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ZnGeP2 is one of the most useful nonlinear optical crystals for generation of high power and high energy infrared radiation in the 2 to 8 m spectral range because of its high nonlinear coefficient along with high thermal conductivity. Growth technology has advanced over the years and samples with improved optical quality arising from fewer defects, impurities and free carrier concentration are now available. But there have not been many direct measurements of the refractive indices of ZnGeP2, especially recently. One of the first papers describing nonlinear optical frequency conversion in ZnGeP2 also provided the values of temperature dependent refractive indices [1]. The temperature dependent birefringence in ZnGeP2 was measured by Fischer et al [2] and the room temperature indices were measured by Zelmon [3] with samples available in the 1990s and early 2000s. Temperature dependent Sellmeier coefficients were obtained by Ghosh [4] based on the data from 1971 – but there have been no new temperature dependent measurements since the work by Fischer in 1997 on the birefringence.
We have measured the ordinary and extraordinary indices of ZnGeP2 for the wavelength range of 0.9 to 12 m, over a temperature range of 90 K to 475 using high resolution FTIR spectra of a recently grown sample (ZGP 391Q) of thickness 150 m with the c-axis lying on the sample face. The spectra were taken with light polarized along and perpendicular to the c-axis. The temperature dependent Sellmeier coefficients are obtained, expressed in the form given by Kato [5]. The values of the coefficients at 296 K vary a little from those in [5]. The index values obtained from the current Sellmeier equations were used to determine the dependence of the signal and idler wavelengths as a function of the crystal cut angle (theta) for various values of pump wavelengths, for birefringent phase matching of Types I and II. The results obtained show good match with experimental values.
REFERENCES
1. G. D. Boyd, E. Buehler, and F. G. Storz, “Linear and nonlinear optical properties of ZnGeP2 and CdSe,” Appl. Phys. Lett. 18, 301–304, 1971.
2. D. W. Fischer, M. C. Ohmer, P. G. Schunemann, and T. M. Pollak, “Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 m using polarized light interference,” J. Appl. Phys. 77, 5942–5945, 1995, D. W. Fischer and M. C. Ohmer, “Temperature dependence of ZnGeP2 birefringence using polarized light interference,” J. Appl. Phys. 81, 425–431, 1997.
3. D. E. Zelmon, E.A. Hanning and P.G. Schunemann, “Refractive-index measurements and Sellmeier coefficients for zinc germanium phosphide from 2 to 9 m with implications for phase matching in optical frequency-conversion devices”, J. Opt. Soc. Am. B, 1307-1310, 18(9), 2001.
4. G. Ghosh, “Sellmeier coefficients for the birefringence and refractive indices of ZnGeP2 nonlinear crystal at different temperatures”, Appl. Opt., 37(7), 1205-1212, 1998.
5. K. Kato, ‘‘Second harmonic and sum frequency generation in ZnGeP2,’’ Appl. Opt. 36, 2506–2530, 1997.
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