Effect of interface grading on the optical performance of distributed Bragg reflector multilayers in Fabry-Pérot optical filters

Abstract

The design, fabrication, and characterization of abrupt interface and graded interface distributed Bragg reflector (DBR) multilayers is reported. Electron-beam deposited zinc selenide (ZnSe) and Barium Fluoride (BaF2) thin films were chosen as the high and low refractive index dielectric materials respectively, for the DBRs in the mid-wavelength infrared (MWIR) region. In this region, DBRs comprised of 3 stacks of abrupt interface ZnSe and BaF2 thin films with central wavelength of 4 µm were found to suffer from defects and absorption loss that later affect the optical transmission and spectral resolution of assembled Fabry-Perot optical filters. Interfacial grading of the DBRs with a thin layer of amorphous silicon monoxide (SiO) ~ 80 nm, reduced the absorption loss and the interdiffusion of defect atoms at the interface between alternating layers. Fabry-Perot filters assembled from abrupt interface DBRs showed 22% optical transmission, whereas those assembled from graded interface DBRs acheived 26.5% optical transmission. The Full-width at half maximum (FWHM) of the transmission peak for graded interface Fabry-Perot optical filter decreased significantly compared to the abrupt interface optical filter, improving its spectral resolution.