Breaking Kirchhoff's Law in Nonlinear Thermal Emission

TL;DR

This paper demonstrates broadband nonreciprocal thermal radiation by breaking Kirchhoff's law via nonlinear optical frequency conversion, enabling advanced thermal management applications such as cooling and energy harvesting.

Contribution

It introduces a novel experimental method to achieve broadband nonreciprocal thermal emission by breaking Kirchhoff's law through nonlinear optical processes in scattering media.

Findings

Successful demonstration of nonreciprocal broadband thermal radiation.

Evidence of active radiation cooling via nonlinear thermal emission.

Potential applications in thermal management and infrared camouflage.

Abstract

Thermal radiation is strictly governed by Kirchhoff s law to reach thermal equilibrium. The violation of Kirchhoff s law decouples nonreciprocally the equity between absorptivity and emissivity, enabling exotic thermal engineering applications. However, achieving broadband nonreciprocal thermal emissivity and absorptivity remains a challenge. Here we experimentally demonstrate nonreciprocal and broadband thermal radiation by breaking Kirchhoff s law through nonlinear optical frequency conversion in a scattering medium. Thermal blackbody radiation is upconverted through sum-frequency generation with an intense infrared pump, while broadband conversion is enabled by the critical random quasi-phase-matching condition in the nonlinear nanocrystals. Moreover, a temporal transient measurement also indicates a possible active radiation cooling through such nonlinear thermal radiation. These results may pave a new way for nonlinear and active thermal management in critical applications like radiation cooling, energy harvesting, and infrared camouflage.