Correlation of Optical Luminescence with Radiation Hardness in Doped LiNbO 3 Crystals
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Correlation of Optical Luminescence with Radiation Hardness in Doped LiNbO3 Crystals William J. Thomes, Jr.1, Kelly Simmons-Potter2, Barrett G. Potter, Jr.2, and Louis S. Weichman1 1 2
Sandia National Laboratories, Albuquerque, NM 87185 University of Arizona, Tucson, AZ 85721
ABSTRACT Transient ionizing radiation fields have been observed to cause substantial optical loss in undoped LiNbO3 crystals operating at 1.06 microns. This loss is slow to recover and makes the selection of this material for Q-switch applications in radiation environments unfeasible. We have studied the effects of Mg doping on the radiation response of LiNbO3 crystals and have investigated the optical luminescence of doped and undoped samples. Our results indicate a strong correlation between crystal defects, formed primarily during crystal growth, and the radiation-induced optical loss exhibited by these materials. These findings have enabled us to produce radiation-hard LiNbO3 crystals for use in high gamma-field environments. INTRODUCTION Many space-based systems rely on laser devices to perform critical optical functions. Based on their unique acousto-optical and electro-optical properties, LiNbO3 crystals can be used as the basis for frequency doubling, Q-switching, mode-locking, and information storage, among other applications. One of the challenges inherent in the deployment of optical systems in space is the potential for radiation-induced damage to materials and devices during their exposure to both high-flux transient and low-flux continuous-wave radiation environments. It is well known, for example, that point defects in the material crystal structure can participate in a number of electronic and optical radiation-induced effects, including changes in electrical conductivity, photoconductivity, optical absorption, and optical luminescence.[1-3] Such point defects and point defect complexes are generally associated with both atomic displacements (e.g., vacancy-interstitial pairs) and electron rearrangement. The latter can result in the formation of diamagnetic and paramagnetic centers with unique absorption and luminescence characteristics. In the presence of high-energy photons, such as x-ray or gammaray radiation, incoming photons will excite electronic defects, giving rise to transient and/or permanent changes in the absorption and luminescent spectra. In the case of LiNbO3, it has been shown that congruent growth of the bulk crystal leads to a nonstoichiometric lithium-deficient material.[4-11] The advantage of this composition lies in the high level of compositional homogeneity obtained over the boule length, arising from the identical Li concentrations in both the melt and in the evolving crystal.[5,8,11] There is agreement in the literature that the Li deficiency results in antisite Nb atoms sitting in Li sites (NbLi). The debate in the literature, however, has been over the nature of the charge compensation of the NbLi. The Li vacancy model, in which each NbLi is charge compensated by four lithium vacancies, is co
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