Compact coherent perfect absorbers using topological guided-mode resonances.

We propose a topological coherent perfect absorber that enables almost ideal performance with remarkably compact device footprint and tight incident beams. The proposed structure is based on a topological junction of two guided-mode-resonance gratings. The structure provides robust systematic ways of remarkably tight lateral confinement of the absorbing resonance mode and near-perfect mode-match to arbitrary incident beams, which are unavailable with the conventional approaches. We demonstrate an exemplary amorphous Si thin-film structure that enables near-perfect absorptance modulation between 1.7 and 99% with device footprint width of 30-μm and 10-μm-wide incident Gaussian beams. Therefore, our proposed approach greatly improves practicality of guided-mode-resonance coherent perfect absorbers.