Rapid assessment of mid-infrared refractive index anisotropy using a prism coupler: chemical vapor deposited ZnS.

A state-of-the-art mid-infrared prism coupler was used to study suspected anisotropy in the refractive index of forward-looking-infrared grade chemical vapor deposited (CVD) zinc sulfide. Samples were prepared with columnar grain structure in and perpendicular to the sample plane, as well as from different depths in the CVD growth body. This study was motivated by the growing industry concern among optical design engineers, as well as developers of mid-infrared systems, over the reliability of historically accepted index data. Prior photoluminescence and x-ray diffraction measurements have suggested that refractive index may vary according to sample orientation with respect to the grain structure. Measurements were conducted to provide optical dispersion and thermal index (dn/dT) data at discrete laser wavelengths between 0.633 and 10.591 µm at two temperature set points (30 °C and 90 °C). Refractive index measurements between samples exhibited an average standard deviation comparable to the uncertainty of the prism coupler measurement (0.0004 refractive index units), suggesting that the variation in refractive index as a function of sample orientation and CVD deposition time is negligible and should have no impact on subsequent optical designs. Measured dispersion data at mid-infrared wavelengths were also found to agree well with prior published measurements.

Measurement of the refractive index dispersion of As2Se3 bulk glass and thin films prior to and after laser irradiation and annealing using prism coupling in the near- and mid-infrared spectral range.

The prism coupling technique has been utilized to measure the refractive index in the near- and mid-IR spectral region of chalcogenide glasses in bulk and thin film form. A commercial system (Metricon model 2010) has been modified with additional laser sources, detectors, and a new GaP prism to allow the measurement of refractive index dispersion over the 1.5-10.6 µm range. The instrumental error was found to be ±0.001 refractive index units across the entire wavelength region examined. Measurements on thermally evaporated AMTIR2 thin films confirmed that (i) the film deposition process provides thin films with reduced index compared to that of the bulk glass used as a target, (ii) annealing of the films increases the refractive index of the film to the level of the bulk glass used as a target to create it, and (iii) it is possible to locally increase the refractive index of the chalcogenide glass using laser exposure at 632.8 nm.