Critical features of nonlinear optical isolators for improved nonreciprocity

TL;DR

This paper investigates the physical mechanisms behind nonreciprocity in nonlinear optical isolators, highlighting the roles of asymmetry, optical nonlinearity, and scattering processes, and compares two atomic models to optimize nonreciprocal behavior.

Contribution

It introduces a comparative analysis of two atomic models for nonlinear optical isolators, identifying key factors influencing maximum nonreciprocity and the impact of additional weak light.

Findings

Model (b) exhibits higher nonreciprocity than model (a).

Maximum nonreciprocity occurs near a critical incident light intensity.

Coherent elastic scattering mainly causes maximum nonreciprocity.

Abstract

Light propagation in a nonlinear optical medium is nonreciprocal for spatially asymmetric linear permittivity. We here examine physical mechanism and properties of such nonreciprocity (NR). For this, we calculate transmission of light through two models of a nonlinear optical isolator consisting of (a) a two-level atom and (b) a driven $\Lambda$-type three-level atom coupled asymmetrically to light inside open waveguides. We find a higher NR in the model (b) than in the model (a) due to a stronger optical nonlinearity in the former. We determine the critical intensity of incident light for maximum NR and a dependence of the corresponding NR on asymmetry in the coupling. Surprisingly, we find that it is mainly coherent elastic scattering compared to incoherent scattering of incident light which causes maximum NR near the critical intensity. We also show a higher NR of an incident light in the presence of an additional weak light at the opposite port.