Dynamic diffraction-efficiency control of grating couplers in dielectric microcavities

Dynamic switching of the light propagation is a key enabler for integrated nanophotonic circuits. Optical gratings are a widely used, versatile component for coupling light into and out of waveguides. However, their diffraction efficiency is generally set and limited well below unity by static design and process parameters such as etch depths or geometric profile quality. We experimentally demonstrate the use of a dielectric-mirror cavity for enhancing the diffraction efficiency of grating couplers beyond these intrinsic limits. Using a second counterpropagating input beam achieves near-perfect dynamic control of the coupling amplitude. The experiments are backed by analytical and numerical modeling of the cavity-enhanced diffraction efficiency and the interaction of light waves in optical-gratings multiport devices. These findings extend the concept of coherent perfect absorption (CPA) from thin absorbing layers to diffractive structures, providing an all-optical propagation switching mechanism for efficient signal input and control in compact photonic circuits and nano-optic systems.