# Nonreciprocal Negative Refraction Enabled by Photonic Time Crystals

## TL;DR

Researchers developed a new way to control light using time-varying structures, achieving one-way negative refraction in both optical and microwave systems.

## Contribution

A general framework for nonreciprocal negative refraction using photonic time crystals across multiple frequency regimes.

## Key findings

- Isolation exceeding 46 dB was achieved in the optical device.
- A 11 dB isolation was demonstrated in the microwave counterpart.
- Floquet harmonic expansions and a custom solver validated the designs.

## Abstract

We propose and theoretically demonstrate nonreciprocal
negative
refraction enabled by time-varying photonic structures. By engineering
temporal modulations at the interfaces of hyperbolic media, we achieve
isolation between forward and backward beams while preserving the
hallmark property of negative refraction. Two complementary approaches
are developed: in the optical regime, a multilayer AZO/ZnO hyperbolic
slab is sandwiched between permittivity-modulated dielectric layers
(3D time crystals); in the microwave regime, a wire medium is sandwiched
between time-modulated resistive metasurfaces (2D time crystals).
Both designs exploit Floquet harmonic expansions and are validated
with a customized harmonic-balance finite-element solver. We report
isolation exceeding 46 dB in the optical device and 11 dB in the microwave
counterpart. This work introduces a general framework for nonreciprocal
negative refraction across frequency regimes, expanding the design
space of time-varying metasurfaces and photonic time crystals.