In this work, we investigate the optical reflectivity of deeply etched vertical mirrors using the effective medium approximation and the transfer matrix method. The reflectivity is studied versus the incident light wavelength for different metal film thickness ranging from 10 nm to 200 nm, grain to air volume ratio (or fill factor) ranging from 10 % to 100 %, and for 1, 2 or 3 effective metallic layers with different grain size. The model predicts that the reflectivity of the vertical mirror can be about 55% of its nominal expected value of the bulk metal reflectivity for a fill factor of 35% and a film thickness of 24 nm, which is equal to 4 times the skin depth at a wavelength of 1550 nm. A vertical mirror is etched and metallized on a silicon-on-insulator (SOI) wafer and its reflectivity is measured in the wavelength range of 1300 nm to 2100 nm, showing good agreement with the theoretical predictions.
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