Michael P. Scripsick

Effect of crystal growth on Ti3+ centers in KTiOPO4

A series of Ti3+ centers have been formed in hydrothermally grown and flux‐grown potassium titanyl phosphate (KTiOPO4 or KTP). These 3d1 defects (S=1/2) were created with 60 kV x rays at 77 K, and electron paramagnetic resonance and electron‐nuclear double‐resonance (ENDOR) data were taken below 30 K. The ENDOR spectra show that the two Ti3+ centers having the largest concentrations in hydrothermally grown KTP have a neighboring proton, presumably in the form of an adjacent OH− ion. In contrast, ENDOR spectra show that neither of the two Ti3+ centers having the largest concentrations in flux‐grown KTP have a neighboring proton. These significant differences in the local environment of the Ti3+ centers may help explain why KTP crystals have shown differing susceptibilities to gray tracking.

Point defects in lithium triborate (LiB3O5) crystals

Electron paramagnetic resonance (EPR), electron‐nuclear double resonance, optical absorption, and thermoluminescence have been used to investigate radiation‐induced point defects in a single crystal of lithium triborate (LiB3O5). Two prominent defects are observed after irradiation near liquid‐nitrogen temperature with 60 kV x rays. A four‐line EPR spectrum, with 12.2 G splittings, is assigned to a trapped‐hole center, and another four‐line EPR spectrum, with 120 G splittings, is assigned to a trapped‐electron center. In each case, the nucleus responsible for the observed hyperfine is 11B. The trapped hole is localized on an oxygen ion and has a weak hyperfine interaction with one neighboring boron nucleus, whereas the trapped electron is localized primarily on a boron ion with a correspondingly larger hyperfine interaction. Both defects become thermally unstable near 125 K, and their decay (i.e., recombination) correlates with an intense thermoluminescence peak at this same temperature. An optical absorp...

Identification of point defects in hydrothermally grown KTiOPO4

Electron paramagnetic resonance has been used to investigate radiation‐induced point defects in hydrothermally grown single crystals of potassium titanyl phosphate ( KTiOPO4 or KTP). The crystals were irradiated at 77 K with 60–kV x rays. A trapped‐hole center, a Ti3+ center, and a Pt3+ center were observed between 10 and 30 K after the initial 77‐K irradiation. These defects become thermally unstable near 160 K. The trapped hole is localized on an oxygen ion adjacent to a potassium vacancy and has resolved hyperfine interactions with three phosphorus nuclei. The Ti3+ ion is the intrinsic self‐trapped electron center in KTP and has weak hyperfine interactions with four phosphorus nuclei.

Role of silicon impurities in the trapping of holes in KTiOPO4 crystals

Electron paramagnetic resonance (EPR) has been used to characterize a new hole trap in flux-grown KTiOPO4 crystals. This center is formed at room temperature when the crystals are exposed to either 60 kV x rays or a pulsed 355 nm laser beam. Principal g values measured at room temperature are 2.0030, 2.0102, and 2.0320. The intensity of the EPR spectrum is considerably larger in a silicon-doped sample, thus suggesting that the responsible defect consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. Also, a broad optical absorption band peaking near 500 nm has been observed in the irradiated samples. The silicon-associated hole centers thermally decay over a period of several days at room temperature as electrons are released from Ti3+ traps. Analysis of hole-center decay curves obtained at three temperatures (291, 300, and 311 K) has shown that the kinetics of this electron-release process are primarily second order. The activation energy is approximately...

Dose-rate dependence in the production of point defects in quartz

Abstract A model has been developed which generates the complex shapes of the experimentally observed production curves for aluminium-associated trapped hole centers inm, hydrogen-swept quartz. All of the aluminum ions are initially charge-compensated by an adjacent hydrogen ion (in the form of an OH-molecule). Radiation dissociates the hydrogen from the aluminum and forms two separate defects, a trapped hole center and an interstitial hydrogen atom. The hydrogen atom rapidly migrates along a c-axis channel and either (1) becomes trapped or (2) recombines with a hole center to restore the original lattice. Hydrogen atoms become trapped as di-interstitials (i.e., as H 2 molecules) in our model. Rate equations have been developed for this sequence of events, and computer-generated solutions are compared with experimental ESR data. A variation of this model in which the hydrogen atoms are assumed to be singly trapped did not agree with experiment.

Point defects in KTP and their possible role in laser damage

Potassium titanyl phosphate (KTiOPO4 or KTP) has applications in nonlinear optics and electro-optics. It is most commonly employed in the second harmonic generation of .530 um light from 1.06 m Nd:YAG laser radiation. However, applications of KTP are limited by optical damage in the form of thin gray tracks produced by high-power, high-repetition-rate laser pulses. It is difficult to obtain samples of KTP with laser-induced gray tracks that are suitable for quantitative measurements. The gray coloration absorbs both the fundamental and second harmonic, and continued operation after the formation of these defects may quickly lead to catastrophic failure. Another complication arises because the gray tracks characteristic of laser damage are not stable at room temperature (they decay in a matter of days). Even if gray-tracked samples were readily available, it is questionable whether the concentration of responsible defects would be sufficient to provide definitive results. These difficulties have led researchers to investigate alternative methods for producing the defects responsible for laser-induced optical damage in KTP.

Spectra of tetravalent chromium in calcium fluorophosphate

The spectroscopic properties of Cr‐doped Ca5(PO4)3F known as Cr:FAP were examined using several different chemical and physical means. Results from analytical titration measurements indicate that chromium is tetravalent in the samples investigated. Based on electron‐spin resonance measurements at temperatures as low as 11 K, no trivalent chromium and only a trace of pentavalent chromium (0.5–10 ppm) was detected in the samples used for optical studies. We postulate that Cr4+ is stabilized in the crystal as a result of the small amount of Y2O3 that was added during the crystal growth process and that charge balance is achieved when Y3+ replaces Ca2+ and Cr4+ replaces P5+. Assuming tetravalent chromium, absorption, and emission spectra were interpreted by analyzing phonon sidebands and zero‐phonon transitions observed at various temperatures. The observed crystal‐field splitting is compared with calculated levels based on a Tanabe–Sugano diagram for quartet and doublet states of Cr4+(3d2) in tetrahedral sym...

Role of point defects in optical damage of nonlinear crystals

We have initiated a program at West Virginia University to establish the properties of point defects that are relevant to the optical damage phenomena in KTP, BBO, and LBO crystals. Defects have been characterized using optical absorption, electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and luminescence techniques. Among the defects which have been observed are impurity ions (iron, platinum, hydrogen, etc.), trapped hole centers, and trapped electron centers.