Periodic trapezoidal VO2-Ge multilayer absorber for dynamic radiative cooling

Nowadays, the requirement for achieving dynamic radiative cooling is more and more intense, so a cooling system is proposed and developed to meet the demand in this paper. This cooling system is composed of a filter and a periodic trapezoidal VO₂-Ge multilayer absorber (VGMA). The filter on the top enables the VGMA to reflect most of the solar irradiation at daytime and the absorptance or emittance of the VGMA is very different in the spectrum band of 8-13 μm for insulating and metallic VO₂ due to the phase transition characteristic of VO₂. With this cooling system, close-to-zero absorptance in the range of 0.3-2.5 μm and high (low) absorptance from 8 to 13 μm are achieved for metallic (insulating) VO₂. Based on changing the temperature and absorptivity or emissivity simultaneously, radiative heat can be transferred dynamically to the outer space. When VO₂ is in the insulating phase, the absorption mechanism of the absorber is magnetic resonance and surface plasmon polariton resonance, and broadband high absorptivity is achieved by exciting slowlight waveguide mode at broadband wavelengths when VO₂ is in metallic phase. The spectral absorptance characteristics of the absorber in the two phase states are investigated as a function of the layer number and the incident angle of the electromagnetic waves. The results show that the absorber designed is insensitive to the incident angle. Moreover, the net cooling power of the VGMA of metallic VO₂ is instantly 4 times more than that of insulating VO₂ once the phase change temperature is reached. This work will be beneficial to the advancement of dynamic radiative cooling.