Superchirality induced enhanced circular dichroism spectroscopy via multi-beam superposition

Abstract

Circular dichroism (CD) spectroscopy is a typical technique to detect the chirality of an object. Due to the mismatch between light wavelength and the size of molecules, CD signal generally tends to be very weak in natural materials. In this work, we demonstrate the generation of superchiral field in free space through multi-beam superposition, without plasmonic/dielectric substrate that required in traditional methods. Through a systematic study of the superposition effect of up to six beams of light, we found that even achiral light fields can obtain optical chirality through superposition, and circularly polarized light fields can most effectively enhance the optical chirality density in the interference field. Generally, a maximal superchiral factor equivalent to the number of superposed beams can be achieved through optimizing the orientation of the illumination. However, when the number of superposed beams is more than three, the initial phase of superposed beams would play a significant role in determining the characteristics of the interference field, which must be carefully treated in modulating both the shape and the optical chiral density of the synthesized subwavelength chiral hotspot. Furthermore, we demonstrate that this technique is suitable to combine optical tweezers and CD spectroscopy. Based on three-beam superposition configuration, we explore the optical force and CD effects from single chiral nanoparticle, which are introduced into the optical standing wave formed by a flat mirror. It is found out that the nanoparticle can be stably trapped by the superchiral hotspot and gives rise to 2.92-fold enhancement of CD signal. This work may serve as a guideline for establishing applications in chirality sorting, chiral imaging, and CD spectroscopy.