Optical chiral metamaterials: a review of the fundamentals, fabrication methods and applications

Chirality is inherently a 3D structural property. One classical example of chiral metamatrials is a 3D helix structure (figure 1). In [38], a uniaxial photonic metamaterial composed of 3D gold helices (figure 1(a)) was investigated as a broadband circular polarizer in the mid-infrared regime. These nanostructures are fabricated by direct laser writing of helical pores followed by electrochemical deposition of gold. To fabricate an array of helical pores, a positive-tone photoresist is selected and spun onto a glass substrate. Only those regions that are sufficiently exposed by light are removed. Deposition of gold is then controlled by the applied current density and the growth time. Plasma etching is finally used to remove the polymer and realize a square array of 3D gold helices on a glass substrate. The measured transmission spectra in the wavelength region from 3.5 to 7.6 μm reveal the anticipated functionality of blocking one direction of circularly polarized light, while transmitting the other circular polarization (figure 1(a)). N-helical [52] and tapered chiral metamaterials [69] have also been designed to reduce the polarization conversion and broaden the operation bandwidth. However, this approach turns out to be impractical for bichiral structures, which consist of both left-handed and right-handed spirals arranged along three orthogonal spatial axes [70]. This is because certain parts of the horizontal helical cavities are cut off from the electrolyte before they can be completely filled, leaving behind gaps in the final structures. Radke et al have fabricated bichiral plasmonic crystals by combining direct laser writing with electroless silver plating (figure 1(b)) [71]. First, two-photon femtosecond direct laser writing in a negative-tone photoresist fabricates a 3D bichiral crystal. Next, the dielectric template is coated with a conformal silver film via electroless plating. Electroless plating does not require any external current source and possesses the advantage of coating all exposed surfaces. This feature is very challenging to achieve with standard deposition methods, such as vacuum evaporation and sputter coating, because of the self-shadowing of the complex 3D structures.

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Glancing-angle deposition is another approach for fabricating 3D chiral plasmonic nanostructures [72–75]. Its working principle is based on the geometric shadow effect that materials do not deposit in the shadow created by existing structures. Plasmonic nanohelices have been fabricated via the technique of low-temperature shadow deposition [74, 76]. Through precise control over the nanoseed pattern, substrate and temperature, nanohelices of opposite handedness have been successfully fabricated. The measured circular dichroism spectra of the nanohelices in sodium citrate solution show a peak for LCP light and a dip for RCP light around 600 nm (figure 1(c)). Chiral gold nanoshells have also been made by vacuum evaporation on achiral nanopillars with tilted deposition angles [72]. Large-area 3D gold spirals have also been manufactured by colloidal hole-mask lithography in combination with tilted-angle rotation evaporation (figure 1(d)) [77]. Control over both the rotating speed and direction of the samples can flexibly switch the handedness and working frequency of chiral enantiomers. Circular dichroism signals up to 13% have been measured in the 100–400 THz frequency range. This method offers a simple low-cost manufacturing method for cm²-sized chiral plasmonic metamaterials. Another method for fabricating 3D chiral materials is on-edge lithography [78]. In this approach, a template with a corrugated profile is first generated. Then, a thin chromium layer is deposited by physical vapor deposition, followed by spin coating of a layer of electron beam resist. The resist is subsequently exposed and the resulting mask is evaporated with a layer of gold. The 3D L-shape is finally obtained after lift-off (figure 1(e)). This fabrication approach offers an accurate method for generating 3D chiral nanostructures within a single lithography step.

Focused ion beam induced deposition (FIBID) has recently been employed for realizing broadband chiral metamaterials in the near-infrared region [79]. Platinum nanospirals are formed by FIBID and scanning electron microscopy (SEM) in a dual configuration system. Precursor metal–organic compounds are decomposed by an ion beam and absorbed onto the substrate. The handedness of nanospirals is controlled via sweeping the ion beam in a clockwise or counterclockwise direction. Surface charge effects on different substrates can influence the growth and evolution of nanostructures and induce a dimensional gradient of the wire diameter of the spiral structure. Multiple-helical structures have been created using tomographic rotatory growth, which combines the basic principles of tomography with the FIBID technique (figure 1(f)) [80]. Rotational symmetry is ensured by a packed multi-helix arrangement in a single cell, which eliminates cross-polarization conversion from anisotropy. These triple-helical nanowires show up to 37% of circular dichroism in a broad range from 500–1000 nm.

Focused electron beam induced deposition (FEBID) is also a suitable technique for the realization of three-dimensional nanostructures for optical applications [81–83]. Nanoscaling of chiral structures has been achieved by comparing the FIBID and FEBID approaches [84]. Some physical factors affecting the fabrication processes, including resolution limits, growth control and 3D proximity effects, have been carefully analyzed.

Stacked-planar structures are an alternative approach to accomplishing strong chiroptical responses. In general, such structures can be fabricated by standard electro-beam lithography along with alignment in a layer-by-layer manner. Figure 2(a) shows a chiral structure of twisted split rings [85]. The transmittance spectrum can be tuned by adjusting the twisted angle. Specifically, when the angle is 90°, the electric fields in the gaps of the two split rings are perpendicular to each other and thereby no electric dipole–dipole interaction occurs. In addition, as the higher-order multipolar interaction is negligible in a first approximation, the electric coupling can be ignored. Therefore, the resonances are determined by magnetic dipole–dipole coupling. For the 90° twisted dimer, the magnetic dipoles in the two split rings at the two resonances (${\omega }_{90}^{-}$ and ${\omega }_{90}^{+})$ are aligned parallel and antiparallel, respectively, and thus produce chiroptical responses. This strategy is similar to the concept of stereoisomers in chemistry in that isomers have identical chemical formula but different spatial arrangements of atoms in a molecule. It is known that the electronic properties of a molecule are generally influenced by its composite as well as its configuration. These properties determine how the electronic wave functions mix and hybridize, as well as the interaction and binding between the atoms in the molecule [86]. Similarly, a cluster composed of several nanoparticles provides the possibility of configuring the hybridization of plasmonic resonances, thus enhancing the overall chiroptical responses in plasmonic oligomers (figure 2(b)) [87–89]. Diverse chiral nanostructures have been proposed based on this methodology, including bilayer L-shapes (figure 2(c)), arcs (figure 2(d)) and so on [32, 60, 90–94].

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Broadband circular dichroism can also be achieved by cascading multilayer identical nanorod metasurfaces with a relative rotational twist [95]. As shown in the schematic of figure 2(e), each layer of nanorod is intrinsically achiral. Nevertheless, the chiroptical response emerges when neighboring layers are stacked in a helical-like fashion. The transmission spectra of the four-layer twisted structure demonstrate a broadband circular dichroism in the visible regime. Some constraints can be relaxed in such planar, twisted chiral structures, including the complexity of shape requirements and the lateral alignment between neighboring layers. Moreover, a disposable plasmonic chirality has been recently proposed, where a metafilm is grown on a nanoindented polycarbonate substrate fabricated by high-throughput injected molding [96]. The generated structure consists of a solid nanostructure and an identical shaped void directly above it (figure 2(f)). Babinet's principle implies that the switch of electric and magnetic fields between the solid and inverse structure gives rise to the chiroptical response. The optical properties can be tuned by changing the film thickness. This method offers a cost-effective and flexible way of manipulating the chirality in plasmonic structures.

While top-down fabrication technologies are capable of precisely making nanostructures with a feature size of or below 100 nm, they are normally non-scalable, time-consuming and expensive. By contrast, self-assembly technology provides an alternative bottom-up approach for realizing structures at the truly nanometer scale yet in a cost-efficient, highly-tunable and fast manner. Over the past few years, self-assembled chiral nanostructures have attracted extensive interest and shown significant progress.

Self-assembly technology takes advantage of the fundamental forces of nature to synthesize building blocks into multi-atom functional systems. Objects at the nanometer scale can be organized into equilibrium structures due to the delicate balance among various forces [97, 98], which include Van der Waals forces, capillary forces, static and/or transient electromagnetic forces, convective forces, friction forces, and so on. Based on such self-assembly methods, various metallic nanoparticles with different chemical compositions, geometries and sizes can be precisely positioned and controlled. As a result, plasmonic structures with tunable and controllable dimensions can be assembled in a programmable manner with nanometer precision.

The DNA base-pair interaction is widely used for the synthesis of plasmonic chiral nanostructures. DNA-based formation of silver nanoparticles with CD properties has been demonstrated in some early work [99, 100]. Metallic nanoparticles can also be assembled into pyramid geometries with DNA to enhance the chiral responses [101, 102]. In [101], Mastroianni et al demonstrated chiral pyramidal nanostructures, in which DNA strands function as a scaffold for controlling the position of gold nanoparticles with different diameters. The TEM images of synthesized chiral pyramids are shown in figure 3(a). This work provides a way of creating artificial multi-atom molecules with controllable optical properties. The building blocks in such multi-atom molecules are not limited to gold nanocrystals. It is possible to link other metallic and semiconductor particles via this method. Theoretical modeling for such pyramid structures has been performed [103, 104]. The pyramidal structure consists of two pairs of nanoparticles with different sizes. The intensity, spectral characteristics and handedness of the CD response can be tuned by changing the separation along the symmetry-breaking arm. Simulations predict significant CD tunability when the length of the symmetry-breaking arm is varied [104].

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Bifacial DNA origami templates can also be used to fabricate chiral nanostructures with metallic nanoparticles [105–107]. For example, it has been reported that bifacial DNA origami templates can be used to assemble 3D chiral metamaterials with four gold nanoparticles [106]. The experimental schema is shown in figure 3(b). The rectangular DNA origami template is predesigned with three binding sites on the top surface and the fourth site on the bottom surface, right below one of the three top binding sites. Left-handed and right-handed structures can be obtained by setting the fourth binding sites in the left-handed or right-handed geometry. A characteristic bisignate CD spectrum is observed for left-handed and right-handed chiral nanostructures, compared with achiral nanostructures.

The idea of folding DNA strands to create artificial nanostructures [108–110] has stimulated the rapid development of plasmonic chiral metamaterials [111–114]. One experimental study of DNA-based chiral metamaterial is shown in figure 3(c) [112]. The gold nanoparticles coated with special DNA strands are able to bond with complementary DNA strands on the rectangular template at predesigned positions. Upon the use of folding strands, the rectangular DNA template will roll up and eventually become a 3D DNA origami tube. This DNA 'origami' method enables the synthesis of both left-handed and right-handed metamaterials [113], which exhibit pronounced CD responses as shown in figure 3(d). As expected, the signal of the left-handed chiral plasmonic metamaterials and the signal of the right-handed ones are complementary to each other. The CD spectrum can be controlled by depositing additional silver or silver–gold alloy shells on the gold nanoparticles. Since silver has a plasmonic resonance at a shorter wavelength, a blue shift of the CD resonance can be achieved when changing the metal composition.

Assembling metallic nanoparticles with other chiral templates has also been widely studied, such chiral templates are organogel [115], cysteine [116], peptide [117–119], cellulose nanocrystals [120–123], cholesteric liquid crystals [124–128], chiral mesoporous silica [129, 130], supramolecular fibers [131] and block copolymer templates [132, 133]. For example, one work demonstrated the self-assembly of cysteine (CYS) and gold nanorods [116]. Opposite CD responses were observed for L- and D-CYS assembled gold nanorods, while no chiral responses were observed for DL-CYS. The CD responses can be manipulated from visible to near-infrared wavelengths by changing the aspect ratios of the assembled CYS and gold nanorods. Plasmonic chiral nanostructures can be also synthesized through a peptide-based methodology [119]. As shown in figure 3(e), mixing a gold nanoparticle precursor solution and a HEPES buffer with C₁₂–L-PEP_au or C₁₂–D-PEP_au will form left-handed or right-handed double-helix structures, respectively, which produce strong CD responses. The CD signal can be controlled by varying the thickness of the silver coating. In another work [120], plasmonic chiral metamaterials with cellulose nanocrystals have been assembled. The cellulose nanocrystals are rod-like nanoparticles that were self-assembled into cholesteric liquid crystalline phases. The nanoparticles were highly crystalline and negatively charged with high concentration in suspensions. Chiral plasmonic films were prepared by incorporating gold nanorods in self-assembled cellulose nanocrystals that have chiral arrangements, as shown in figure 3(f). A blue shift in the CD spectrum was observed as the concentration of nanorods increases. The chiral optical activity can be tuned by changing nanorod dimensions and the helical pitch of the cellulose nanostructure. The helical pitch can be tuned by adding NaCl to the nanorod-cellulose nanocrystal suspensions.

Last but not least, plasmonic chiral nanostructures can be realized by binding chiral molecules with metallic nanoparticles [134–137]. In one study [136], plasmonic enhancement of the CD spectrum was demonstrated through the attachment of l-GS-bimane chromophore to the surfaces of the silver nanoparticles. The two orders of enhancement in magnitude made it possible to observe a very weak induced CD signal of the bimane chromophore [136]. It was also found that chiral dichroism can be induced by binding peptide molecules on gold nanoparticles [134], as shown in figure 3(g). Two types of hybrid structures have been studied. One is E5–AuNPs, in which E5 is a 38 amino acid helical peptide that links with the gold nanoparticles through a thiol linkage. The other is FlgA3–AuNPs, where the FlgA3 peptide is an unstructured random coil peptide that binds gold nanoparticles via noncovalent interactions. The chiral responses of two kinds of hybrid structures have been observed.

It is worth noting that optical activity can be observed even when the structures are intrinsically achiral in geometry (figure 4). Such extrinsic chirality arises from the mutual orientation of the achiral metamaterials and the incident beam. This mechanism was detected in liquid crystals in the past [138], but only revisited recently in achiral metamaterials [139]. As shown in figure 4(a), using asymmetric split rings, researchers have observed strong circular dichroism and birefringence indistinguishable from those of chiral 3D materials. The underlying mechanism is the simultaneously induced electric and magnetic dipoles that are parallel to each other. The oblique incidence is crucial for observing extrinsic chirality, because it breaks the mirror symmetry for any plane containing the wave vector (propagating direction). Extrinsic chirality can also induce a large circular dichroism in individual single-walled carbon nanotubes, with the degree of polarization reaching 65% [140]. Moreover, electromagnetic chirality can occur in 1D patterned composites whose components are achiral [141]. A schematic diagram is given in figure 4(b). Electromagnetic waves propagate through a 1D metamaterial whose unit cell is obtained by stacking layers of different media of different thickness along the x-axis. In the epsilon-near-zero regime, the nonlocal effect can be comparable or even greater than the local linear part of the dielectric response and results in enhanced chiroptical responses.

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Extrinsic chirality in anisotropic plasmonic metasurfaces can be greatly boosted by interaction with evanescent excitations [142]. Figure 4(c) schematically illustrates the process in which the incoming light of opposite handedness impinges onto an array of gold nanoparticles. When illuminated slightly above the critical angle, no transmission is allowed. For incident wavelengths close to the plasmonic resonance of the nanoparticles, the metasurface can efficiently absorb the LCP light while the other spin state is almost totally reflected. The effect of photon spin selectivity originates from the mutual orientation of the achiral metasurface, so-called extrinsic chirality, and highly depends on both the incident angle and the relative orientation of the nanoparticle. Almost perfect selectivity (∼90%) of the incident photon spin has been observed at visible frequencies. Since polarization manipulation has intriguing applications in photonics, the enhanced chiroptical effects from evanescent waves may pave a new way for efficient on-chip chiroptical devices.

Molecules of opposite chirality, called enantiomers, always show identities in almost all physical properties (e.g., density, weight, electronic and vibrational frequencies). Opposite enantiomers become distinguishable only when they interact with other chiral objects, such as circularly polarized light. Therefore chirality-sensitive spectroscopic techniques, including circular dichroism, optical rotatory dispersion and Raman optical activity, are widely used in biomolecular science [1, 143]. However, the inherently weak chiroptical effect of molecules imposes significant challenges in biomolecular detection, especially when the concentration of molecules is low. In recent years, chiral plasmonic metamaterials have aroused a lot of interest because their chiroptical effects can be several orders of magnitude higher than those of common biomolecules [34, 144]. Furthermore, chiral plasmonic metamaterials can efficiently enhance the optical signals from biomolecules [145–148]. This is because of the enhancement of the chiral field in the near-field region, known as the superchiral field, which may provide a method for detecting the handedness of a single molecule.

The optical chirality C, a time-even pseudoscalar, is used to quantify the enhancement of this light–matter interaction [149–151]

$$\begin{eqnarray}&&C(\bar{r},t)=\displaystyle \frac{1}{2}\phantom{\rule{0ex}{1.5em}}[{\varepsilon }_{0}\bar{E}(\bar{r},t)\cdot {\rm{\nabla }}\times \bar{E}(\bar{r},t)\\ &&\quad \quad \quad \quad \left.+\displaystyle \frac{1}{{\mu }_{0}}\bar{B}(\bar{r},t)\cdot {\rm{\nabla }}\times \bar{B}(\bar{r},t)\right],\end{eqnarray} \tag{ 9 }$$

where $\bar{E}$ and $\bar{B}$ are the time-dependent electric and magnetic fields, respectively. Although the quantity of chirality C was introduced in 1964, the physical meaning was disclosed very recently [151]. It has been demonstrated that this quantity is closely related to the rate of excitation of a chiral molecule. If one can increase the chirality C at the location of chiral molecules, the sensitivity for the detection of the handedness can also be enhanced [151, 152]. For time-harmonic fields, chirality C can be written as a simpler version [43]

$$\begin{eqnarray}&&C(\bar{r})=-\displaystyle \frac{\omega {\varepsilon }_{0}}{2}{\rm{Im}}[{\bar{E}}^{* }(\bar{r})\cdot \bar{B}(\bar{r})].\end{eqnarray} \tag{ 10 }$$

Since the maximum optical chirality in free space is obtained for circularly polarized light ${C}_{{\rm{CPL}}}^{\pm }=\pm \tfrac{\omega {\varepsilon }_{0}}{2c}{|\bar{E}|}^{2},$ where ω is the angular frequency and c is the velocity of light in vacuum, the relatively local enhancement of chirality can be calculated by ${\hat{C}}^{\pm }={C}^{\pm }/|{C}_{{\rm{CPL}}}^{\pm }|.$ The positive and negative signs correspond to the right-handed and left-handed circular polarizations defined from the point of view of the source, respectively.

Recent work has shown that plasmonic nanostructures, whose geometry directly influences the distributions of their near fields, can be utilized to generate chiral fields with strong optical chirality. Interestingly, this strong optical chirality exists not only in 2D and 3D chiral structures under circularly polarized illuminations [43], but also in achiral structures illuminated by linearly polarized light [153–155]. For example, figure 5(a) shows the optical chirality of a left-handed helix with left-handed circularly polarized light. The absolute values of optical chirality are much smaller for the nonmatching configurations, in which the same structure is illuminated with the opposite spin state. Planar gammadion also exhibits a similar behavior for circularly polarized light, as illustrated in figure 5(b). However, the gammadion shows both positive and negative optical chirality with similar strength, while for the helix the values corresponding to the nonmatching configuration are much smaller. Moreover, in gammadion structures, regions with enhanced optical chirality are at the same position and have same signs for both spin states. Therefore, a change of incident polarization cannot provide a large change in the optical chirality at a certain position, which makes this design less practical in applications. Interestingly, the chiral near fields can also be formed in achiral structures [153, 154]. For example, optical chirality occurs in the near field of nanoslit pairs illuminated with linear polarized light (figure 5(c)) [153].

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An experimental study of superchiral fields for biosensing was reported in [145], which employed gold gammadion arrays working at visible and near-infrared frequencies. As shown in figure 5(d), three resonant modes exist in this planar chiral metamaterial. The spectra of the left-handed and right-handed gammadions are mirror images of each other, as expected. By coupling the incident light into localized surface plasmon resonances of the gold nanostructures, the local chiral field is greatly enhanced and thereby molecules adsorbed on the surface of the chiral metamaterials will strongly influence the resonant wavelength. The property of resonance shifts is caused by the change in the refractive index, which is determined by the chirality of the molecules and that of the fields. According to the first line in figure 5(e), almost no shift is observed for an achiral adsorption such as ethanol. Specifically, large dissymmetry is observed in the difference in resonance shifts of tryptophan and β-sheet proteins. The measured results also provide indirect information on the geometry of chiral molecules, which can be used to explain the asymmetric difference in the resonance shifts. As shown in figure 5(f), biomacromolecules rich in α-helices are more isotropically distributed at the interface, while the β-sheet proteins result in anisotropic aggregation in lateral directions.

The application of chirality in biochemical engineering is not limited to the linear regime. In fact, chiroptical effects in second-harmonic generation (SHG) are typically orders of magnitude larger than their linear counterparts [156–158]. In the second-harmonic process, two photons at a fundamental frequency are annihilated into a single photon at twice the frequency [159]. This response can be described by a nonlinear polarization, which can be written in the electric dipole approximation as ${P}_{i}^{{\rm{NL}}}(2\omega )={\chi }_{ijk}^{(2)}{E}_{j}(\omega ){E}_{k}(\omega ),$ where ω is the angular frequency, ${\chi }_{ijk}^{(2)}$ is the second order susceptibility tensor, E is the electric field and i, j, and k are the Cartesian indices. By applying the inversion symmetry to this equation, we can find that SHG only exists in noncentrosymmetric materials or matter that lacks inversion symmetry. Thus chiral metamaterials are naturally suitable for SHG because all chiral structures are intrinsically noncentrosymmetric. Based on this concept, researchers have performed SHG measurements to detect the handedness of planar G-shape nanostructures [144, 160, 161], as well as twisted-cross gold nanodimers [162]. It has been demonstrated that the chirality can be distinguished with linearly polarized light regardless of the polarization direction and the polarization state of the second-harmonic light [160].

To better understand the underlying physics of the nonlinear process in chiral nanostructures, the relationship between second-harmonic responses and superchiral fields was investigated in [163]. Traditional Slavic symbols (gammadions) were chosen in the experiments, and the separation distance between neighboring units was gradually decreased from 493 nm to 64 nm. Simulation results indicated that the strong density of optical angular momentum at interfaces can give rise to the enhancement of a localized superchiral field and thus strong second-harmonic signals are observed in experiments. In addition, whereas conservation under space and time reversal causes the linear response in chiral metamaterials to be reciprocal, it has also been indicated that the nonlinear ones are in fact non-reciprocal [163]. CD spectra show that CD does not change its sign upon flipping the sample, which is generalized as reciprocal chiroptical behavior. In the nonlinear regime, however, the SHG-CD does change its sign. These findings provide the means for experimental mapping of the local superchiral fields and design guidance for plasmonic devices with strong chiral light–matter interactions. Based on this fact, the separation distance of neighboring units was optimized to enhance the macroscopic circular dichroism in the second-harmonic field (figure 6(a)). Strong SHG-CD signals have also been observed in twisted-arc structures that lack the four-fold rotational symmetry [61]. An image of the 'GT' logo has been created by a point-by-point calculation of the SHG-CD (figure 6(b)). In addition, anisotropic SHG-CD measurements in four-fold symmetric G-shaped nanostructures have demonstrated that both the value and the sign of the CD response are highly dependent on the incident angles of the fundamental illuminations [161]. The SHG probe can thereby also serve as an extremely sensitive probe of the structural symmetry, and allow us to distinguish between chirality and anisotropic effects.

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Chiroptical effects in the second-harmonic field have also been investigated in anisotropic achiral structures [164, 165]. Figure 6(c) shows the measured circular dichroism in the second-harmonic field. Blue circles show the measured SHG-CD for the sample with the curvature of nanowires pointing down. The SHG-CD increases as the incident angle becomes larger. The situation is reversed when the sample is rotated by 180 degrees (red circles). Specifically, the achiral structure of curved nanowires has shown more than 50% visibility by exploiting the extrinsic chirality in the second-harmonic field [165].

The enhancement of chiral light–matter interaction is one of the research focuses in chiral metamaterials. Superchiral fields, as discussed before, have been suggested to overcome the problem of weak chiroptical signals in nature. Subsequently, significant effort has been given to find various nanostructures that enhance chiroptical signals, such as plasmonic structures [43, 145, 153, 155], dielectric nanoparticles [166] and negative-index metamaterials [167]. The chiral Purcell factor has also been proposed for characterizing the ability of optical resonators to enhance chiroptical signals [168]. In [169], the effect of parity-time symmetric potentials on the radiation of chiral dipole sources has been theoretically investigated. Through appropriate design of parity-time symmetric potentials, chiral quantum emitters can be distinguished by their decay rates.

The interaction between quantum emitter and chiral metamaterials has been experimentally investigated in [170]. In this work, chiral arc metamaterials were adopted as the two enantiomers, as shown in figure 7(a). Achiral quantum dots occupied the entire space around the upper arc and only a slight part of the lower arc. In the linear regime, the circular dichroism of the arc metamaterials reached as high as 50% at the wavelength of resonance (∼780 nm), which led to the highest contrast of the transmission images of the chiral metamaterials patterned into the Georgia Tech mascot. The light confinement at the resonant wavelength was largely boosted and thereby implied a chiral-selective enhancement in nonlinear light–matter interaction. Specifically, the chiral contrast of the two-photon luminescence was witnessed when a circularly polarized light was incident on the embedded quantum emitters. The two-photon luminescence from the chiral system was remarkably enhanced by over 40× with respect to a reference case without the metamaterials. Moreover, the enhancement was chirality-enabled and was sensitive to the handedness of the illuminations. The enhancement factor for the two spin states achieved a contrast as large as 3 in the experiment. These findings manifest the potential for applications in chiral-selective imaging, sensing, and spectroscopy.

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Apart from the enhancement of quantum emitters discussed above, another fascinating application is chirality-selective optoelectronics. In the photoemission process, light absorption in solids can produce hot carriers whose energies are larger than those of thermal excitations at ambient temperatures [171]. Hot carrier science has played an important role in the development of quantum mechanics in the past, and nowadays offers exciting opportunities for fundamental research and applications in optoelectronics, catalysis, and photochemistry. Hot electrons generated from light absorption in a metal can be emitted over a Schottky barrier to produce current. Properly designed plasmonic nanostructures may greatly enhance light absorption and provide valuable control over the emission of the hot electrons in practical applications [172–174]. Chiral plasmonic nanostructures enable disparate light absorption for different circular polarizations and result in chiral-responsive hot-electron devices. To prove this concept, an ultracompact device for circularly polarized light detection has been proposed [175]. As shown in figure 7(b), the designed chiral metamaterial consists of Z-shaped silver stripes on top of a dielectric spacer and an optically thick silver backplane. The chiral metamaterial acts as a perfect LCP light absorber at the resonant wavelength and it reflects nearly 90% of the light of the other spin state. The simulated circular dichrosim (CD = A_LCP−A_RCP) reaches as high as 0.9, promising enhanced discrimination between LCP and RCP in photodetection. In experiment, a Schottky barrier is formed by integrating a semiconductor layer (n-type silicon) on top of the chiral plasmonic structure. Photoresponse spectra match well with the measured absorption spectra, and demonstrate peak photoresponsivity of the resonant state up to 2.2 mA W⁻¹. The corresponding quantum efficiency reaches as high as 0.2%. This efficiency is two times that of chiral organic semiconductor transistors [176]. Furthermore, large circular dichroism gives rise to a significant distinction in photocurrents for the two spin states, with a polarization discrimination ratio of 3.4 and a difference in photoresponsivity of 1.5 mA W⁻¹.

Active control over the chirality of metamaterials has the potential of serving as a key element of future optical systems such as polarization sensitive imaging and interactive display. However, achieving active control over chiral metamaterials is challenging since it involves the reconfiguration of the metamolecule from a left-handed enantiomer to a right-handed counterpart and vice versa. In the terahertz regime, the generation of charge carriers via an optical pump in silicon has been utilized to actively switch the overall handedness of chiral structures [177–180]. A pulsed laser working at 800 nm is utilized to excite electron–hole pairs across the 1.12 eV bandgap of silicon. Under the illumination of a 500 mW pump laser, the photoconductivity of silicon can be as high as 50 000 S m⁻¹. This feature enables all-optical switching of the handedness in chiral metamaterials in the THz regime. An all-optical tunable chirality has also been realized in a double-layer metamaterial, which consists of a nonlinear nano-Au:polycrystalline indium-tin oxide layer between two L-shaped nano-antennas in the building block [181]. A 45 nm shift of the peak in the circular dichroism spectrum has been observed under a 40 kW cm⁻² weak pump. This work opens up the possibility for ultralow-power and ultrafast all-optical tunable chirality at visible frequencies.

Switchable chirality can also be achieved by micro-electro-mechanical systems. In [182], a planar spiral structure is vertically deformed into its three-dimensional counterparts once a pneumatic force is applied (figure 8(a)). The pneumatic force is supplied through air channels with the pressure source of N₂ gas regulated by a pressure injector. Enantiomer switching is realized by selecting the deformation direction, which allows the alteration of the polarity of the optical activity without changing the spectral shape. To eliminate the cross-polarization conversion from birefringence, spiral arrays with C₄ symmetry have been further developed. A polarization rotation of up to 28 degrees has been experimentally observed, which provides a compact polarization modulator in the THz regime.

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Phase-change materials [183–186], whose refractive index can be modified by temperature, voltage or light pulses, have been integrated with metamaterials to dynamically control the properties of metamateirals [187, 188]. Very recently, active control of chirality has been experimentally verified in [189], in which a layer of phase-change material Ge₃Sb₂Te₆ is sandwiched between two stacked nanorods (figure 8(b)). Such sandwich structures can ensure an optimum interaction between Ge₃Sb₂Te₆ and the plasmonic near field, and does not require complex lithography. A thermally induced transition from the amorphous state to the crystalline state occurs when the phase-change layer is heated up to the transition temperature (160 °C), and leads to a change in refractive index from 3.5 + 0.01i to 6.5 + 0.06i. Simulation results demonstrate that a giant spectral shift of around 20% at mid-infrared frequencies occurs when the phase transition happens. The measured circular dichroism spectrum is highly consistent with the theoretical prediction and a large spectral shift of 18% is realized. The wavelength tunability has also been utilized to obtain a reversal in the sign of the circular dichroism. Thermal switching of the handedness at a fixed wavelength is realized by cascading an active right-handed chiral dimer with a passive left-handed one. In the amorphous state, the strong positive CD signals from the active right-handed chiral dimer dominate and make the entire structure right-handed. In the crystalline state, the CD peak red shifts and the signals from the passive left-handed chiral dimer determines the entire chirality of the cascade system. Experiments have demonstrated that for cascade chiral dimers, the CD signal at 4200 nm in the amorphous state is exactly opposite to that in the crystalline state. These findings pave a new way towards thermal-controlled polarization modulation in the mid-infrared region and may find potential applications in thermal imaging and detection.

DNA-directed assembly has recently been proven to be a viable method for the controlled arrangement of plasmonic nanostructures, thus offering reconfigurable chirality [190–192]. As shown in figure 8(c), two gold nanorods are hosted on a reconfigurable DNA template, which consists of two connected bundles folded from a long single-stranded DNA scaffold. The relative angle between nanorods can be controlled by two DNA locks that extended from the sides of the DNA origami bundles. By adding designed DNA fuel strands, the 3D metamolecules can be switched between different conformational states, thus giving distinct CD spectra. Both of the DNA locks are open in an initially relaxed state. In the left cyclic process, a removal strand is added to dissociate the DNA strand from the arm through a branch migration process. Unreactive waste is produced during this process. The two DNA bundles are joined together, and therefore a left-handed system is established. With the addition of a return strand, the DNA lock is opened again through a second brand migration process. Hence, the system returns to its relax state. Figure 8(d) presents the measured CD signal over time between three distinct states: relaxed, left-handed and right-handed. The wavelength here is fixed at 725 nm. This CD signal clearly describes the cycling between three states of this system. The combination of plasmonics with DNA technology provides a versatile platform for designing active devices and may advance the development of smart probes for life science and biochemistry.