Small-aperture guided-mode-resonance filter with cavity resonators

A cavity-resonator-integrated guided-mode-resonance filter (CRIGF) consisting of a grating coupler (GC) and a pair of distributed-Bragg-reflectors (DBRs) on a thin film waveguide has been recently proposed and investigated to provide a narrow-band reflection spectrum for an incident wave of a small beam width from the free space. A CRIGF demonstrated so far shows polarization dependence because propagation constants of guided waves excited by GC are different between TE and TM incident waves. In order to construct a polarization-independent guided-mode resonance filter with small aperture, an integration of two CRIGFs crossed each other was proposed and discussed in this paper. A device was designed for a resonance wavelength of 1550 nm and its reflection and transmission spectra were predicted by a newly developed analysis based on the coupled-mode theory. A reflectance of 96 % with 1 nm bandwidth was expected for an incident beam diameter of 10 μm. A test sample working at 846 nm was fabricated and characterized. A Ge:SiO2 guiding core layer was deposited on a SiO2 glass substrate, and GC and DBRs were formed by the electron-beam direct writing lithography. Measured reflection spectra for TE and TM incident beams were well coincident with each other.