Diffraction engineering for silicon waveguide grating antenna by harnessing bound state in the continuum

Abstract

Silicon waveguide grating antennas (SWGAs) have been widely employed to interface the guided and radiation modes in various integrated photonic systems. However, ultrasmall feature sizes or heteromaterial integrations are usually required to obtain long propagation length and small far-field divergence. Moreover, for conventional SWGAs, the diffraction strength is wavelength sensitive, so the output power and far-field divergence will deviate in the beam steering process. In this paper, we propose and demonstrate a novel approach to engineer the diffraction in SWGA by harnessing the bound state in the continuum (BIC). A new degree of freedom is attained in diffraction engineering by introducing the “modified” diffraction formula. The side-wall emission can be dramatically depressed by building the quasi-BIC at critical waveguide width, leading to ultrauniform diffraction. The extremely weak diffraction strength (~3.3 × 10⁻³ dB/μm) has been experimentally realized for the fabricated device with a large feature size (~60 nm). From the measurement results, one can predict a centimeter-scale propagation length and an ultrasmall divergence (~0.027°). Moreover, the diffraction strength dispersion can be flattened for SWGA with critical waveguide width. Such effect has also been experimentally verified. Our proposed design is the first one that introduces the BIC effect into SWGA optimization, paving the way for precise diffraction engineering and high-performance integrated optical antennas.