Hybrid Mid-Infrared Supermirrors with Ion-Beam-Sputtered Coatings

Highly reflective (HR) optical coatings for the mid-infrared (MIR) spectral region are a vital component for a wide variety of applications, especially cavity-enhanced spectroscopy methods [1], [2]. However, HR coatings deposited by physical vapor deposition techniques often suffer from high excess loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S$</tex>+<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$A$</tex> (scatter/absorption) in the MIR. Recently, monocrystalline substrate-transferred supermirrors have emerged as an excellent alternative in the MIR region [1]–[3]: the latest 44.5-period GaAs/AlGaAs distributed Bragg reflectors (DBR) grown via molecular beam epitaxy (MBE) yielded extremely low S+A = 4.27 ppm and total loss 1-R = 13.6 ppm (two-mirror cavity finesse <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$F$</tex> = 231000) at a center wavelength (CWL) <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{0}=4.45\ \mu \mathrm{m}$</tex>[1]. Still, the low refractive index contrast of GaAs/AlGaAs requires a high number of quarter-wave layers for optimal reflectivity R, leading to thick coatings. This impedes scaling of MBE-grown DBRs to CWLs well above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5\ \mu \mathrm{m}$</tex> due to manufacturing constraints.