Topological properties of reflection zeros of optical differentiators based on layered metal-dielectric-metal structures

TL;DR

This paper explores the topological nature of reflection zeros in layered metal-dielectric-metal structures used as optical differentiators, revealing their topological protection and behavior under parameter changes.

Contribution

It uncovers the topological charges of reflection zeros in layered structures and demonstrates their robustness and dynamics under parameter perturbations.

Findings

Reflection zeros have non-zero topological charges, making them topologically protected.

Small parameter changes shift reflection zeros without eliminating them.

Opposite topological charges can merge and annihilate under parameter variation.

Abstract

We investigate the topological properties of reflection zeros of three-layer structures consisting of a dielectric layer sandwiched between two metal layers, which can be used as optical differentiators. We show that the reflection zeros possess non-zero topological charges, which makes them topologically protected. With a small perturbation of the parameters of the structure (e.g., a change in one of the layer thicknesses), the reflection zero does not disappear, but shifts in the parameter space, i.e., appears at different wavelength and angle of incidence. We demonstrate that with a further parameter change, two zeros with opposite topological charges (+1 and -1) approach each other, merge, and then disappear. We believe that the obtained results give useful insight regarding the operation of layered metal-dielectric-metal structures possessing reflection zeros.