Design of highly reflective subwavelength diffraction gratings for use in a tunable spectrometer

The design of highly reflective subwavelength gratings (SWGs) for use in a micro-electromechanical system (MEMS) tunable spectrometer is presented. The SWGs are designed to be polarization independent at an incident wavelength of 1.5 ¿m with high reflectivity over a 200 nm bandwidth. Two designs are considered; Model 1: a silicon layer with periodic air holes and Model 2: a stacked Si-SiO₂-Si design with the last Si layer a periodic array of columns. The designs are simulated using a commercial rigorous coupled wave analysis (RCWA) software package. The RCWA software aids in the design of SWGs that have higher reflectance than traditional dielectric mirrors. Model 1 has a reflectance (R)>0.99 for lambda 1.37-1.6 ¿m. Model 2 has a R>0.99 for lambda 1.46-1.69 ¿m. Finally, both designs are modeled to create a Fabry-Perot cavity, and at an incident wavelength 1.5 ¿m, the designs have a reflection finesse of 1707 and 4452 for Model 1 and Model 2, respectively.