A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal-Insulator Transition

Thermal radiation from a black body increases with the fourth power of absolute temperature (T4 ), an effect known as the Stefan-Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (eint ), is insensitive to temperature (|deint /dT| ≈ 10-4 °C-1 ). The resultant radiance bound by the T4 law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which eint can be engineered to decrease in an arbitrary manner near room temperature (|deint /dT| ≈ 8 × 10-3 °C-1 ), enabling unprecedented manipulation of infrared radiation. As an example, eint is programmed to vary with temperature as the inverse of T4 , precisely counteracting the T4 dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.