HCG reflectivity characteristics for a small-sized beam spot

A high contrast grating (HCG) mirror was investigated in the case of the wavelength-scale small beam spot system by the numerical simulations. It was found that a reflectivity dependence on the spacer thickness showed a large reflectivity oscillation unlike a plane wave system. A thin spacer thickness was indispensable for a high reflectivity by suppression of the beam diffraction. The number of the HCG grating was also discussed and the number of covering the waveguide core area or more was important for high reflectivity. A 70% of reflectivity can be obtained by a single grating.