Cavity-resonator-integrated guided-mode resonance filter with reflection phase variation

A cavity-resonator-integrated guided-mode resonance filter (CRIGF) consisting of a grating coupler and a pair of distributed Bragg reflectors on a thin film waveguide has been recently proposed and demonatrated to provide a narrow-band reflection spectrum for an incident wave of a small beam width from the free space. CRIGF is attractive for constructing a very compact laser system. Since a reflection phase of CRIGF is a crucial factor in designing it as a laser mirror, CRIGF with a wavelength dependence of the reflection phase is discussed in this paper. A reflection spectrum was calculated by the finite-difference time-domain (FDTD) method for CRIGF with a resonance wavelength of 848 nm and an aperture size of 10 μm. The reflectance has the maximum of about 60% at the resonance wavelength but decreases to 0% at a slightly different wavelength. Whereas the reflection of the maximum reflectance can be explained by the cancellation of transmission with anti-phase relation of the radiation wave to the direct transmitted wave, the reflection of 0% would indicate in-phase relation. It is predicted that the reflection phase changes between the two wavelengths. A reflection phase variation depending on wavelength was confirmed and estimated to be π/2 by FDTD simulation and optical measurement.