Chiral sensing with achiral anisotropic metasurfaces

TL;DR

This paper introduces an anisotropic metasurface-based chiral sensing method that significantly enhances signals, allows complete chirality measurements, and enables absolute, reversible detection without sample removal, applicable across a broad spectrum.

Contribution

The work presents a novel anisotropic metasurface design for chiral sensing, providing analytical insights, practical measurement schemes, and broadband operation capabilities.

Findings

Enhanced chiroptical signals by over two orders of magnitude.

Complete measurement of chirality including magnitude and sign.

Broadband operation from near-infrared to near-ultraviolet.

Abstract

Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation - from near-infrared to near-ultraviolet frequencies - opening the way for highly sensitive nanoscale chiroptical spectroscopy.