Guided-mode Resonance in Waveguide Cavity

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consisting of a grating coupler (GC) in two distributed Bragg reflectors constructing a waveguide cavity resonator shows a narrowband reflection spectrum for an incident free-space wave with a small aperture. Modification of grating lines of the GC in the cavity resonator are discussed. CRIGF with a nonuniform GC was investigated in order to improve reflectance for an incident Gaussian beam. Fill factors of grating lines were varied to give a Gaussian distribution in the coupling coefficient. The improvement of reflectance was experimentally confirmed with 11.5- m aperture. Shift of grating lines reduces the coupling efficiency. CRIGFs with various line-shifted GCs of a 10-pm aperture are designed and fabricated. The bandwidth of CRIGF reduced until the line shifted to a quarter of GC period. In particular, a line shift of a quarter of GC period nullifies the guided-mode resonance, and no peak in reflection spectrum was observed.