Topologically protected chiral sensing using Synthetic Chiral Light

Abstract

Chirality underlies molecular function in living matter, yet its optical detection remains challenging because conventional chiroptical spectroscopies rely on weak corrections to the dominant electric-dipole light-matter interaction, making desired optical signals weak and fragile. Topology offers a route to robustness, enabling observables whose defining properties survive disorder and imperfections. However, experimental realization of a practical topological observable for chiral spectroscopy has remained elusive. Here we realize chiral topological light and demonstrate such an observable. A tightly focused, phase-locked, counter-rotating two-colour field encodes chirality in the three-dimensional electric-field trajectory while its dominant topological charge resides in the longitudinal electric-field component where it remains hidden from direct far-field detection. An isotropic chiral medium acts as a topological transducer, converting the latent topology of the driving field into a propagating nonlinear response whose topological charge becomes directly observable in the far field while remaining strongly suppressed in achiral media. Using randomly oriented chiral single-crystal powders, we experimentally detect the enantio-sensitive response through the topological charge of the emitted field. Directly accessible in the far field and robust against experimental imperfections, this observable allows us to track and control chiral signal on attosecond timescales through the relative phase of the driving two-colour fields. Our results establish topology as a practical resource for ultrafast chiral optical spectroscopy.