Bifacial omnidirectional and band-tunable light absorption in free-standing core–shell resonators

Effective optical absorption is highly desirable for numerous applications in energy harvesting and optoelectronics. Bifacial absorbers can significantly enhance light absorption by capturing albedo light from the environment. Here, we experimentally demonstrate that free-standing silica-silver core–shell nano-resonator arrays allow bifacial and omnidirectional optical absorption across the visible spectrum. Specifically, resonator arrays can highly absorb light (>80%) with all polarizations from a directional range (−40° to 40°) on both front and rear sides of a surface. Numerical simulations reveal that such bifacial and omnidirectional light absorption results from hybridized excitation of surface plasmons and whispering gallery modes in a symmetrical configuration. The absorption band can be flexibly adjusted by changing the silica core size. In addition, the absorbed optical energy quickly decays as the excitation of plasmonic hot electrons as observed using transient absorption spectroscopy. Our work provides a bifacial absorber for many optoelectronic applications in photodetection, photovoltaics, and photocatalysis.