Ultra-broadband solar light wave trapping by gradient cavity-thin-film metasurface

Despite the fact that solar energy has been widely used as a renewable and clean energy source for decades, when designing solar irradiation absorbers one is generally confronted with the dilemma of choosing between higher absorption but narrowband or broadband but lower absorption, which has greatly limited the development of the solar energy industry. In this work, a gradient cavity-thin-film metasurface (GCM) made up of alternating multiple layers of titanium (Ti) and silicon dioxide (SiO2) exhibits ultra-broadband strong absorption in 354-2980 nm. The operating bandwidth covers the dominating portion of the solar irradiation spectrum. The absorption spectrum can be manipulated by adjusting the structural parameters of the unit cell. It is worth noting that the spectrally weighted solar absorption efficiency reaches 98.28% under the AM 1.5G illumination. This impressive near-unity absorption could be attributed to multiple light-matter interactions including surface plasmon resonances, cavity resonance, and the intrinsic spectral responses of multi-layer refractory material. In addition, the absorption response is insensitive to the incident angle and polarization states. These high performances provide the GCM with great potential for practical applications in solar thermal energy harvesting and photothermal conversion, etc.