In a recent review article published in the journal Engineering, researchers from the University of Shanghai for Science and Technology discussed various strategies for enhancing chiral optical signals. Adapting optical fields is one strategy. For example, light fields that can generate higher g-factors of chiral molecules than circularly polarized light (CPL) are referred to as “superchiral.”
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Many molecules have the characteristic of chirality, which is important in fields like chemistry, biology, and pharmacology. However, measuring them can be difficult because chiral optical signals are frequently weak.
Researchers have suggested several methods for producing superchiral fields, such as in tightly focused fields or employing standing wave chiral fields created by counterpropagating CPL plane waves. By improving the chiral response, these superchiral fields can make it possible to detect chiral materials with greater sensitivity.
Another important tactic is photonic resonance. Metasurfaces, artificial surfaces with special optical characteristics, can be designed to strengthen chiral optical fields in specific areas. Nanocubes and helices of nanoparticles are examples of plasmonic nanostructures that have demonstrated significant promise in raising the asymmetric enhancement factor of chiral molecules.
Furthermore, high-index dielectric nanoparticles such as silicon nanospheres can intensify enantiomeric excesses through Mie resonances. Stimulating magnetic multipolar Mie resonances in these particles can greatly enhance the dissymmetry factor and CD signal.
An innovative method to improve chiral signals is also provided by using orbital angular momentum (OAM) beams. The angular momentum of OAM beams is associated with the spatial phase distribution's helicity.
According to research, OAM beams can be used to detect helical dichroism and distinguish enantiomers. When they interact with chiral molecules, OAM beams cause chiral absorption, and increasing the OAM of photons can result in larger helical dichroism signals.
The study also investigates metasurfaces with bound states in the continuum (BICs). Chiral metasurfaces based on BICs can show strong chiral responses and high Q factors, which can improve the interaction between light and matter. Achieving chiral BICs requires breaking the symmetry of nanostructures, either in-plane or out-of-plane.
The review concludes by discussing how nonlinear optics can improve chiral signals. Chirality can be detected at low concentrations using nonlinear processes such as second-harmonic generation (SHG) and high-harmonic generation (HHG). For instance, SHG-CD can identify chirality at a substance's submonolayer levels.
Even though much progress has been made, the researchers point out that difficulties still exist. Enhancing optical activity in the ultraviolet range is a promising but challenging field, and designing reconfigurable chiral metamaterials is still challenging.
Furthermore, most recent research focuses on average chiral characteristics, and methods for high-spatial-resolution local chiral detection must be developed. Overall, these novel approaches create new opportunities for chiral optics and its applications research.
Journal Reference:
Cai, H., et al. (2024) Enhancement Methods for Chiral Optical Signals by Tailoring Optical Fields and Nanostructures. Engineering. doi.org/10.1016/j.eng.2024.12.022.