Chirality and chiroptical effects in plasmonic nanostructures: fundamentals, recent progress, and outlook

High-Performance Refractive Index and Temperature Sensing Based on Toroidal Dipole in All-Dielectric Metasurface

This article shows an all-dielectric metasurface consisting of "H"-shaped silicon disks with tilted splitting gaps, which can detect the temperature and refractive index (RI). By introducing asymmetry parameters that excite the quasi-BIC, there are three distinct Fano resonances with nearly 100% modulation depth, and the maximal quality factor (Q-factor) is over 104. The predominant roles of different electromagnetic excitations in three distinct modes are demonstrated through near-field analysis and multipole decomposition. A numerical analysis of resonance response based on different refractive indices reveals a RI sensitivity of 262 nm/RIU and figure of merit (FOM) of 2183 RIU-1. This sensor can detect temperature fluctuations with a temperature sensitivity of 59.5 pm/k. The proposed metasurface provides a novel method to induce powerful TD resonances and offers possibilities for the design of high-performance sensors.

Chirality tuning and reversing with resonant phase-change metasurfaces

Dynamic control of circular dichroism in photonic structures is critically important for compact spectrometers, stereoscopic displays, and information processing exploiting multiple degrees of freedom. Metasurfaces can help miniaturize chiral devices but only produce static and limited chiral responses. While external stimuli can tune resonances, their modulations are often weak, and reversing continuously the sign of circular dichroism is extremely challenging. Here, we demonstrate the dynamically tunable chiral response of resonant metasurfaces supporting chiral bound states in the continuum combining them with phase-change materials. Phase transition between amorphous and crystalline phases allows for control of chiral response and varies chirality rapidly from -0.947 to +0.958 backward and forward via the chirality continuum. Our demonstrations underpin the rapid development of chiral photonics and its applications.

Nonlinear Optical Activity of a Chiral Organic-Inorganic ([(NH3CH2CH2)3NH])2[MnBr5]Br5 Photoluminescent and Piezoelectric Crystal

The family of Mn-based organic-inorganic hybrids has greatly expanded due to their advantages in applications. They also show superior bright and size-tunable photoluminescence and can be considered a perfect alternative to toxic lead-based compounds. In this work, we present the detailed structural, optical, and electrical characterization of ([(NH3CH2CH2)3NH])2[MnBr5]Br5. The title compound exhibits a unique type of inorganic arrangement created by the trigonal bipyramids. It crystallizes in noncentrosymmetric space group R32, indicating its optical activity, piezoelectricity, and second-order optical nonlinearity proven by the second harmonic of light measurements. The studied crystals exhibit intense photoluminescence originating from the Mn(II) ion 4T1(G) → 6A1 transition. The measured lifetime of the photoluminescence emission is ≤1.5 ms, while the measured quantum yield for both powder and crystal samples reaches ∼70%.

Chiral plasmonic metasurface assembled by DNA origami

Chiral materials are essential to perceive photonic devices that control the helicity of light. However, the chirality of natural materials is rather weak, and relatively thick films are needed for noticeable effects. To overcome this limitation, artificial photonic materials were suggested to affect the chiral response in a much more substantial manner. Ideally, a single layer of such a material, a metasurface, should already be sufficient. While various structures fabricated with top-down nanofabrication technologies have already been reported, here we propose to utilize scaffolded DNA origami technology, a scalable bottom-up approach for metamolecule production, to fabricate a chiral metasurface. We introduce a chiral plasmonic metamolecule in the shape of a tripod and simulate its optical properties. By fixing the metamolecule to a rectangular planar origami, the tripods can be assembled into a 2D DNA origami crystal that forms a chiral metasurface. We simulate the optical properties but also fabricate selected devices to assess the experimental feasibility of the suggested approach critically.

Phototransformation of achiral metasurfaces into handedness-selectable transient chiral media

Chirality is a geometric property describing the lack of mirror symmetry. This unique feature enables photonic spin-selectivity in light-matter interaction, which is of great significance in stereochemistry, drug development, quantum optics, and optical polarization control. The versatile control of optical geometry renders optical metamaterials as an effective platform for engineered chiral properties at prescribed spectral regimes. Unfortunately, geometry-imposed restrictions only allow one circular polarization state of photons to effectively interact with chiral meta-structures. This limitation motivates the idea of discovering alternative techniques for dynamically reconfiguring the chiroptical responses of metamaterials in a fast and facile manner. Here, we demonstrate an approach that enables optical, sub-picosecond conversion of achiral meta-structures to transient chiral media in the visible regime with desired handedness upon the inhomogeneous generation of plasmonic hot electrons. As a proof of concept, we utilize linearly polarized laser pulse to demonstrate near-complete conversion of spin sensitivity in an achiral meta-platform-a functionality yet achieved in a non-mechanical fashion. Owing to the generation, diffusion, and relaxation dynamics of hot electrons, the demonstrated technique for all-optical creation of chirality is inherently fast, opening new avenues for ultrafast spectro-temporal construction of chiral platforms with on-demand spin-selectivity.

Direct-write 3D printing of plasmonic nanohelicoids by circularly polarized light

Chiral plasmonic surfaces with 3D "forests" from nanohelicoids should provide strong optical rotation due to alignment of helical axis with propagation vector of photons. However, such three-dimensional nanostructures also demand multi-step nanofabrication, which is incompatible with many substrates. Large-scale photonic patterns on polymeric and flexible substrates remain unattainable. Here, we demonstrate the substrate-tolerant direct-write printing and patterning of silver nanohelicoids with out-of-plane 3D orientation using circularly polarized light. Centimeter-scale chiral plasmonic surfaces can be produced within minutes using inexpensive medium-power lasers. The growth of nanohelicoids is driven by the symmetry-broken site-selective deposition and self-assembly of the silver nanoparticles (NPs). The ellipticity and wavelength of the incident photons control the local handedness and size of the printed nanohelicoids, which enables on-the-fly modulation of nanohelicoid chirality during direct writing and simple pathways to complex multifunctional metasurfaces. Processing simplicity, high polarization rotation, and fine spatial resolution of the light-driven printing of stand-up helicoids provide a rapid pathway to chiral plasmonic surfaces, accelerating the development of chiral photonics for health and information technologies.

Origination of the chiroptical effect in plasmonic nano-structures in the view of quasi-normal mode theory

General chiroptical effects describe all of the interaction differences between light carrying opposite spins and chiral matters, such as circular dichroism, optical activity, and chiral Raman optical activity, and have been proven to hold great promise for extensive applications in physics, chemistry, and biology. However, the underlying physical mechanism is usually explained intangibly by the twisted currents in chiral geometry, where the cross coupling between the electric and magnetic dipoles breaks the degeneracy of the helicity eigenmodes. In this Letter, we construct a clear sight on the origination of the chiroptical effect in the view of the eigenstates of a non-Hermitian system, i.e., quasi-normal modes (QNMs). The intrinsic chiroptical effect comes from the chiral QNMs, which have distinct excitation and emission differences in both phase and intensity for lights carrying opposite spins, while the extrinsic chiroptical effect coming from the achiral QNMs requires specific illumination and observation conditions, where the low symmetrical QNM can generate chiroptical effects in both absorption and scattering, but the highly symmetrical QNMs can only generate chiroptical effects in scattering through the coherent superposition of several QNMs. Our findings offer an in-depth understanding of the chiroptical effect and have the potential to bring broad inspiration to the design and applications of chiroptical effects.

Polarization and incident angle independent multifunctional tunable terahertz metasurface based on graphene

Motivated by the imperative demand for design integration and miniaturization within the terahertz (THz) spectrum, this study presents an innovative solution to the challenges associated with singular functionality, limited application scope, and intricate structures prevalent in conventional metasurfaces. The proposed multifunctional tunable metasurface leverages a hybridized grapheme-metal structure, addressing critical limitations in existing designs. Comprising three distinct layers, namely a graphene-gold resonance layer, a Topas dielectric layer, and a bottom gold film reflective layer, this terahertz metasurface exhibits multifunctionality that is both polarization and incident-angle independent. The metasurface demonstrates a broadband circular dichroism (CD) function when subjected to incident circularly polarized waves. In contrast, under linear incidence, the proposed design achieves functionalities encompassing linear dichroism (LD) and polarization conversion. Remarkably, graphene's chemical potential and the incident light's state can be manipulated to tune each functional aspect's intensity finely. The proposed tunable multifaceted metasurface showcases significant referential importance within the terahertz spectrum, mainly contributing to advancing CD metamirrors, chiral photodetectors, polarization digital imaging systems, and intelligent switches.

Bulk Electrosynthesis of Patchy Particles with Highly Controlled Asymmetric Features

Asymmetric modification of particles with various patches of different composition and size at predefined positions is an important challenge in contemporary surface chemistry, as such particles have numerous potential applications, ranging from materials science and (photo)catalysis to self-assembly and drug delivery. However, approaches allowing the synthesis of this kind of complex objects in the bulk of a solution in a straightforward way are currently lacking. In this context, bipolar electrochemistry (BE) is a powerful technique for the asymmetric modification of conducting objects. Herein, this approach is used for the highly controlled modification of particles with different metal patches, generated at specific locations of isotropic objects. The synthesis is carried out in the bulk of the solution and leads to predefined patterns of increasing complexity, including even a specific chiral arrangement of the patches.

Low-Dimensional-Materials-Based Photodetectors for Next-Generation Polarized Detection and Imaging

The vector characteristics of light and the vectorial transformations during its transmission lay a foundation for polarized photodetection of objects, which broadens the applications of related detectors in complex environments. With the breakthrough of low-dimensional materials (LDMs) in optics and electronics over the past few years, the combination of these novel LDMs and traditional working modes is expected to bring new development opportunities in this field. Here, the state-of-the-art progress of LDMs, as polarization-sensitive components in polarized photodetection and even the imaging, is the main focus, with emphasis on the relationship between traditional working principle of polarized photodetectors (PPs) and photoresponse mechanisms of LDMs. Particularly, from the view of constitutive equations, the existing works are reorganized, reclassified, and reviewed. Perspectives on the opportunities and challenges are also discussed. It is hoped that this work can provide a more general overview in the use of LDMs in this field, sorting out the way of related devices for "more than Moore" or even the "beyond Moore" research.

Key Role of Asymmetric Photothermal Effect in Selectively Chiral Switching of Plasmonic Dimer Driven by Circularly Polarized Light

Strong interaction between circularly polarized light and chiral plasmonic nanostructures can enable controllable asymmetric photophysical processes, such as selective chiral switching of a plasmonic nanorod-dimer. Here, we uncover the underlying physics that governs this chiral switching by theoretically investigating the interplay between asymmetric photothermal and optomechanical effects. We find that the photothermally induced local temperature rises could play a key role in activating the dynamic chiral configurations of a plasmonic dimer due to the temperature-sensitive molecular linkages located at the gap region. Importantly, different temperature rises caused by the opposite handedness of light could facilitate selective chiral switching of the plasmonic dimer driven by asymmetric optical torques. Our analyses on the wavelength-dependent selectively chiral switching behaviors are in good agreement with the experimental observations. This work contributes to a comprehensive understanding of the physical mechanism involved in the experimentally designed photoresponsive plasmonic nanosystems for practical applications.

Metasurface-based circular polarizer with a controllable phase and its application in holographic imaging

A diatomic circular polarizer based on a single-layered metasurface is proposed. This metasurface circular polarizer carries the controllable phase besides the desired circular dichroism, which is different from the existing circular polarizers. The diatoms contain two nanoholes equivalent to half-wave plates with a specified cross angle and a fixed phase difference. The alternative circular polarization transmission of this circular polarizer depends on the relative angular position of diatoms, and the controllable phase of this circular polarizer can be adjusted through rotating nanoholes. The generation of the optical vortex and holographic imaging verifies the polarization and phase manipulation of the diatomic circular polarizer. The numerical simulations and the experimental measurement give the powerful verification. Simple design, compact structure, and poly-functionality enable the wide applications of circular polarizer in integrated and polarized optics.

Reversible Ultrafast Chiroptical Responses in Planar Plasmonic Nano-Oligomer

Ultracompact chiral plasmonic nanostructures with unique chiral light-matter interactions are vital for future photonic technologies. However, previous studies are limited to reporting their steady-state performance, presenting a fundamental obstacle to the development of high-speed optical devices with polarization sensitivity. Here, a comprehensive analysis of ultrafast chiroptical response of chiral gold nano-oligomers using time-resolved polarimetric measurements is provided. Significant differences are observed in terms of the absorption intensity, thus hot electron generation, and hot carrier decay time upon polarized photopumping, which are explained by a phenomenological model of the helicity-resolved optical transitions. Moreover, the chiroptical signal is switchable by reversing the direction of the pump pulse, demonstrating the versatile modulation of polarization selection in a single device. The results offer fundamental insights into the helicity-resolved optical transitions in photoexcited chiral plasmonics and can facilitate the development of high-speed polarization-sensitive flat optics with potential applications in nanophotonics and quantum optics.

Chiral Plasmonic Hybrid Nanostructures: A Gateway to Advanced Chiroptical Materials

Chirality introduces a new dimension of functionality to materials, unlocking new possibilities across various fields. When integrated with plasmonic hybrid nanostructures, this attribute synergizes with plasmonic and other functionalities, resulting in unprecedented chiroptical materials that push the boundaries of the system's capabilities. Recent advancements have illuminated the remarkable chiral light-matter interactions within chiral plasmonic hybrid nanomaterials, allowing for the harnessing of their tunable optical activity and hybrid components. These advancements have led to applications in areas such as chiral sensing, catalysis, and spin optics. Despite these promising developments, there remains a need for a comprehensive synthesis of the current state-of-the-art knowledge, as well as a thorough understanding of the construction techniques and practical applications in this field. This review begins with an exploration of the origins of plasmonic chirality and an overview of the latest advancements in the synthesis of chiral plasmonic hybrid nanostructures. Furthermore, representative emerging categories of hybrid nanomaterials are classified and summarized, elucidating their versatile applications. Finally, the review engages with the fundamental challenges associated with chiral plasmonic hybrid nanostructures and offer insights into the future prospects of this advanced field.

Electron Beam Induced Circularly Polarized Light Emission of Chiral Gold Nanohelices

Chiral plasmonic nanostructures possess a chiroptical response orders of magnitude stronger than that of natural biomolecular systems, making them highly promising for a wide range of biochemical, medical, and physical applications. Despite extensive efforts to artificially create and tune the chiroptical properties of chiral nanostructures through compositional and geometrical modifications, a fundamental understanding of their underlying mechanisms remains limited. In this study, we present a comprehensive investigation of individual gold nanohelices by using advanced analytical electron microscopy techniques. Our results, as determined by angle-resolved cathodoluminescence polarimetry measurements, reveal a strong correlation between the circular polarization state of the emitted far-field radiation and the handedness of the chiral nanostructure in terms of both its dominant circularity and directional intensity distribution. Further analyses, including electron energy-loss measurements and numerical simulations, demonstrate that this correlation is driven by longitudinal plasmonic modes that oscillate along the helical windings, much like straight nanorods of equal strength and length. However, due to the three-dimensional shape of the structures, these longitudinal modes induce dipolar transverse modes with charge oscillations along the short axis of the helices for certain resonance energies. Their radiative decay leads to observed emission in the visible range. Our findings provide insight into the radiative properties and underlying mechanisms of chiral plasmonic nanostructures and enable their future development and application in a wide range of fields, such as nano-optics, metamaterials, molecular physics, biochemistry, and, most promising, chiral sensing via plasmonically enhanced chiral optical spectroscopy techniques.

Terahertz Metamaterials for Biosensing Applications: A Review

In recent decades, THz metamaterials have emerged as a promising technology for biosensing by extracting useful information (composition, structure and dynamics) of biological samples from the interaction between the THz wave and the biological samples. Advantages of biosensing with THz metamaterials include label-free and non-invasive detection with high sensitivity. In this review, we first summarize different THz sensing principles modulated by the metamaterial for bio-analyte detection. Then, we compare various resonance modes induced in the THz range for biosensing enhancement. In addition, non-conventional materials used in the THz metamaterial to improve the biosensing performance are evaluated. We categorize and review different types of bio-analyte detection using THz metamaterials. Finally, we discuss the future perspective of THz metamaterial in biosensing.

Chiral Gold Nanorods with Five-Fold Rotational Symmetry and Orientation-Dependent Chiroptical Properties of Their Monomers and Dimers

Chiral plasmonic nanoparticles have attracted much attention because of their strong chiroptical responses and broad scientific applications. However, the types of chiral plasmonic nanoparticles have remained limited. Herein we report on a new type of chiral nanoparticle, chiral Au nanorod (NR) with five-fold rotational symmetry, which is synthesized using chiral molecules. Three different types of Au seeds (Au elongated nanodecahedrons, nanodecahedrons, and nanobipyramids) are used to study the growth behaviors. Different synthesis parameters, including the chiral molecules, surfactant, reductant, seeds, and Au precursor, are systematically varied to optimize the chiroptical responses of the chiral Au NRs. The chiral scattering measurements on the individual chiral Au NRs and their dimers are performed. Intriguingly, the chiroptical signals of the individual chiral Au NRs and their end-to-end dimers are similar, while those of the side-by-side dimers are largely reduced. Theoretical calculations and numerical simulations reveal that the different chiroptical responses of the chiral NR dimers are originated from the coupling effect between the plasmon resonance modes. Our study enriches chiral plasmonic nanoparticles and provides valuable insight for the design of plasmonic nanostructures with desired chiroptical properties.

Chiral Mesostructured Inorganic Materials with Optical Chiral Response

Fabricating chiral inorganic materials and revealing their unique quantum confinement-determined optical chiral responses are crucial tasks in the multidisciplinary fields of chemistry, physics, and biology. The field of chiral mesostructured inorganic materials started from the synthesis of individual nanocrystals and evolved to include their assembly from metals, semiconductors, ceramics, and inorganic salts endowed with various chiral structures ranging from atomic to micron scales. This tutorial review highlights the recent research on chiral mesostructured inorganic materials, especially the novel expression of mesostructured chirality and endowed optical chiral response, and it may inspire us with new strategies for the design of chiral inorganic materials and new opportunities beyond the traditional applications of chirality. Fabrication methods for chiral mesostructured inorganic materials are classified according to chirality type, scale, and symmetry-breaking mechanism. Special attention is given to highlight systems with original discoveries, exceptional phenomena, or unique mechanisms of optical chiral response for left- and right-handedness.

Maximum helical dichroism enabled by an exceptional point in non-Hermitian gradient metasurfaces

Helical dichroism (HD) utilizing unbounded orbital angular momentum degree of freedom, has provided an important means of exploring chiral effects in diverse wave systems, surpassing the two-state constraint in circular dichroism that relies on intrinsic spin. However, the naturally feeble chiral signals that arise during wave-matter interactions pose significant challenges to the effective enlargement of HD. Here, we introduce a new paradigm for realizing maximum HD through non-Hermitian gradient metasurfaces by engineering a chiral exceptional point (EP) in intrinsic topological charge. The non-Hermitian gradient metasurfaces are empowered by the asymmetric coupling feature at the EP, enabling flexible construction to realize versatile chirality control in extreme circumstances where one chiral vortex is totally reflected and the opposite counterpart is completely absorbed or transmitted into the customized vortex modes. As the manifestation of the maximum HD, we present the first experimental demonstration of perfect chirality-selected vortex transmission in acoustics. Our findings open new venues to achieve maximum chirality and explore chiral physics of wave-matter interactions, which can boost many vortical applications in asymmetric chirality manipulation, one-way propagation, and information multiplexing.

A reprogrammable mechanical metamaterial with origami functional-group transformation and ring reconfiguration

Recent advancements in reprogrammable metamaterials have enabled the development of intelligent matters with variable special properties in situ. These metamaterials employ intra-element physical reconfiguration and inter-element structural transformation. However, existing mono-characteristic homo-element mechanical metamaterials have limited reprogramming functions. Here, we introduce a reprogrammable mechanical metamaterial composed of origami elements with heterogeneous mechanical properties, which achieves various mechanical behavior patterns by functional group transformations and ring reconfigurations. Through the anisotropic assembly of two heterogeneous elements into a functional group, we enable mechanical behavior switching between positive and negative stiffness. The resulting polygonal ring exhibits rotational deformation, zero Poisson's ratio stretching/compression deformation, and negative Poisson's ratio auxetic deformation. Arranging these rings periodically yields homogeneous metamaterials. The reconfiguration of quadrilateral rings allows for continuous fine-tunability of the mechanical response and negative Poisson's ratio. This mechanical metamaterial could provide a versatile material platform for reprogrammable mechanical computing, multi-purpose robots, transformable vehicles and architectures at different scales.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Creation of galaxy-shaped vortex relief structures in azo-polymers with petal-like beams

We demonstrate the formation of surface relief structures in azo-polymers which exhibit multiple spiral arms, through irradiation of a rotating petal-like beam formed by the coherent superposition of Laguerre-Gaussian modes with opposite handedness. Intriguingly, the fabricated relief structures reflect full geometric parameters of the irradiated petal beam, such as handedness, topological charge, initial azimuthal phase and even ellipticity, corresponding to azimuthal and polar angles along equator and meridian planes of an orbital Poincaré sphere. The handedness, or direction of rotation, of the fabricated structures with multiple spiral arms could be controlled via the rotation and polarization directions of the irradiating laser field. This effect highlights an exotic coupling between the optical intensity gradient induced mass transport of the irradiated material and the spin angular momentum characteristics of the irradiating optical field. The azimuthal orientation of the surface relief structures could also be tuned by altering the initial relative phase between the coherently superposed Laguerre-Gaussian modes with opposite handedness, constituting the irradiating petal laser field. This work offers new insights into fundamental interactions which occur between light and matter, and we believe, will pave the way towards advanced technologies, such as ultrahigh density optical data storage.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Plasmonic Nanostructure Engineering with Shadow Growth

Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.

Dense Arrays of Nanohelices: Raman Scattering from Achiral Molecules Reveals the Near-Field Enhancements at Chiral Metasurfaces

Against the background of the current healthcare and climate emergencies, surface enhanced Raman scattering (SERS) is becoming a highly topical technique for identifying and fingerprinting molecules, e.g., within viruses, bacteria, drugs, and atmospheric aerosols. Crucial for SERS is the need for substrates with strong and reproducible enhancements of the Raman signal over large areas and with a low fabrication cost. Here, dense arrays of plasmonic nanohelices (≈100 nm in length), which are of interest for many advanced nanophotonics applications, are investigated, and they are shown to present excellent SERS properties. As an illustration, two new ways to probe near-field enhancement generated with circular polarization at chiral metasurfaces are presented, first using the Raman spectra of achiral molecules (crystal violet) and second using a single, element-specific, achiral molecular vibrational mode (i.e., a single Raman peak). The nanohelices can be fabricated over large areas at a low cost and they provide strong, robust and uniform Raman enhancement. It is anticipated that these advanced materials will find broad applications in surface enhanced Raman spectroscopies and material science.

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of achiral gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the surface lattice resonances (SLRs) become optically active, i.e., showing handedness-dependent excitation. Numerical simulations with varying lattice parameters show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of single-molecular and plasmonic resonances rationalizes this chirality transfer.

Atomically precise gold nanoclusters at the molecular-to-metallic transition with intrinsic chirality from surface layers

The advances in determining the total structure of atomically precise metal nanoclusters have prompted extensive exploration into the origins of chirality in nanoscale systems. While chirality is generally transferrable from the surface layer to the metal-ligand interface and kernel, we present here an alternative type of gold nanoclusters (138 gold core atoms with 48 2,4-dimethylbenzenethiolate surface ligands) whose inner structures are not asymmetrically induced by chiral patterns of the outermost aromatic substituents. This phenomenon can be explained by the highly dynamic behaviors of aromatic rings in the thiolates assembled via π - π stacking and C - H···π interactions. In addition to being a thiolate-protected nanocluster with uncoordinated surface gold atoms, the reported Au₁₃₈ motif expands the size range of gold nanoclusters having both molecular and metallic properties. Our current work introduces an important class of nanoclusters with intrinsic chirality from surface layers rather than inner structures and will aid in elucidating the transition of gold nanoclusters from their molecular to metallic states.

Chirality Analysis of Complex Microparticles using Deep Learning on Realistic Sets of Microscopy Images

Nanoscale chirality is an actively growing research field spurred by the giant chiroptical activity, enantioselective biological activity, and asymmetric catalytic activity of chiral nanostructures. Compared to chiral molecules, the handedness of chiral nano- and microstructures can be directly established via electron microscopy, which can be utilized for the automatic analysis of chiral nanostructures and prediction of their properties. However, chirality in complex materials may have multiple geometric forms and scales. Computational identification of chirality from electron microscopy images rather than optical measurements is convenient but is fundamentally challenging, too, because (1) image features differentiating left- and right-handed particles can be ambiguous and (2) three-dimensional structure essential for chirality is 'flattened' into two-dimensional projections. Here, we show that deep learning algorithms can identify twisted bowtie-shaped microparticles with nearly 100% accuracy and classify them as left- and right-handed with as high as 99% accuracy. Importantly, such accuracy was achieved with as few as 30 original electron microscopy images of bowties. Furthermore, after training on bowtie particles with complex nanostructured features, the model can recognize other chiral shapes with different geometries without retraining for their specific chiral geometry with 93% accuracy, indicating the true learning abilities of the employed neural networks. These findings indicate that our algorithm trained on a practically feasible set of experimental data enables automated analysis of microscopy data for the accelerated discovery of chiral particles and their complex systems for multiple applications.

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.

Single-Layer Transmissive Chiral Plasma Metasurface with High Circular Polarization Extinction Ratio in Visible Wavelength

Chiral metamaterials are extensively applied in the fields of photoelectric detection, biomedical diagnostics and micro-nano polarization imaging. Currently, single-layer chiral metamaterials are unfortunately limited by several issues, such as a weaker circular polarization extinction ratio and circular polarization transmittance difference. To tackle these issues, a single-layer transmissive chiral plasma metasurface (SCPMs) suitable for visible wavelength is proposed in this paper. Its basic unit is composed of double orthogonal rectangular slots and a spatial π/4 inclined arrangement of the rectangular slot to constitute a chiral structure. Each rectangular slot structure has characteristics that enable the SCPMs to easily achieve a high circular polarization extinction ratio and strong circular polarization transmittance difference. Both the circular polarization extinction ratio and circular polarization transmittance difference of the SCPMs reach over 1000 and 0.28 at a wavelength of 532 nm, respectively. In addition, the SCPMs is fabricated via the thermally evaporated deposition technique and focused ion beam system. This compact structure coupled with a simple process and excellent properties enhances its applicability for the control and detection of polarization, especially during integration with linear polarizers, to achieve the fabrication of a division-of-focal-plane full-Stokes polarimeter.

Chiral hybrid waveguide-plasmon resonances

We investigate the chiroptical responses of the hybrid systems consisting of metal-insulator-metal (MIM) gammadion arrays on top of a dielectric slab waveguide. We demonstrate that both the transverse magnetic (TM) and transverse electric (TE) waveguide modes could be coupled to the antisymmetric localized surface plasmon resonances (LSPRs) of the individual MIM-gammadions, leading to the formation of narrow hybrid waveguide-plasmon resonances (WPRs), of which the TM-WPR is less dependent while the TE-WPR is highly dependent on the handedness of the incident light. Associated with the excitation of the TE-WPRs, strong negative and positive circular dichroism (CD) peaks with high quality factors could be obtained on the short-wavelength and long-wavelength side of the LSPRs of the MIM-gammadion, respectively. Moreover, we show that the variation on either the lattice period or slab waveguide thickness allows for easily tuning the TE-WPRs based CD peaks over a relative wide spectral range. Our proposed hybrid system provides tunable and strong CD responses with narrow linewidth, which may have applications in chiral selective imaging, chiral plasmonic bio-sensing and spectroscopy.

Robust Helical Dichroism on Microadditively Manufactured Copper Helices via Photonic Orbital Angular Momentum

Three-dimensional chiral metallic metamaterials have already attracted extensive attention in the wide research fields of chiroptical responses. These artificial chiral micronanostructures, possessing strong chiroptical signals, show huge significance in next-generation photonic devices and chiroptical spectroscopy techniques. However, most of the existing chiral metallic metamaterials are designed for generating chiroptical signals dependent on photonic spin angular momentum (SAM). The chiral metallic metamaterials for generating strong chiroptical responses by photonic orbital angular momentum (OAM) remain unseen. In this work, we fabricate copper microhelices with opposite handedness by additively manufacturing and further examine their OAM-dominated chiroptical response: helical dichroism (HD). The chiral copper microhelices exhibit differential reflection to the opposite OAM states, resulting in a significant HD signal (∼50%). The origin of the HD can be theoretically explained by the difference in photocurrent distribution inside copper microhelices under opposite OAM states. Moreover, the additively manufactured copper microhelices possess an excellent microstructural stability under varying annealing temperatures for robust HD responses. Lower material cost and noble-metal-similar optical properties, accompanied with well thermal stability, render the copper microhelices promising metamaterials in advanced chiroptical spectroscopy and photonic OAM engineering.

Observation of intrinsic chiral bound states in the continuum

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics¹, quantum optics², topological photonics³ and chiroptics⁴. Intrinsic chirality is weak in natural materials, and recent theoretical proposals^5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies⁸. Therefore, most of the experimental attempts^9-11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence^9,10 or structural anisotropy¹¹. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

Chiral Au Nanorods: Synthesis, Chirality Origin, and Applications

Chiral Au nanorods (c-Au NRs) with diverse architectures constitute an interesting nanospecies in the field of chiral nanophotonics. The numerous possible plasmonic behaviors of Au NRs can be coupled with chirality to initiate, tune, and amplify their chiroptical response. Interdisciplinary technologies have boosted the development of fabrication and applications of c-Au NRs. Herein, we have focused on the role of chirality in c-Au NRs which helps to manipulate the light-matter interaction in nontraditional ways. A broad overview on the chirality origin, chirality transfer, chiroptical activities, artificially synthetic methodologies, and circularly polarized applications of c-Au NRs will be summarized and discussed. A deeper understanding of light-matter interaction in c-Au NRs will help to manipulate the chirality at the nanoscale, reveal the natural evolution process taking place, and set up a series of circularly polarized applications.

Reconfigurable chirality with achiral excitonic materials in the strong-coupling regime

We introduce and theoretically analyze the concept of manipulating optical chirality via strong coupling of the optical modes of chiral nanostructures with excitonic transitions in molecular layers or semiconductors. With chirality being omnipresent in chemistry and biomedicine, and highly desirable for technological applications related to efficient light manipulation, the design of nanophotonic architectures that sense the handedness of molecules or generate the desired light polarization in an externally controllable manner is of major interdisciplinary importance. Here we propose that such capabilities can be provided by the mode splitting resulting from polaritonic hybridization. Starting with an object with well-known chiroptical response-here, for a proof of concept, a chiral sphere-we show that strong coupling with a nearby excitonic material generates two spectral branches that retain the object's high chirality density, which manifest most clearly through anticrossings in circular-dichroism or differential-scattering dispersion diagrams. These windows can be controlled by the intrinsic properties of the excitonic layer and the strength of the interaction, enabling thus the post-fabrication manipulation of optical chirality. Our findings are further verified via simulations of circular dichroism of a realistic chiral architecture, namely a helical assembly of plasmonic nanospheres embedded in a resonant matrix.

Enhanced Chiral Sensing at the Few-Molecule Level Using Negative Index Metamaterial Plasmonic Nanocuvettes

Chirality is a fundamental property of biological molecules and some pharmaceutical molecules. Chiral molecules have a pair of chiral isomers (enantiomers) with opposite handedness. Although both enantiomers of the same molecule have identical chemical and physical properties, one enantiomer may be toxic to living organisms while the other one is harmless. The detection of these enantiomers is done using their small differential absorption between right and left circularly polarized light, known as circular dichroism (CD). Considering the macroscopic size of these molecules, combined with their small differential absorption, the obtained CD signal is very small, imposing a severe limitation on the minimal concentration that can be detected. Chiral plasmonic and metamaterial structures have been used to enhance the sensitivity of CD measurements by orders of magnitude through chiral density hot spots (super chiral fields). However, the large background signal due to these structures' intrinsic chirality limits the effectiveness of these methods. Contrary to absorption-based chiral sensing measurements (CD), fluorescence detection circular dichroism (FDCD) sensing can greatly improve chiral measurement sensitivity, down to the ultimate limit of a few and even a single chiral molecule. Like differential absorption, differential fluorescence also produces a weak signal at the few-chiral-molecule limit. However, here we demonstrate a negative-index metamaterial (NIM) cavity that acts as a "plasmonic nanocuvette" with globally enhanced volume super chiral fields. Moreover, the achiral structure of the plasmonic nanocuvette allows for completely background-free chiral sensing. We show that with NIM-cavity-enhanced FDCD, we can detect as low as a few tens of chiral molecules, well within the zeptomole range.

Multiple resonant modes coupling enabled strong CD response in a chiral metasurface

The chiral structures with strong circular dichroism (CD) response and narrow linewidth are desirable in chiral sensing, circularly-polarized light detection, and polarization imaging. Here, we theoretically proposed a hybrid chiral metasurface for differential absorption of circularly polarized light. Based on the multiple resonant modes coupling effect in a two-dimensional dielectric slab, it is realizable then to achieve a nearly perfect absorption for right circularly polarized light and simultaneously reflects 90% of left circularly polarized light, suggesting the generation of strong CD of 0.886 within a narrowly spectral linewidth of 4.53 nm. The multipole analysis reveals that the electric dipole, the magnetic dipole, and the electric quadrupole make dominant contributions to chiral absorption and the high CD response in this metsurface. The excitation of guided mode resonance enhances the ability of this metasurface to absorb electric field. Moreover, the optical chirality response can be further manipulated through the geometry features. These findings pave a powerful way to realize the narrowing and strong CD platform for single-band and multiband chirality behaviors.

Coexistence of circular dichroism and asymmetric transmission in Babinet-complementary metamaterials

Chiral metamaterials with circular dichroism (CD) or asymmetric transmission (AT) draw enormous attention for their attractive applications in polarization transformers, circular polarizers, and biosensing. In this study, a feasible trilayer chiral metamaterials (TCM) is designed and investigated in theory and simulation. The proposed TCM is composed of a nanoslit layer and a Babinet-complementary nanorod layer separated by a nanoslit spacer. Owing to symmetry breaking by the tilted nanoslit in metal film, the TCM shows simultaneous CD and AT effects in the near-infrared region. The simulated electric charge distributions prove that the chirality arises from the excitation of asymmetric electric dipole resonant modes due to the coupling of adjacent unit cells. Moreover, CD and AT can be tuned by the tilted angle of the nanoslit and the thickness of the spacer, the fitting functions of which are consistent with the theoretical formulas based on transmittance matrix analysis. The proposed nanostructure offers a potential strategy for manipulating metamaterials with simultaneous CD and AT effects, allowing a multitude of exciting applications such as ultra-sensitive polarization transformer and biosensor.

Planar chiral metasurfaces with maximal and tunable chiroptical response driven by bound states in the continuum

Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters.

Here, the authors employ the physics of chiral bound states in the continuum and suggest planar chiral metasurfaces with simultaneous ultrahigh quality factor and near-perfect circular dichroism in both linear regime and nonlinear regime.

DNA-Assembled Chiral Satellite-Core Nanoparticle Superstructures: Two-State Chiral Interactions from Dynamic and Static Conformations

A significant challenge exists in obtaining chiral nanostructures that are amenable to both solution-phase self-assembly and solid-phase preservation, which enable the observation of unveiled optical responses impacted by the dynamic or static conformation and the incident excitations. Here, to meet this demand, we employed DNA origami technology to create quasi-planar chiral satellite-core nanoparticle superstructures with an intermediate geometry between the monolayer and the double layer. We disentangled the complex chiral mechanisms, which include planar chirality, 3D chirality, and induced chirality transfer, through combined theoretical studies and thorough experimental measurements of both solution- and solid-phase samples. Two distinct states of optical responses were demonstrated by the dynamic and static conformations, involving a split or nonsplit circular dichroism (CD) line shape. More importantly, our study on chiral nanoparticle superstructures on a substrate featuring both a dominant 2D geometry and a defined 3D represents a great leap toward the realization of colloidal chiral metasurfaces.

Methylpiperazine based 0D chiral hybrid lead halides for second harmonic generation

Hybrid organic-inorganic metal halides (HOMHs) have recently attracted broad research interest for their structural tunability and remarkable optoelectronic properties. Among them, chiral HOMHs have demonstrated promising applications in second-order nonlinear optics (NLO) on account of their inherent noncentrosymmetric structures. Herein, we synthesized two new chiral HOMHs, (S-/R-2-C₅H₁₄N₂)₂PbI₆, based on S-/R-2-methylpiperazine chiral amines. They feature a band gap close to 2.9 eV with high phase purity as well as environmental and thermal stability. The induction of the chiral optical properties of (S-/R-2-C₅H₁₄N₂)₂PbI₆ by chiral organic cations was verified by circular dichroism (CD) spectroscopy. Moreover, the resulted HOMHs materials demonstrate a strong second harmonic generation response with a large laser damage threshold (∼2.97 mJ cm^-2), showing promising applications in NLO photonic devices.

Ultra-narrow-band circular dichroism by surface lattice resonances in an asymmetric dimer-on-mirror metasurface

Narrow-linewidth circular dichroism (CD) spectroscopy is a promising candidate to push the limits of molecular handedness detection toward a monolayer or even to a single molecule level. Here, we designed a hybrid metasurface consisting of a periodic array of symmetry-breaking dielectric dimers on a gold substrate, which can generate strong CD of 0.44 with an extremely-narrow linewidth of 0.40 nm in the near-infrared. We found that two surface lattice resonance modes can be excited in the designed metasurface, which can be superimposed in the crossing spectral region, enabling a remarkable differential absorption with a high Q-factor for circular polarizations. The multipole decomposition of the resonance modes shows that the magnetic dipole component contributes most to the CD. Our simulation results also show that the CD response of the chiral structure can be engineered by modulating the structural parameters to reach the optimal CD performance. Ultra-narrow-linewidth CD response offered by the proposed metasurface with dissymmetry provides new possibilities towards design of the high-sensitive polarization detecting, chiral sensing and efficient chiral light emitting devices.

Dual-band terahertz all-silicon metasurface with giant chirality for frequency-undifferentiated near-field imaging

Chiral metasurfaces are widely used in imaging and biosensing due to their powerful light field control capabilities. Most of the work is devoted to achieving the goals of chirality enhancement and tunability, but lacks consideration of design complexity, loss, cost, and multi-band operation. In order to alleviate this situation, we propose a pair of dual-frequency giant chiral structures based on all-silicon, which can achieve excellent and opposite spin-selective transmission around 1.09 THz and 1.65 THz. The giant chirality derives from the in-plane electric and magnetic dipole moments excited in different degrees. Theoretically, the maximum circular dichroism at the two frequencies are both as high as 0.34, and the coverage bandwidths of the two giant chirality are 85.5 GHz and 41.4 GHz, respectively. The experimental results are in good agreement with the simulation results. Based on the dual-band giant chiral patterns, the terahertz near-field imaging of different Chinese character images is demonstrated at two frequencies. The frequency-undifferentiated characteristics, good intensity contrast and three-dimensional imaging information are shown by the results. This work provides new ideas for the design of terahertz devices with simple structure and multi-functions, which are expected to be applied in the field of terahertz imaging or multi-channel communication.

Dual-band chiral metasurface for independent controls of spin-selective reflections

The development of chiral metasurfaces with spin-selective reflection or transmission provides a new way to control the circularly polarized (CP) waves. However, it is still a great challenge to independently manipulate the polarization, frequency, and phase of the spin-selective reflected waves in different operating bands, which may have potential applications in improving the data capacity of microwave and optical communication systems. Here, a dual-band chiral metasurface is proposed to generate gigantic intrinsic chirality with strong circular dichroism (CD) in two different frequency bands by piecing two typical mono-chiral units together. The polarization, frequency and phase of the spin-selective reflected waves can also be independently designed in the two operating bands by adjusting the configuration of the chiral unit structures. Based on the proposed chiral structures, a dual-band chiral metasurface with spin-selective anomalous reflections is designed and demonstrated by both simulations and experiments. The results show that the polarization of spin-selective reflected waves can be customized by selecting appreciate chiral structures, while the wavefront of the spin-selective reflected waves can be further controlled by designing their arrangement.

Chirality Inversion in Self-Assembled Nanocomposites Directed by Curvature-Mediated Interactions

Nanoscale curvature-dependent interactions are of paramount importance in biological systems. Here, we report that nanoscale curvature plays an important role in regulating the chirality of self-assembled nanocomposites from chiral organic molecules and achiral nanoparticles. Specifically, we show that the supramolecular chirality of the nanocomposites markedly depends on the nanoparticle curvature, where small-sized nanoparticles of high curvature and large-sized nanoparticles of low curvature lead to nanocomposites with opposite chirality. Quantitative kinetic experiments and molecular dynamics simulations reveal that nanoparticle curvature plays a key role in promoting the pre-nucleation oligomerization of chiral molecules, which consequently regulates the supramolecular chirality of the nanocomposites. We anticipate that this study will aid in rational design of an artificial cooperative system giving rise to emergent assembling phenomena that can be surprisingly rich and often cannot be understood by studying the conventional noncooperative systems.

Characterization of Chirality in Diffractive Metasurfaces by Photothermal Deflection Technique

Chirality, a lack of mirror symmetry, is present in nature at all scales; at the nanoscale, it governs the biochemical reactions of many molecules, influencing their pharmacology and toxicity. Chiral substances interact with left and right circularly polarized light differently, but this difference is very minor in natural materials. Specially engineered, nanostructured, periodic materials can enhance the chiro-optical effects if the symmetry in their interactions with circular polarization is broken. In the diffraction range of such metasurfaces, the intensity of diffracted orders depends on the chirality of the input beam. In this work, we combine a photothermal deflection experiment with a novel theoretical framework to reconstruct both the thermal and optical behavior of chiro-optical behavior in diffracted beams.

Tunable circular dichroism through absorption in coupled optical modes of twisted triskelia nanostructures

We present a system consisting of two stacked chiral plasmonic nanoelements, so-called triskelia, that exhibits a high degree of circular dichroism. The optical modes arising from the interactions between the two elements are the main responsible for the dichroic signal. Their excitation in the absorption cross section is favored when the circular polarization of the light is opposite to the helicity of the system, so that an intense near-field distribution with 3D character is excited between the two triskelia, which in turn causes the dichroic response. Therefore, the stacking, in itself, provides a simple way to tune both the value of the circular dichroism, up to 60%, and its spectral distribution in the visible and near infrared range. We show how these interaction-driven modes can be controlled by finely tuning the distance and the relative twist angle between the triskelia, yielding maximum values of the dichroism at 20° and 100° for left- and right-handed circularly polarized light, respectively. Despite the three-fold symmetry of the elements, these two situations are not completely equivalent since the interplay between the handedness of the stack and the chirality of each single element breaks the symmetry between clockwise and anticlockwise rotation angles around 0°. This reveals the occurrence of clear helicity-dependent resonances. The proposed structure can be thus finely tuned to tailor the dichroic signal for applications at will, such as highly efficient helicity-sensitive surface spectroscopies or single-photon polarization detectors, among others.

Terahertz circular polarization sensing for protein denaturation based on a twisted dual-layer metasurface

Protein denaturation has very important research value in nutrition, biomedicine, and the food industry, which is caused by the changes in the molecular structure of the protein. Since the collective vibrational and torsional modes of protein molecules are within the terahertz (THz) frequency range, THz spectroscopy can characterize the protein denaturation with several advantages of non-contact, label-free, real-time, and non-destructive. Therefore, we proposed a reflective THz time-domain polarization spectroscopy sensing method, and use a flexible twisted dual-layer metasurface film as a sensor to realize the thermal denaturation sensing, concentration sensing, and types identification of protein aqueous solutions. The experiment tested three proteins (bovine serum albumin, whey protein, and ovalbumin), and the results show that: for the thermal denaturation sensing, its detection sensitivity can reach 6.30 dB/% and the detection accuracy is 0.77%; for the concentration sensing, the detection sensitivity and detection accuracy reach 52.9 dB·mL/g and 3.6·10^-5 g/mL, respectively; in addition, different protein types can be distinguished by the difference of the circular polarization spectra.