Broadband Reflectance and Transmittance Characterization of Crystalline Supermirrors

The thesis at hand describes reflectance R and transmittance T measurements per-formed on a pair of substrate-transferred crystalline supermirrors centered at a wavelength of 4.54 μm. For this purpose, two low-cost, flexible and spectrally broadband measurement setups were built. For the reflectance measurements, a cavity ring-down scheme, using a Fabry-Pérot quantum cascade laser, was implemented. This setup exploited direct passive feedback effects of the measurement cavity on the source laser in a simple linear configuration. The transmittance was determined using a direct approach, comparing the intensity of both, the light incident on and the transmitted by the mirror coating; using a globar type white light source for power stability and lock-in detection to enhance the signal-to-noise ratio and dynamic range. In both setups, a reflection-grating monochromator was used to achieve spectrally resolved measurements. We measured a maximum reflectance R=(0.9998215−(33)+(31)), and minimum transmittance T=(142±15) ppm. Since R+T+l=1, this allowed for an estimation of residual loss (absorption and scatter) in the order of l=(37±18) ppm. Alongside these measurement results, the needed theoretical basics and models are derived and explained.