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.
In this work, we report a novel notch optical filter based on the imaging properties of a MEMS-based Multimode Interference (MMI) waveguide. The concept is based on the dependence of the imaging lengths on the different wavelengths, where each wavelength exits the waveguide at a different lateral position. Thus, by properly choosing the output waveguide position, it is possible to have a good selective optical filter as well as a good notch optical filter (the complementary response). To validate this concept an MMI structure is fabricated using Deep Reactive Ion Etching (DRIE) technology on a silicon-on-insulator (SOI) wafer. The walls of the waveguide are metalized with Aluminum to decrease the insertion loss. The design makes use of the compactness of the parabolic butterfly shape to reduce the MMI length. The structure is fed by a 9/125 single-mode fiber and the Amplified Spontaneous Emission ASE out of a Semiconductor Optical Amplifier is used as a wideband source for the optical response characterization. The output is measured on an optical spectrum analyzer demonstrating a notch filter response around 1550 nm with about 20-dB rejection ratio. The reported results open the door for integrated, low-cost and fabrication insensitive optical MEMS notch filter.