Photonic Thermal Switch with Positive Differential Emissivity

Controlling temperature in solid-state devices is crucial as high temperatures can negatively impact their functionality, efficiency, speed, reliability, and lifespan. One way to manage this is by adjusting their radiative surface emission properties. Metal–insulator transition (MIT) materials, which have a critical temperature (T_c), have been proposed for thermal switches that drastically change their radiative properties around T_c. Typically, MIT materials become less emitting at high temperatures (negative differential emitter). This study introduces a broadband thermal switch with positive differential emitter behavior. Using a genetic algorithm, a composite multilayer structure based on vanadium dioxide (VO₂) (an MIT material with a phase transition at T_c = 68 °C) films has been designed. The analysis of local losses within the switch reveals that its behavior is related to the soft metallic behavior of VO₂ films in the infrared spectrum beyond T_c, reducing electric field screening. The developed switch emits a heat flux of 59 W m^–2 below T_c and about six times more just above this temperature, which is 60% of the blackbody emission.