Optimal cavity length in cavity-resonator-integrated guided-mode resonance filter

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) has been recently proposed to provide a narrow-band reflection spectrum for an incident wave of a small diameter from the free space. CRIGF consists of a grating coupler (GC) between two distributed Bragg reflectors (DBRs) on a waveguide. A vertically incident wave from the air at a coupling wavelength of GC is partially coupled to a guided wave by GC in a waveguide cavity formed by a pair of DBRs, and is coupled out by the same GC to radiation waves into the air and substrate. The radiation overlaps the direct transmission of the incident wave. In order to get high reflectance of CRIGF, a length of the waveguide cavity needs to be set so that a resonance wavelength of the waveguide cavity is equal to the coupling wavelength of GC. In this paper, the optimal cavity length is experimentally investigated. CRIGF of 11.5-μm aperture was designed for operation at 1540-nm wavelength and its maximum reflectance was predicted to be 90%. A waveguide was formed by depositing a GeO₂:SiO₂ guiding core layer and Si-N cladding layer on a SiO₂ glass substrate. GC and DBR patterns were formed in the Si-N layer by electron-beam direct-writing lithography and reactive ion etching. CRIGFs with slightly different cavity length were fabricated on the same substrate. It was demonstrated experimentally that the reflectance of CRIGF depends on the phase-adjusting gap and the gap giving the optimal length of the waveguide cavity differs from the basic design.