Managing thermal emission using ENZ polar dielectrics for self-cooling window application

The passive emission of thermal radiation by utilizing the natural ability of materials to emit thermal radiation within the atmospheric transparency window (8-13 µm) can emerge as a promising strategy to enhance energy efficiency in cooling applications. Here, we designed a broadband thermal emitter consisting of four polar dielectric materials (<i>S</i><i>i</i><i>O</i>/<i>S</i><i>i</i><i>N</i>/<i>A</i><i>l</i>₂<i>O</i>₃/<i>T</i><i>a</i>₂<i>O</i>₅) layered on an ITO-coated polycarbonate substrate. In this design, the polar dielectrics act as epsilon-near-zero (ENZ) materials, causing the emitter to exhibit angle-dependent thermal emission due to the Berreman mode, a characteristic feature commonly observed in ENZ materials. This emitter demonstrates high emissivity in p-polarization at large oblique angles (70°-80°), effectively covering the critical 8-13 µm wavelength range. Furthermore, the emitter structure offers over 70% transparency in the visible spectrum while also providing substantial reflection in the near-infrared (NIR) region. The optimized structures produce a good radiative cooling power density up to 199<i>W</i>/<i>m</i>² at night and a mean daytime radiative cooling power density of 60<i>W</i>/<i>m</i>². All these properties can make this emitter a good candidate for self-cooling windows in buildings for energy-saving purposes. The directional emission capability of the windows ensures efficient heat dissipation, even in scenarios where it is obstructed by nearby objects. In addition, this structure can show significant implications for diverse applications, including photovoltaic systems, waste heat management, and the encryption of information.