Dielectric super-absorbing metasurfaces via PT symmetry breaking

Dielectric super-absorbing ( > 50 % ) metasurfaces, born of necessity to break the 50% absorption limit of an ultrathin film, offer an efficient way to manipulate light. However, in previous works, super absorption in dielectric systems was predominately realized via making two modes reach the degenerate critical coupling condition, which restricted the two modes to be orthogonal. Here, we demonstrate that in nonorthogonal-mode systems, which represent a broader range of metasurfaces, super absorption can be achieved by breaking parity-time (PT) symmetry. As a proof of concept, super absorption (100% in simulation and 71% in experiment) at near-infrared frequencies is achieved in a Si-Ge-Si metasurface with two nonorthogonal modes. Engineering PT symmetry enriches the field of non-Hermitian flat photonics, opening-up new possibilities in optical sensing, thermal emission, photovoltaic, and photodetecting devices.