhibit a broad spectral range of optical chiral responses. However, exploiting a high quality (Q) (

Q = ω_{0} / γ

ω_{0}

is the resonance frequency;

γ

is the resonance linewidth) to manipulate chirality within narrow frequency bands is essential for practical applications such as high-sensitivity and high Q factor chirality sensing. Achieving effective chirality manipulation within narrow frequency bands remains challenging.

The emergence of metasurfaces with BICs offers viable solutions to significant challenges in optical chirality within narrow bands. BICs denote localized states in the radiation continuum above the light cone, which decouple completely from the far-field radiation continuum, producing no radiation leakage and possessing an infinitely high Q factor [168]. In theory, the linewidth of a BIC would vanish, making it unobservable in the spectrum. However, by introducing perturbations in the structure, such as symmetry breaking, an unobservable BIC can be transformed into a sharp Fano resonance with a high Q factor, referred to as a quasi-BIC, which is observable [169], [170], [171], [172]. BICs exist as eigenstates of the electromagnetic field within the continuous radiation spectrum while retaining localization, thereby achieving the maximum Q factor [59], [173], [174]. This effect can enhance the interaction between light and matter, promoting the development of applications such as biomedical sensors [175], [176], [177], nonlinear frequency converters [178], [179], [180], [181], [182], [183], and surface-emitting lasers [184], [185], [186]. Breaking the mirror and rotational symmetries in the structure enables to realize chiral metasurfaces, facilitating the creation of optical chiral nanostructures controlled by BICs. Such structures exhibit singular chirality and extremely high Q factors [187], [188], which greatly expand the applicability of optical chirality [189], [190], [191], [192], [193], [194].

The design of all-dielectric chiral metasurfaces based on the BIC effect has emerged as a hot topic in physics. Breaking the mirror symmetry of nanostructures is a key aspect of achieving chiral BICs. There are two primary types of symmetry breaking: in-plane symmetry breaking and out-of-plane symmetry breaking. Currently, 2D chiral BICs formed by breaking the in-plane symmetry are used to reflect the CPL with near-unity efficiency and ultra-narrow bandwidth [187]. Shi et al. [195] proposed a planar chiral metasurface based on BICs, as shown in Fig. 10(a), simultaneously achieving high Q factor and strong CD responses in the optical frequency range. The experiment showed that the planar chiral response nearly reached levels with a nonlinear CD of up to 0.81 and a linear CD of up to 0.93. Ma et al. [196] proposed an all-dielectric chiral metasurface controlled by quasi-BIC states. The unit cell comprises a cross-sharped meta-atom, as shown in Fig. 10(b) [196]. By introducing in-plane symmetry breaking, the maximum CDs in transmission and refraction reached 0.9996 and −0.9804, respectively. Moreover, optical chirality responses are more easily achievable in 3D photonic structures. Recently, Kühner et al. [197] proposed a nanofabrication technique that enables the arbitrary control of resonator heights, as shown in Fig. 10(c). The intensity of the chirality spectra ΔT experimentally reached ±0.7 at a wavelength of approximately 900 nm, because of the chiral-selective behaviors of the chiral quasi-BIC. Although these chiral metasurfaces involved only single-layer rectangular blocks carefully placed on a planar substrate, they achieved stronger—and, more importantly, more controllable—optical chirality compared to that induced by complex bilayer arrangements.

Implementing out-of-plane symmetry breaking provides an additional degree of freedom in designing optical structures with strong chiral quasi-BICs, enabling selective coupling with light of specific helicity. This effect has laid a theoretical foundation for the chirality manipulation of light fields based on BICs. Early theoretical studies revealed that strong chiral BICs can be achieved by breaking the out-of-plane [198] or in-plane [180] symmetry of the structure. The photonic crystal (PHC) slab is an ideal platform for studying the relationship between structural symmetry breaking and chiral optical response. Liu et al. [199] eliminated vortex polarization singularities (PSs) by breaking the in-plane inversion symmetry within a 2D PHC slab (Figs. 11(a) and (b)), resulting in pairs of circular polarization states. The ratio of the reduced upper base length 2ΔL to the lower base length is defined as the asymmetry parameter α (Fig. 11(c) [199]). Numerous methods have been proposed to break both out-of-plane and in-plane mirror symmetries; however, the remaining symmetry planes hindered the emergence of intrinsic chiral BICs. In 2023, Chen et al. [200] innovatively proposed breaking the out-of-plane mirror symmetry of TiO₂ dielectric metasurfaces with a tilt-etching structure. Combined with in-plane trapezoidal nanoaperture design, this approach simultaneously broke both the out-of-plane and in-plane symmetries, achieving true 3D chirality. The results for this metasurface revealed a CD value of up to 0.93, approaching the theoretical limit of 1. The Q factor of chiral resonance reached 2663.

An extreme chirality concept based on OAM helical chirality has been proposed, with a chiral exceptional point (EP) being observed within a BIC [201]. Using the intrinsic OAM selective coupling characteristics of metasurfaces, the system achieves a perfect chiral EP. The left-handed incident OAM is completely absorbed, whereas its right-handed counterpart is entirely reflected, enabling extreme asymmetric OAM modulation, as shown in Fig. 12 [201]. The wavefronts of the reflected acoustic field for right- and left-handed incidence are shown in Fig. 12(c), which effectively demonstrates the selective coupling of quasi-BICs with incident vortices carrying specific OAM.

Increasing interest is being directed towards the generation of chiral optical fields utilizing achiral structures. Compared with chiral structures, achiral structures have simpler design and fabrication processes, with broad prospects for applications [35], [161], [202], [203], [204]. An achiral structure, characterized by its mirror symmetry, undergoes a transformation when interacting with light. This interaction alters the symmetry properties of the system compared with the structure only, resulting in extrinsic chirality. Under oblique incident circularly and linearly polarized light, extrinsic chirality can induce optical activity on achiral interfaces [76], [205], [206], [207], [208], [209], [210]. Recently, the combination of chiral and singular optics has provided new vitality to optical chirality. Under plane wave illumination, the surface of an achiral structure can generate PSs, where the optical field is circularly polarized and the major axis of the elliptically polarized light cannot be determined [211], [212]. These PSs, also known as “C lines” [213], can detect chiral particles in achiral structures. García-Etxarri [214] excited highly refractive nanoparticles with linearly polarized light, generating PSs. Chen et al. [215] investigated the chiral response of structures through PSs of quasi-normal modes and far-field radiation. They found that modifying the propagation direction of light interacting with chiral structures can result in varying chiral responses, including no chirality or even opposite chirality, as shown in Fig. 13 [215]. Subsequently, Peng et al. [216] explored the connection between the topology of metallic structures and the topological properties of the scattering fields, focusing particularly on the generation and evolution of PSs in the near field.

Jia et al. [217] reported the generation of a chiral optical field by exciting achiral structures with LP light at oblique incidence, as shown in Fig. 14(a). The origin of the optical chirality stems from asymmetric C lines in the near-field of the metasurfaces, which exhibit greater optical chirality than CPL. Fig. 14(b) [217] shows the results of normalized optical chirality at different wavelengths, under oblique and normal incidence conditions, respectively. The researchers presented the normalized optical chirality under different incident angles θ and polarization angles φ of LP light, as shown in Fig. 14(c) [217]. Evidently, the degree of optical chirality escalates as θ and φ increases, indicating that optical chirality arises from symmetry breaking under the oblique incidence of plane waves.

3.5. Chiral signal enhancement via nonlinear optics

The first observation of nonlinear optical phenomena dates back over 60 years [218]. Today, platforms for nonlinear generation have evolved from nonlinear crystals to more miniature components such as optical fibers, waveguides, and metasurfaces [219], [220], [221], [222], [223], [224], [225]. HHG is an extremely nonlinear process that converts the frequency of the incident field into higher-integer multiples, which are then emitted as the frequency of the outgoing field [226]. So far, numerous studies have reported methods based on metasurfaces to enhance the nonlinear processes [183], [227], [228], [229], [230], [231], [232]. Such nonlinear enhancement can be used to enhance the chirality signal when detecting the chirality of biological samples. Compared with linear light interactions, nonlinear detection can achieve high sensitivity sensing at low concentrations. Metasurfaces aid in detecting the nonlinear interaction between light and matter in biological samples, expanding the applications of biosensing or bioimaging [233], [234], [235]. Second-harmonic-generation (SHG) biosensing utilizes the intrinsic nonlinear characteristics of biological materials with surface specificity, making it highly suitable for characterizing molecular interactions on surfaces [236]. Byers et al. [237] reported an SHG signal on a chiral metasurface, whose intensity was sensitively dependent on whether the incident light was LCP or RCP. They referred to this phenomenon as SHG-CD. This method can detect chirality at submonolayer concentrations of a material and enable the acquisition of larger CD spectra. The SHG-CD effect was subsequently observed in anisotropic materials [238], [239]. Valev et al. [240] constructed a device for measuring SHG-CD, as shown in Fig. 15(a). These researchers demonstrated the presence of SHG-CD signals in G-shaped nanostructures, emphasizing that the arrangement of these nanostructures is a decisive factor in the SHG-CD effect.

Recently, Guo et al. [241] achieved chiral SHG using a single plasmonic vortex metalens at specified visible wavelengths (Fig. 15(b)). The researchers combined plasmon-enhanced SHG with CD to probe the chirality of plasmonic vortex metasurfaces. Under excitation by RCP and LCP light, the chiral SHG of the bare aluminum vortex lens exhibited strong CD, such that SHG-CD ≈ 120% at room temperature. Moreover, Yan et al. [242] proposed a method for synthesizing chiral CdTe nanohelices. Subsequently, Ohnoutek et al. [243] reported a new chiroptical effect (third-harmonic-Mie scattering optical activity).

Compared with the linear regime, nonlinear CD (NLCD) can exhibit substantially higher contrast, as demonstrated in plasmonic structures. Recently, resonant dielectric nonlinear metasurfaces have also enabled high NLCD with associated high conversion efficiencies. Zograf et al. [244] reported a dramatic enhancement of the efficiency of HHG in dielectric metasurfaces due to BIC resonance. Subsequently, Koshelev et al. [245] implemented a system consisting of L-shaped silicon nanoparticles with in-plane symmetry breaking, as shown in Fig. 16(a). This system supported both Mie and quasi-BIC resonant modes in the wavelength range from 1240 to 1500 nm, as shown in Figs. 16(b) and (c) [245]. Based on these studies, Gandolfi et al. [246] designed a chiral silicon metasurface supporting a quasi-BIC mode with high Q factor in the near-infrared to enhance third-harmonic-generation and its CD.

In the field of nonlinear optical scattering, hyper-Rayleigh optical activity (HROA) and ROA are becoming increasingly popular for studying the stereochemical configuration of chiral molecules. HROA and ROA are chiroptical methods that evaluate chiral signals from HRS and RS processes, respectively. Similar to linear optical activity, both HROA and ROA are extremely weak, primarily owing to the minuscule optical activity of chiral molecules.

As mentioned above, HROA is a chiroptical technique based on HRS, an incoherent second-order nonlinear optical process. In HRS, incident light with a fundamental frequency is scattered at the second harmonic frequency and is used to determine the symmetry of molecules in solution [247]. In an experiment reported in 2019, Collins et al. [248] observed for the first time optical activity in second-harmonic HRS (Fig. 17(a)). The researchers specifically observed HRS optical activity of chiral metal nanohelices in water, with an intensity five orders of magnitude higher than that of linear optical activity. Subsequently, Verreault et al. [249] experimentally demonstrated that HRS can be applied to the chiroptical field of molecular systems in solution. Later, Rodriguez and Verreault [250] utilized HROA to distinguish simple chiral molecules using LP incident light (Fig. 17(b)). Recent theoretical studies reported by Forbes [251] discussed the nonlinear optical activity of optical vortices. The researchers emphasized that CVDS signal can be detected using both HROA and ROA techniques.

ROA-based spectroscopy is a spectroscopic method technique that is sensitive to the structure and properties of chiral molecules in a sample. It is used to identify chiral molecules in solution by detecting the slight difference in RS intensity between LCP and RCP incident light [252], [253], [254]. To overcome the inherent limitation of weak ROA signals, efforts are currently focused on enhancing ROA signals through surface-enhanced ROA (SEROA), such as nanodisk arrays [255], nanoparticles [256], [257], nanogap antennas [258], and spiral nanoflowers [259]. Er et al. [260] deeply discussed the recent progress in ROA and SEROA. Raman spectroscopy is a powerful technique for material analysis; however, achieving single-molecule (SM) detection using this discusses has long been a challenge. Currently, two main approaches for using Raman spectroscopy to detect chiral signals from single molecules exist: electromagnetic enhancement (EME) [261], [262] and chemical enhancement (CME) [263], [264]. Notably, a recent work employed both EME and CME simultaneously to achieve SM Raman detection [265]. Under these two enhancement mechanisms, Raman spectroscopy can be clearly observed at extremely low concentrations down to 10⁻¹⁸ mol·L⁻¹, with a total enhancement factor reaching 16 orders of magnitude. The synergistic interaction of these two enhancement mechanisms holds tremendous potential for ROA spectroscopic technology.

4. Conclusions and prospective

Since Lord Kelvin first proposed a general definition of chirality in the 1840s, significant progress has been made in understanding the interaction between chiral materials and light. This progress has led to the development of optical techniques for exploring the properties of natural chiral materials. These technological advancements have brought about important scientific developments in numerous fields. In this work, we reviewed a portion of research conducted in the field of chiroptics. Particularly in recent decades, with the rapid advancement of nanoscale manufacturing technologies, optical chirality has become possible to deliberately engineer and manufacture deliberately. Technologies for manipulating optical fields have also seen unprecedented development. Optical chirality signals can be enhanced by introducing metasurfaces in the near-field and manipulating light in the far-field. Combining these two strategies offers further potential for enhancement.

This paper discussed recent strategies to enhance chirality in light–matter interactions, aiming to deepen our understanding of the optical chirality of molecules from various perspectives. Chirality holds significant fundamental scientific importance for both molecules and optics and presents broad application prospects in fields such as biomedicine, chemical synthesis, sensing and more. While significant progress has been made in using the techniques (e.g., CD) mentioned in this review for detecting natural and synthetic chiral materials, notable challenges and opportunities remain.

First, designing reconfigurable chiral metamaterials is still a challenging. The reconfigurability of metamaterials enables the adjustment of their optical chirality in response to external stimuli. Given the numerous metamaterials that have been reported for their capacity to amplify the chiral signals of biological molecules, the potential application of reconfigurable chiral metamaterials in biological detection is highly anticipated. Further research into the design theory and manufacturing methods for reconfigurable metamaterials is essential to enhance their chirality response. Additionally, most research is focused on the infrared and visible spectra. Nevertheless, the enhancement of optical activity in the ultraviolet range is a field with considerable promise, as most biomolecules produce strong chiral signals within this frequency range. However, achieving the goal of enhancing chiral signals in the ultraviolet range presents significant challenges. For example, positioning the resonant optical response of nanostructures within the ultraviolet spectrum requires smaller feature sizes, which in turn increases the difficulty of fabrication processes. Finally, in most current studies, the focus has been on characterizing the average chiral properties of entire samples, rather than measuring local chiral features with high spatial resolution. To achieve localized chiral detection with enhanced sensitivity, further development of light- manipulation techniques—particularly in the realm of tightly focused light fields—is essential. Implementing chiral imaging through precise scanning of the detection area could significantly broaden the scope of current chiral measurements. This advancement would pave the way for more selective molecular detection methods.

Acknowledgments

This research was funded by the National Natural Science Foundation of China (62005168, 62075132, and 92050202) and the Natural Science Foundation of Shanghai (22ZR1443100).

Compliance with ethics guidelines

Hanqing Cai, Liangliang Gu, Haifeng Hu, and Qiwen Zhan declare that they have no conflict of interest or financial conflicts to disclose.

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