Flat photonics for broadband light-trapping

The ability to trap light in an ultrathin photoactive layer has been of great significance for applications ranging from optoelectronics, energy to spectroscopy. However, the current broadband light trapping suffers from undesirable direct Joule heat output with plasmonics or bulkiness with photonics. Here, we report a light-trapping photonic structure using an ultrathin all-dielectric super-absorbing metasurface. This presented photonic structure features asymmetrically coupled magnetic resonances, which eliminate reflection and transmission simultaneously by introducing destructive interference between the backscattered field of the resonance and the direct reflected field of the highly reflective Fabry–Pérot background. In particular, this photonic structure enables broadband light trapping by placing nanostructures of different sizes in a supercell. As a proof of concept, we experimentally demonstrate broadband (550–1280 nm) super absorption (>50%) within an ultrathin (∼200 nm) all-dielectric germanium metasurface. This work provides a design paradigm for harvesting light through flat photonic structures at the nanoscale and paves the way for cost-effective light management.