Mesoporous Organosilica-Based Infrared Metamaterials for Radiative Cooling of Windows.

Efficient radiative cooling requires an unobstructed view of the sky. However, the vertical facades of buildings see terrestrial features, which act as broadband infrared emitters of heat in the summer. As a result, for conventional materials, including building windows, terrestrial heat gain in the summer can negate or overwhelm a vertically-oriented surface's narrowband long-wave infrared (LWIR) emission to space. To address this challenge, here we develop a visibly transparent, LWIR-selective mesoporous metamaterial suitable for use on windows that is designed to emit selectively within the 8-13 µm atmospheric transparency window. This material, a highly porous organosilica network dried under ambient conditions known as an ambigel, leverages phonon-polariton resonances to achieve high and selective emittance in the transparency window. Experimental characterization and modeling suggest that the bond angle and length distributions of amorphous SiO2 in this ambigel differs significantly from those in fused silica, highlighting a new degree of freedom for optical design. Field tests demonstrate that the ambigel maintains surface temperatures approximately 0.5°C lower than a conventional broadband emitter in representative conditions. Our results demonstrate a material design strategy for a visibly transparent infrared selective material that can enable effective radiative cooling for windows and building facades.