High‐Contrast Switching of Light Enabled by Zero Diffraction

Abstract

Diffraction allows to change the direction of light. Therefore, controlling the diffraction efficiency with high contrast enables controlling the pathway of light within optical systems. However, a high contrast requires that the diffraction efficiency is tunable close to zero. Probably the most prominent example for zero diffraction in a waveguide grating is a bound state in the continuum (BIC). Herein, zero diffraction of two plane waves under symmetric incidence to a leaky symmetric waveguide grating is found. The phenomenon not only occurs at singular spectral positions but on continuous curves in the energy–momentum space. The relative phase of the two waves enables large contrast control over diffraction in a wide spectral range. The practical meaning of this finding for local switching is demonstrated. Light is trapped into a nonlinear optical waveguide and detrapped at a desired position with electric control. A switching contrast exceeding 1000 is experimentally shown.