# Nonreciprocal Electromagnetic Scattering from a Periodically Space-Time Modulated Slab and Application to a Quasisonic Isolator

## TL;DR

This paper investigates nonreciprocal electromagnetic scattering from space-time modulated slabs, demonstrating their ability to act as harmonic generators and filters, and applying this to develop a microwave quasisonic isolator.

## Contribution

It introduces a novel nonreciprocal scattering mechanism in space-time modulated slabs and demonstrates its application in creating a microwave isolator in the quasisonic regime.

## Key findings

- Space-time modulated slabs act as nonreciprocal harmonic generators.
- Low-frequency harmonics are filtered as surface waves, high-frequency as space waves.
- Experimental microwave demonstration of a quasisonic isolator.

## Abstract

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.