Fortunately, with the emergence of

Fortunately, with the emergence of advanced design and fabricationtechnologies, new classes of selective infrared radiators, includingphotonic structures and metamaterials, have been rapidly developed inrecent studies. Thus, diurnal radiative cooling well below the ambienttemperature has been achieved. These novel radiators exhibit high re-flection within a solar radiation band (i.e., 0.3–4.0μm) while simulta-neously strongly emitting within the atmospheric transparent window,offering substantial diurnal radiative cooling. Here, diurnal radiativecooling for sub-ambient phenomenon under direct sunshine was in-itially experimentally demonstrated by Raman et al. [13] based on aplanar photonic radiator. This radiator, which consists of seven alter-nating layers of hafnium dioxide (HfO2) and silica (SiO2) with varyingthickness on top of 200 nm thick silver (Ag) and 750μm thick silicon(Si) wafer substrate, can reflect approximately 97% of incident solarirradiance and simultaneously emit strong thermal radiation.