All-dielectric chiral-field-enhanced Raman optical activity

Observation of Chirality Transfer in Twisted Few-Layer Graphene

Chiral materials are the focus of research in a variety of fields such as chiroptical sensing, biosensing, catalysis, and spintronics. Twisted two-dimensional (2D) materials are rapidly developing into a class of atomically thin chiral materials that can be effectively modulated through interlayer twist. However, chirality transfer in chiral 2D materials has not been reported. Here, we show that the chirality from the twist interface of graphene can directly transfer to achiral few-layer graphene and lead to a strong chiroptical response probed with circularly polarized Raman spectroscopy. Distinct Raman optical activity (ROA) for the interlayer shear modes in achiral few-layer graphene is observed, with the degree of polarization reaching as high as 0.5. These findings demonstrate the programmability of chiroptical response through stacking and twist engineering in 2D materials and offer insights into the transfer of chirality in atomically thin chiral materials for optical and electronic applications.

Circular Polarization-Resolved Raman Optical Activity: A Perspective on Chiral Spectroscopies of Vibrational States

Circular polarization-resolved Raman scattering methods include Raman optical activity (ROA) and its derivative─surface-enhanced Raman optical activity (SEROA). These spectroscopic modalities are rapidly developing due to their high information content, stand-off capabilities, and rapid development of Raman-active chiral nanostructures. These methods enable a direct readout of the vibrational energy levels of chiral molecules, crystals, and nanostructured materials, making it possible to study complex interactions and the dynamic interfaces between them. They were shown to be particularly valuable for nano- and biotechnological fields encompassing complex particles with nanoscale chirality that combine strong scattering and intense polarization rotation. This perspective dives into recent advancements in ROA and SEROA, their distinction from surface-enhanced Raman scattering, and the potential of these information-rich label-free spectroscopies for the detection of chiral biomolecules.

Colloidal Chiral Carbon Dots: An Emerging System for Chiroptical Applications

Chiral CDots (c-CDots) not only inherit those merits from CDots but also exhibit chiral effects in optical, electric, and bio-properties. Therefore, c-CDots have received significant interest from a wide range of research communities including chemistry, physics, biology, and device engineers. They have already made decent progress in terms of synthesis, together with the exploration of their optical properties and applications. In this review, the chiroptical properties and chirality origin in extinction circular dichroism (ECD) and circularly polarized luminescence (CPL) of c-CDots is briefly discussed. Then, the synthetic strategies of c-CDots is summarized, including one-pot synthesis, post-functionalization of CDots with chiral ligands, and assembly of CDots into chiral architectures with soft chiral templates. Afterward, the chiral effects on the applications of c-CDots are elaborated. Research domains such as drug delivery, bio- or chemical sensing, regulation of enzyme-like catalysis, and others are covered. Finally, the perspective on the challenges associated with the synthetic strategies, understanding the origin of chirality, and potential applications is provided. This review not only discusses the latest developments of c-CDots but also helps toward a better understanding of the structure-property relationship along with their respective applications.

Fabrication of Mie-resonant silicon nanoparticles using laser annealing for surface-enhanced fluorescence spectroscopy

Silicon nanostructures with unique Mie resonances have garnered considerable attention in the field of nanophotonics. Here, we present a simple and efficient method for the fabrication of silicon (Si) nanoparticle substrates using continuous-wave (CW) laser annealing. The resulting silicon nanoparticles exhibit Mie resonances in the visible region, and their resonant wavelengths can be precisely controlled. Notably, laser-annealed silicon nanoparticle substrates show a 60-fold enhancement in fluorescence. This tunable and fluorescence-enhancing silicon nanoparticle platform has tremendous potential for highly sensitive fluorescence sensing and biomedical imaging applications.

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.

Mie-Resonant Nanophotonic-Enhancement of Asymmetry in Sodium Chlorate Chiral Crystallization

Studies on chiral spectroscopy have recently demonstrated strong enhancement of chiral light-matter interaction in the chiral near-field of Mie resonance in high-refractive-index dielectric nanostructures by studies on chiral spectroscopy. This situation has motivated researchers to demonstrate effective chiral photosynthesis under a chiral near-field beyond circularly polarized light (CPL) as a chiral source. However, the effectivity of the chiral near-field of Mie resonance for chiral photosynthesis has not been clearly demonstrated. One major challenge is the experimental difficulty in evaluating enantiomeric excess of a trace amount of chiral products synthesized in the near-field. Here, by adopting sodium chlorate chiral crystallization as a phenomenon that includes both synthesis and the amplification of chiral products, we show that crystallization on a Mie-resonant silicon metasurface excited by CPL yields a statistically significant large crystal enantiomeric excess of ∼18%, which cannot be achieved merely by CPL. This result provides implications for efficient chiral photosynthesis in a chiral near-field.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Expanding chiral metamaterials for retrieving fingerprints via vibrational circular dichroism

Circular dichroism (CD) spectroscopy has been widely demonstrated for detecting chiral molecules. However, the determination of chiral mixtures with various concentrations and enantiomeric ratios can be a challenging task. To solve this problem, we report an enhanced vibrational circular dichroism (VCD) sensing platform based on plasmonic chiral metamaterials, which presents a 6-magnitude signal enhancement with a selectivity of chiral molecules. Guided by coupled-mode theory, we leverage both in-plane and out-of-plane symmetry-breaking structures for chiral metamaterial design enabled by a two-step lithography process, which increases the near-field coupling strengths and varies the ratio between absorption and radiation loss, resulting in improved chiral light-matter interaction and enhanced molecular VCD signals. Besides, we demonstrate the thin-film sensing process of BSA and β-lactoglobulin proteins, which contain secondary structures α-helix and β-sheet and achieve a limit of detection down to zeptomole level. Furthermore, we also, for the first time, explore the potential of enhanced VCD spectroscopy by demonstrating a selective sensing process of chiral mixtures, where the mixing ratio can be successfully differentiated with our proposed chiral metamaterials. Our findings improve the sensing signal of molecules and expand the extractable information, paving the way toward label-free, compact, small-volume chiral molecule detection for stereochemical and clinical diagnosis applications.

Chiral molecular imprinting-based SERS detection strategy for absolute enantiomeric discrimination

Chiral discrimination is critical in environmental and life sciences. However, an ideal chiral discrimination strategy has not yet been developed because of the inevitable nonspecific binding entity of wrong enantiomers or insufficient intrinsic optical activities of chiral molecules. Here, we propose an "inspector" recognition mechanism (IRM), which is implemented on a chiral imprinted polydopamine (PDA) layer coated on surface-enhanced Raman scattering (SERS) tag layer. The IRM works based on the permeability change of the imprinted PDA after the chiral recognition and scrutiny of the permeability by an inspector molecule. Good enantiomer can specifically recognize and fully fill the chiral imprinted cavities, whereas the wrong cannot. Then a linear shape aminothiol molecule, as an inspector of the recognition status is introduced, which can only percolate through the vacant and nonspecifically occupied cavities, inducing the SERS signal to decrease. Accordingly, chirality information exclusively stems from good enantiomer specific binding, while nonspecific recognition of wrong enantiomer is curbed. The IRM benefits from sensitivity and versatility, enabling absolute discrimination of a wide variety of chiral molecules regardless of size, functional groups, polarities, optical activities, Raman scattering, and the number of chiral centers.

Chiral Quantum Metamaterial for Hypersensitive Biomolecule Detection

Chiral biological and pharmaceutical molecules are analyzed with phenomena that monitor their very weak differential interaction with circularly polarized light. This inherent weakness results in detection levels for chiral molecules that are inferior, by at least six orders of magnitude, to the single molecule level achieved by state-of-the-art chirally insensitive spectroscopic measurements. Here, we show a phenomenon based on chiral quantum metamaterials (CQMs) that overcomes these intrinsic limits. Specifically, the emission from a quantum emitter, a semiconductor quantum dot (QD), selectively placed in a chiral nanocavity is strongly perturbed when individual biomolecules (here, antibodies) are introduced into the cavity. The effect is extremely sensitive, with six molecules per nanocavity being easily detected. The phenomenon is attributed to the CQM being responsive to significant local changes in the optical density of states caused by the introduction of the biomolecule into the cavity. These local changes in the metamaterial electromagnetic environment, and hence the biomolecules, are invisible to "classical" light-scattering-based measurements. Given the extremely large effects reported, our work presages next generation technologies for rapid hypersensitive measurements with applications in nanometrology and biodetection.