  Through passively emitting excess heat to the outer space, radiative cooling
has been demonstrated as an efficient way for energy saving applications.
Selective surface with unity emittance only within the atmospheric window as
well as zero absorption within the solar spectrum is sought to achieve the best
sub-ambient radiative cooling performance during the daytime. In this work, we
proposed a bulk radiative cooler consisting of a 1-mm-thick lithium fluoride
crystal coated with silver backing, which exhibits a solar absorptance of 4.7%
and nearly ideal infrared selectivity with high emission exactly within the
atmospheric transmission band (i.e., 8-13 um). Excellent daytime cooling
performance was demonstrated in an outdoor test with stagnation temperature
below the ambient temperature by 5 K under solar irradiance above 900 W/m2 and
a net cooling power of about 60 W/m2 when the cooler is in thermal equilibrium
with the ambient. As a bulk material with the highest ultraviolet-visible-near
infrared transmittance, lithium fluoride crystal has been widely employed as
the optical windows and mirrors in various applications. The proposed simple
selective infrared emitter based on lithium fluoride would open up an
innovative way to radiatively cool optical systems.
