Observation of topologically enabled complete polarization conversion

TL;DR

This paper demonstrates experimentally that topological properties in photonic crystal slabs enable complete polarization conversion and spin-preserved reflection, revealing a new topological route for robust photonic device design.

Contribution

It provides the first experimental observation linking topological scattering matrix effects to polarization conversion and BICs in photonics.

Findings

CPC occurs at vortex singularities in reflection matrices.

Topological CPC is verified through angle-resolved reflection measurements.

CPC and BICs are connected via critical coupling curves.

Abstract

Exploiting topological ideas has been a major theme in modern photonics, which provides unprecedented opportunities to design photonic devices with robustness against defects and flaws. While most previous works in topological photonics have focused on band theory, recent theoretical advances extend the topological concepts to the analysis of scattering matrices and suggest a topological route to complete polarization conversion (CPC), a unique photonic phenomenon without an electronic counterpart. Here, we report on the experimental observation of the topological effect in reflection matrices of a photonic crystal slab, enabling CPC between two linear polarizations over a wide range of frequencies. Using angle-resolved reflection measurements, we observe CPC occurring at vortex singularities of reflection coefficients in momentum space, verifying the topological nature of CPC. In addition, the topological effect also guarantees the spin-preserved reflection of a circularly polarized wave. Remarkably, we experimentally establish a connection between two seemingly unrelated topological phenomena--CPC and bound states in the continuum (BICs): BICs lie on the critical coupling curves that define the condition for CPC. Our work paves the way to exploring the topological properties in scattering matrices for controlling light polarization and spin and creating robust photonic devices.