手 性 光 场 及 其 在 分 子 手 性 探 测 方 面 的 近 期 研 究 进 展（特 邀）

Significance Chirality is a property of an object that cannot be superimposed on its mirror image by translation or rotation operations. The two enantiomers of chiral molecules have the same physical properties but completely different chemical properties. Therefore, effective detection and characterization of chiral molecules are crucial for such fields as pharmaceuticals and biochemistry. Chiral objects exhibit optical activity, and the optical chirality response generated during the interaction with other chiral objects provides a basic strategy for effective enantiomer identification. The light field plays a vital role in the detection of chiral molecules, and according to Curie s asymmetry principle, the chiral response often requires a chiral light field. The light field chirality greatly affects the optical response intensity of chiral media. Therefore, how to enhance the chirality of ordinary light fields is a core issue in chirality research. The circularly polarized light field is the most common type of chiral light, playing an important part in chiral responses such as optical rotation, circular dichroism, and Raman optical activity. However, the chiral response generated by circularly polarized light fields is often weak, which greatly affects the ability of these methods to detect molecular chirality. Therefore, many other optical fields have been proposed, such as superchiral field, optical field with orbital angular momentum, and synthetic chiral light. Progress Currently, extensive theoretical and experimental research has been conducted on the regulation of chiral light fields and their ultra-sensitive detection with the assistance of artificial nanostructures. After the concept of optical chirality (C) was proposed, its physical meaning was improved by Tang and Cohen. By employing the expression of optical chirality, when the optical chirality of a certain light field is greater than that of circularly polarized light, it is called superchiral field. When chiral structures are adopted for chiral detection, the chiral signals of molecules are often influenced by the chiral signal of the structure itself (Fig. 1). To this end, it is proposed that non-chiral structures should be utilized to generate superchiral fields for enhanced spectral detection, such as the study of nanostructure superchiral hotspots, and the construction of vector exceptional points (EPs) and Bound states in the continuum (BICs) to generate strong and uniform superchiral field over a large area, which has been experimentally validated (Fig. 2). In molecular chirality ultra-sensitive detection based on superchiral field, the initial research mostly focuses on enhancing the circular dichroism signal of molecules by obtaining superchiral hotspots with enhanced chirality density (Fig. 3). Initially, it was believed that the significant enhancement of chiral signals by placing molecules in hotspots was due to the enhanced field strength at the hotspots. Later, it was theoretically proven that the peak/valley intensity of plasma-induced circular dichroism enhancement directly corresponds to a larger optical chirality in the near-field, rather than a larger enhanced electromagnetic field intensity at the hotspots (Fig. 4). Additionally, the interaction between orbital angular momentum beams and chiral molecules has been extensively discussed. Placing chiral molecules in the hotspots of dimers can observe a significant enhancement effect of chiral signals (Fig. 3). The study of employing dielectric nanoparticles to enhance molecular Raman optical activity (ROA) signals can provide more comprehensive information on molecular chirality structures, and also investigate the thermal effects of dielectric and metal structures under light irradiation (Fig. 4). Meanwhile, the strength of chiral optical gradient force is related to the gradient of optical field chirality. The utilization of a superchiral field can increase the total optical force difference of enantiomers, achieving the separation of enantiomers and nanoparticles (Fig. 5). Research based on circularly polarized light or superchiral light fields requires consideration of both electric dipole interactions and magnetic dipole interactions (or electric quadrupole interactions) between light and matter. However, the interaction between magnetic dipoles and electric quadrupoles is usually weak, with often small detection signal strength. The interaction between synthetic chiral light and chiral substances can generate significant and freely adjustable enantioselectivity in pure electric dipole effects. Synthetic chiral light is a light field composed of multiple light frequencies, which requires that the polarization of the total light field should not be coplanar, and its chirality is only related to the light field itself at a certain point (Fig. 6). The synthesis of dual color chiral light mainly employs strong field physics to recognize chiral molecules, such as photoexcited circular dichroism (PXCD) and high harmonic generation (HHG) spectroscopy. The research on the tricolor synthesis of chiral light is mainly based on the cyclic three-level model, and researchers have discussed enantiomer specific state transfer (ESST) and enantiomer spatial separation. For chiral detection, different types of optical responses that can be adopted to distinguish left and right hands have been discussed, such as enantioselective light absorption, enantioselective three-wave mixing, enantioselective ac Stark effect, and enantioselective response of cavity molecule mixing systems (Fig. 7). Conclusions and Prospects We introduce chiral light fields and their applications in molecular chirality detection. Firstly, we present the enhancement of superchiral fields based on nanostructures and review their applications in two aspects, including chiral molecule ultra-sensitive detection based on superchiral hotspots and chiral optical force enhancement based on vector exceptional points. Then the relevant research on synthetic chiral light is discussed. Currently, the main focus is on the detection of chiral molecules using bicolor and tricolor synthetic chiral light. Meanwhile, we have developed methods such as photoexcited circular dichroism, high harmonic generation spectroscopy, and enantioselective ac Stark effect. In addition, research on ESST has also been conducted based on synthetic chiral light. The research on the chiral light field and its applications in molecular chirality detection is still in the development stage. Thus, designing reasonable nanostructures to achieve uniformity and higher optical chirality C in the development of superchiral fields is a challenge and key. A clearer understanding of the interaction mechanism between the superchiral field and the near chiral field of nanostructures can potentially boost the development of higher-precision chiral spectroscopy methods. In addition, the utilization of chiral optical forces is a rapidly developing field where there have been many theoretical studies so far, demonstrating enormous potential in chiral separation. The research on synthetic chiral light is still in its early stage. In the future, we hope to combine synthetic chiral light with nanostructures to develop a new generation of surface-enhanced chiral light.