Omnidirectional high-reflectivity mirror in the 4–20μm spectral range

Abstract

Decreasing-width multilayers (DWMs) can behave as highly-efficient broadband reflectors with wider reflection spectrum than periodic, disordered or other aperiodic structures made of the same materials. In this article, we numerically analyze and optimize the reflectance R of a linear DWM formed by the bilayer $\mathrm{CsBr}\mbox{--}\mathrm{Te}$ from normal to grazing incidence and for a large wavelength spectrum (from mid to far infrared). As a result, when the rate of the thickness decrease of $\mathrm{CsBr}$ layers equals −59.5 nm/layer, an optimized average reflectance for ${\text{}}p$ polarization $\langle {R}_{p}\rangle =1\,-\,6\times {10}^{-6}$ can be obtained in the 4–20 μm spectral range for all angles of incidence. For ${\text{}}s$ incident states, the average reflectance is even greater $\langle {R}_{s}\rangle =1\,-\,3\times {10}^{-7}$. Moreover, a near-zero transmittance region $T\leqslant {10}^{-12}$ occurs in the 12–20 μm wavelength interval constituting (to our knowledge) the widest spectrum for a single omnidirectional mirror where such remarkable reflectance values can be achieved.