Refractive Index Measurements and New Sellmeir Coefficients of Zinc Germanium Phosphide From 2-9 Microns With Implicatio
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451 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
which doubling is observed is wider than that predicted. The discrepancy between the results of modem frequency conversion experiments and those predicted by the older data is easily understandable as the ZnGeP 2 being grown today is far superior to that available in the early 70s. Recent advances in compound synthesis, crystal growth and post processing techniques have vastly improved the optical properties of the ZnGeP 2. For instance, the broad absorption shoulder near two microns mentioned by Boyd has been greatly reduced so that pumping ZnGeP 2 at 2 microns is now possible. Crystals are now grown routinely with losses less than. 1 cm- at two microns, which makes ZnGeP 2 suitable for high power devices. In order to try to account for the newest observations of the frequency conversion characteristics of ZnGeP 2, we have made new set of measurements of the refractive indices, calculated new Sellmeir coefficients, and used these to calculate new phase matching loci for various frequency conversion processes. Experiment We have measured the refractive indices of ZnGeP 2 from 2-9 microns using the minimum deviation method on a prism fabricated from a boule grown by the horizontal gradient freeze method. The prism was cut so that the optic axis bisected the prism and the a axis was perpendicular to the triangular faces. At minimum deviation, the input light was propagating perpendicular to the optic axis. This permitted us to measure both the ordinary and extraordinary refractive index on the same prism. Using a modified MollerWedel divided circle spectrometer, we were able to obtain values for the refractive index to an accuracy of±lx10-4 . The refracted beam for the 2-5 micron spectral range was imaged using a Cincinnati Electronics model 160 IRRIS infra red camera which uses a cooled indium antimonide array for imaging. For the spectral region between 7.8 and 9 microns, an AVIO model 2000 infra red imaging camera was used. The apex angle of the prism, which was measured using an autocollimater attached to the divided circle spectrometer, was 29.948 degrees. The result of ten separate measurements of this angle resulted in a standard deviation of approximately 3 seconds of arc. Five measurements of the refractive index were made at each wavelength. The refracted beam was located on each side of the prism and the minimum deviation angle calculated as half the difference between the two readings. The refractive index was then calculated for each measured minimum deviation angle using the standard formula6 . Standard deviation of the 5 minimum deviation angle measurements taken at each wavelength was less than 15 seconds of arc. As a result, the error of the refractive index calculated from the standard equation was approximately 1X104 at any individual wavelength. The temperature for all measurements was 21-22' C. Results The refractive indices of ZnGeP 2 are shown in fig. 1. Our results differ significantly from the findings of earlier work
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