Revealing the Physical Origin of Chirality Signals with Optical Vortices

— How Twisted Light Distinguishes Structural Handedness —

A research group led by Assistant Professor Shun Hashiyada and Professor Yoshito Y. Tanaka at the Research Institute for Electronic Science (RIES), Hokkaido University, has clarified the physical origin of chirality-dependent optical responses observed with optical vortices. Chirality refers to a property of structures that cannot be superimposed onto their mirror images, like left and right hands. It appears widely in nature, from molecules to nanoscale architectures. Conventional chiral spectroscopy relies on circular polarization, which probes local electromagnetic chirality. In contrast, optical vortices possess a spatially twisted wavefront associated with orbital angular momentum, offering the possibility of probing more extended structural chirality.

Previous studies reported small transmission differences when left- and right-handed optical vortices illuminate chiral materials. However, the physical origin of these differences ̶ and whether they genuinely arise from material chirality ̶ remained unclear.

In this study, the researchers employed a high-speed vortex switching technique (50 kHz) to precisely measure transmission asymmetries while systematically varying the relative position between the optical vortex and a chiral nanostructure. Remarkably, they found that the strongest chirality-dependent signal from a single twisted gold nanorod dimer as a minimal chiral platform emerges near the vortex core, where the light intensity is nearly zero ̶ a counterintuitive result.

Through theoretical analysis based on their recently developed multipolar framework, the team demonstrated that this asymmetry originates from the coupling between the spatial twisting of the optical vortex (orbital angular momentum) and the three-dimensional structural chirality of the material. This distinguishes the effect from conventional circular polarization responses.

By establishing the physical mechanism behind vortex-induced chirality signals, this work provides a rigorous foundation for structured-light-based chiral spectroscopy. The findings open new possibilities for selectively probing hierarchical chiral structures, such as molecular assemblies and nanoscale architectures, using light.

This research was published in Optica on February 17, 2026.

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