# Breakdown of the Kirchhoff's law of thermal radiation by a spatiotemporally modulated nonreciprocal metasurface

## TL;DR

This paper demonstrates a graphene-based spatiotemporally modulated metasurface that violates Kirchhoff's law of thermal radiation at room temperature by breaking reciprocity, enabling unequal absorption and emission.

## Contribution

It introduces the first experimental and theoretical framework for nonreciprocal metasurfaces that violate Kirchhoff's law at mid-infrared frequencies.

## Key findings

- Nonreciprocal reflection observed at gigahertz modulation frequencies.
- Theoretical model relates nonreciprocal scattering to unequal absorptivity and emissivity.
- Effective decoupling of absorption and emission channels achieved.

## Abstract

Kirchhoff's law of thermal radiation, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, which dictates that the emissivity of a surface equals its absorptivity under thermal equilibrium, fundamentally limits the efficiency of photonic systems by enforcing reciprocal energy exchange between source and detector. Breaking this reciprocity is particularly important for advancing photonic devices for energy conversion, radiative cooling, and mid-infrared sensing and imaging. Driven by the growing need for photonic platforms to overcome reciprocity constraints, we present the first demonstration of spatiotemporally modulated nonreciprocal metasurfaces operating at mid-infrared frequencies suitable for the violation of the Kirchhoff's law at room temperature. We fabricate a graphene-based integrated photonic structure and experimentally demonstrate nonreciprocal reflection from a metasurface modulated at gigahertz frequencies. We develop a theoretical framework to relate nonreciprocal scattering under spatiotemporal modulation with unequal absorptivity and emissivity for violation of the spectral directional Kirchhoff's law. Our experiment and theory imply effective decoupling of absorption and emission channels by breaking time-reversal symmetry at thermal wavelengths.

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Source: https://tomesphere.com/paper/2509.00463