Optical Performance of E-beam Deposited ZnSe/BaF2-based Distributed Bragg Reflectors in the MWIR Region

Abstract

The distributed Bragg reflectors (DBRs) are important optical components in a wide range of applications, such as Fabry-Perot optical resonators, hyperspectral sensors, micro-cavity structures, and waveguide lasers. In this thesis, we have developed, designed, and fabricated highly reflecting, and mechanically stable DBRs comprised of electron-beam deposited zinc selenide (ZnSe) and barium fluoride (BaF2) thin films and explored its use in optical filtering and hyperspectral sensing applications. Most published results are in the short-wavelength infrared (SWIR) and near-infrared (NIR) with limited results in the mid-wavelength infrared (MWIR). In this work, we have fabricated ZnSe/BaF2-based DBRs in the MWIR region. The fabricated DBRs showed a central wavelength of 4 µm and reflection exceeding 93%. A Fabry-Perot optical filter, that is comprised of two parallel DBR mirrors with half-wavelength optical cavity thickness was used to partially transmit and reflect the incident light at the target wavelength. The assembled fixed-cavity Fabry-Perot optical filters achieved a full-width half maximum (FWHM) of ~ 450 nm. We further showed that the FWHM of the optical filters can be reduced by ~ 50 % with the addition of nano-thick silicon monoxide (SiO) interface grading layer between every pair of the quarter-wave optical thick (QWOT) layers in the stack. The SiO interface grading layer acts as an anti-reflection coating layer to reduce reflection loss in the DBR multilayer structure. The SiO interface grading layer, ~ 80 nm thick, also has the potential to stop atomic inter-diffusion between the layers to minimize free carrier absorption loss. The principal thesis goal is to design and assemble fixed-cavity interface-graded Fabry-Perot optical filters (with SiO interfacial layers) that optimized optical transmission and spectral resolution. The thesis also proposes a low-cost micro-cavity structure that can be formed by embedding two-dimensional atomic crystals, such as black phosphorous into a DBR cavity structure. The proposed design can be used to increase absorption in 2D materials, which has the potential to increase the efficiency of next generation MWIR photodetectors and infrared sensors.