Zinc telluride is a semiconductive material that has been found to become photorefractive when it is suitably doped with vanadium or with manganese and vanadium. The combination of photorefractivity and semiconductivity make this material attractive for use in a variety of applications, including optical power limiting (for shielding eyes or delicate sensors against intense illumination), holographic interferometry, providing reconfigurable optical interconnections for optical computing and optical communication, and correcting for optical distortions and combining laser powers via phase conjugation. In comparison with other important photorefractive materials based on III-V and II-VI binary compounds, ZnTe:V offers superior photorefractive performance at wavelengths from 0.6 to 1.3 μm.

Undoped or doped ZnTe can be grown by physical vapor transport in a closed ampoule. The source material lies at one end of the ampoule and is made to sublimate by heating that end to a suitable temperature. The resulting vapor is transported to the other, cooler end of the ampoule, where it condenses to form a boule of the material. Ideally, the boule thus formed should be a single crystal (as distinguished from a polycrystalline mass). For successful growth of a single crystal, it is necessary to adjust the thermal gradient and other conditions to make the rate of transport commensurate with the rate of integration of condensing molecules and atoms into the growing crystal.

In an effort to learn how to optimize conditions for single-crystal growth, the diffusive and convective effects of heat and mass transfer have been investigated both experimentally and theoretically. Topics addressed in these investigations have included effects of process parameters, effects of buoyancy-driven convection on transport properties, growth fluxes, and crystal-growth rates (deduced from growth fluxes, assuming fast kinetics at growth interfaces). The results of these investigations indicate that for a given gradient of temperature, the ratio between the partial pressures of Zn and Te at the source strongly affects the rate of transport. The rate of growth changes with both the temperature and the gradient of temperature between the source and the growing crystal.

These Interferograms, generated by resonant holographic interferometric spectroscopy, illustrate the distribution of potassium seeded into three butane diffusion flames. Taking advantage of the rapid response of a photorefractive semiconductor like ZnTe:V:Mn, one can acquire such images at video frame rates.

Experiments have been performed to determine the optical absorption spectra, electrical resistivities, photorefractive properties, and microstructures of specimens of doped ZnTe. Among other things, it has been found that the yield of photorefractive crystals is very low when vanadium is the only dopant, but that one can increase the yield, the photorefractive gain, and the diffraction efficiency by doping with manganese in addition to vanadium.

An experiment was performed to investigate optical power limiting in ZnTe:V by the field-shielding effect, which is a nonlinear effect that occurs in the presence of an applied electric field and that results in partial darkening. For example, in one case, the transmission of a specimen at a wavelength of 0.83 μm was 20 percent at an incident radiant flux density of 6 mW/cm2, but decreased to 1 percent when the flux density was increased to 1 W/cm2.

Experiments were performed to investigate the utility of ZnTe:V:Mn for real-time resonant holographic interferometry. These experiments involved, variously, two- or four-wave mixing, using pulsed dye or continuous-wave He/Ne or diode lasers. Holographic image transfer and two-wavelength resonant holographic interferometry were demonstrated; in particular, a ZnTe:V:Mn crystal was used in a demonstration of resonant holographic interferometric spectroscopy, which is a technique for obtaining chemical-species-specific interferograms by recording two holograms simultaneously at two slightly different wavelengths near an absorption spectral peak of the species in question (see figure).

This work was done by Walter M.B. Duval of Lewis Research Center; Sudhir B. Trivedi, G. V. Jagannathan, Xiaolu Wang, Jolanta I. Soos, and Robert D. Rosemeier of Brimrose Corp.; H. Zhou and Abdelfattah Zebib of Rutgers University; and W. H. Steier and Mehrdad Ziari of the University of Southern California. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Machinery/Automation category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 - 3
21000 Brookpark Road
Cleveland
Ohio 44135.

Refer to LEW-16498.


NASA Tech Briefs Magazine

This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine.

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