New strategies to enhance chiral optical signals unveiled

A recent review article published in Engineering delves into the latest research on enhancing chiral optical signals, a topic with significant implications for various scientific fields. Chirality, a property found in many molecules, plays a crucial role in areas such as chemistry, biology, and pharmacology. However, the measurement of chiral optical signals can be challenging because they are often weak.

The review, led by researchers from the University of Shanghai for Science and Technology, covers a range of methods to boost these signals. One approach involves tailoring optical fields. For instance, the concept of “superchirality” has emerged, referring to light fields that can produce greater g-factors of chiral molecules compared to circularly polarized light (CPL). Scientists have proposed different ways to create superchiral fields, like using standing wave chiral fields formed by counterpropagating CPL plane waves or generating them in tightly focused fields. These superchiral fields can enhance the chiral response, enabling more sensitive detection of chiral materials.

Photonic resonance is another key strategy. Metasurfaces, which are artificial surfaces with unique optical properties, can be designed to enhance chiral optical fields in localized regions. Plasmonic nanostructures, such as nanocubes and nanoparticle helices, have shown great potential in increasing the asymmetric enhancement factor of chiral molecules. Additionally, high-index dielectric nanoparticles, like silicon nanospheres, can enhance enantiomeric excesses through Mie resonances. By exciting magnetic multipolar Mie resonances in these particles, the dissymmetry factor and CD signal can be significantly increased.

The use of orbital angular momentum (OAM) beams also offers a novel way to enhance chiral signals. OAM beams carry an angular momentum related to the helicity of their spatial phase distribution. Research has shown that they can be used to distinguish enantiomers and detect helical dichroism. When interacting with chiral molecules, OAM beams can induce chiral absorption, and increasing the OAM of photons can lead to larger helical dichroism signals.

Metasurfaces with bound states in the continuum (BICs) are also explored in the review. BICs can enhance the interaction between light and matter, and chiral metasurfaces based on BICs can exhibit high Q factors and strong chiral responses. Breaking the symmetry of nanostructures, either in-plane or out-of-plane, is crucial for achieving chiral BICs.

Finally, the review discusses the role of nonlinear optics in enhancing chiral signals. Nonlinear processes like high harmonic generation (HHG) and second-harmonic-generation (SHG) can be used to detect chirality at low concentrations. SHG-CD, for example, can detect chirality at submonolayer concentrations of a material.

While significant progress has been made, the researchers note that there are still challenges. Designing reconfigurable chiral metamaterials remains difficult, and enhancing optical activity in the ultraviolet range is a promising but challenging area. Additionally, most current studies focus on average chiral properties, and there is a need to develop techniques for high-spatial-resolution local chiral detection. Overall, these new strategies open up new avenues for further research in chiral optics and its applications.

The paper “Enhancement Methods for Chiral Optical Signals by Tailoring Optical Fields and Nanostructures,” authored by Hanqing Cai, Liangliang Gu, Haifeng Hu, Qiwen Zhan. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.12.022. For more information about the Engineering, follow us on X (https://twitter.com/EngineeringJrnl) & like us on Facebook (https://www.facebook.com/EngineeringJrnl).