Oxygen Vacancies in LiB\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e Crystals and Their Role in Nonlinear Absorption

LiB3O5 (LBO) crystals are used to generate the second, third, and fourth harmonics of near-infrared solid-state lasers. At high power levels, the material’s performance is adversely affected by nonlinear absorption. We show that as-grown crystals contain oxygen and lithium vacancies. Transient absorption bands are formed when these intrinsic defects serve as traps for “free” electrons and holes created by x rays or by three- and four-photon absorption processes. Trapped electrons introduce a band near 300 nm and trapped holes produce bands in the 500-600 nm region. Electron paramagnetic resonance (EPR) is used to identify and characterize the electrons trapped at oxygen vacancies (the unpaired electron is localized on one neighboring boron). Self-trapped holes and lithium vacancies with the hole trapped on an adjacent oxygen are also observed with EPR. At room temperature, we predict that most of the unwanted defect-related ultraviolet absorption created by a short laser pulse will decay with a half-life of 29 µs

Transition-metal ions in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e crystals: Identification of Ni acceptors

Excerpt: Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band

Cu 2+ and Cu 3+ Acceptors in β-Ga 2 O 3 Crystals: A Magnetic Resonance and Optical Absorption Study

Electron paramagnetic resonance (EPR) and optical absorption are used to characterize Cu2+ (3d9) and Cu3+ (3d8) ions in Cu-doped β-Ga2O3. These Cu ions are singly ionized acceptors and neutral acceptors, respectively (in semiconductor notation, they are Cu− and Cu0 acceptors). Two distinct Cu2+ EPR spectra are observed in the as-grown crystals. We refer to them as Cu2+(A) and Cu2+(B). Spin-Hamiltonian parameters (a g matrix and a 63,65Cu hyperfine matrix) are obtained from the angular dependence of each spectrum. Additional electron-nuclear double resonance (ENDOR) experiments on Cu2+(A) ions give refined 63Cu and 65Cu hyperfine matrices and provide information about the nuclear electric quadrupole interactions. Our EPR results show that the Cu2+(A) ions occupy octahedral Ga sites with no nearby defect. The Cu2+(B) ions, also at octahedral Ga sites, have an adjacent defect, possibly an OH− ion, an oxygen vacancy, or an H− ion trapped within an oxygen vacancy. Exposing the crystals at room temperature to 275 nm light produces Cu3+ ions and reduces the number of Cu2+(A) and Cu2+(B) ions. The Cu3+ ions have an S = 1 EPR spectrum and are responsible for broad optical absorption bands peaking near 365, 422, 486, 599, and 696 nm. An analysis of loops observed in the Cu3+ EPR angular dependence gives 2.086 for the g value and 22.18, 3.31, and −25.49 GHz for the principal values of D (the fine-structure matrix). Thermal anneal studies above room temperature show that the Cu3+ ions decay and the Cu2+ ions recover between 75 and 375 °C

Intrinsic Point Defects (Vacancies and Antisites) in CdGeP\u3csub\u3e2\u3c/sub\u3e Crystals

Cadmium germanium diphosphide (CdGeP2) crystals, with versatile terahertz-generating properties, belong to the chalcopyrite family of nonlinear optical materials. Other widely investigated members of this family are ZnGeP2 and CdSiP2. The room-temperature absorption edge of CdGeP2 is near 1.72 eV (720 nm). Cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites are present in as-grown CdGeP2 crystals. These unintentional intrinsic point defects are best studied below room temperature with electron paramagnetic resonance (EPR) and optical absorption. Prior to exposure to light, the defects are in charge states that have no unpaired spins. Illuminating a CdGeP2 crystal with 700 or 850 nm light while being held below 120 K produces singly ionized acceptors (VCd−) and singly ionized donors (GeCd+), as electrons move from VCd2− vacancies to GeCd2+ antisites. These defects become thermally unstable and return to their doubly ionized charge states in the 150–190 K range. In contrast, neutral phosphorous vacancies (VP0) are only produced with near-band-edge light when the crystal is held near or below 18 K. The VP0 donors are unstable at these lower temperatures and return to the singly ionized VP+ charge state when the light is removed. Spin-Hamiltonian parameters for the VCd− acceptors and VP0 donors are extracted from the angular dependence of their EPR spectra. Exposure at low-temperature to near-band-edge light also introduces broad optical absorption bands peaking near 756 and 1050 nm. A consistent picture of intrinsic defects in II-IV-P2 chalcopyrites emerges when the present CdGeP2 results are combined with earlier results from ZnGeP2, ZnSiP2, and CdSiP2

Deep Selenium Donors in ZnGeP\u3csub\u3e2\u3c/sub\u3e Crystals: An Electron Paramagnetic Resonance Study of a Nonlinear Optical Material

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert Se+p donors to Se0p donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior

Electron Traps in Ag-doped Li\u3csub\u3e2\u3c/sub\u3eB\u3csub\u3e4\u3c/sub\u3eO\u3csub\u3e7\u3c/sub\u3e Crystals: The role of Ag Interstitial Ions

Electron paramagnetic resonance (EPR) is used to establish models for electron traps in Ag-doped lithium tetraborate (Li2B4O7) crystals. When exposed at room temperature to ionizing radiation, electrons are trapped at interstitial Ag+ ions and holes are trapped at Ag+ ions on Li+ sites. The trapped electrons occupy a 5s1 orbital on the interstitial Ag ions (some of the unpaired spin density is also on neighboring ions). Three EPR spectra are assigned to electrons trapped at interstitial Ag ions. Their g values are near 1.99 and they have resolved hyperfine structure from 107Ag and 109Ag nuclei. The spectrum representing the largest concentration of trapped electrons has the unpaired spin shared by the interstitial Ag ion and an adjacent boron ion at its regular lattice site. A 10B enriched crystal verifies this assignment and an analysis of spin-Hamiltonian parameters yields information about the Ag and B orbitals occupied by the unpaired spin. The second spectrum has the unpaired spin shared equally by two Ag ions, one at an interstitial site and the other at an adjacent Li site. The third spectrum has a large Ag hyperfine interaction and a weak Li interaction. Optical absorption bands associated with the trapped electrons are observed between 225 and 500 nm. Thermal release of electrons from these traps is responsible for a prominent thermoluminescence peak near 150 °C, whereas optical release of the electrons at room temperature produces intense optically stimulated luminescence. Radiative recombination occurs at Ag2+ ions with emission peaking near 270 nm

Optically Active Selenium Vacancies in BaGa\u3csub\u3e4\u3c/sub\u3eSe\u3csub\u3e7\u3c/sub\u3e Crystals

Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is the production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize these defects. Five distinct EPR spectra, each representing an electron trapped at a selenium vacancy, are observed at low temperature (there are seven crystallographically inequivalent selenium sites in the crystal). One spectrum is stable at room temperature and is present before illumination. The other four are produced at lower temperatures with 532 nm laser light and are thermally unstable at room temperature. Each S = 1/2 singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site. A significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion and gives principal values of the hyperfine matrices in the 3350–6400 MHz range. Broad photoinduced optical absorption bands in the visible and near-infrared are assigned to the selenium vacancies

Residual Optical Absorption from Native Defects in CdSiP\u3csub\u3e2\u3c/sub\u3e Crystals

CdSiP2 crystals are used in optical parametric oscillators to produce tunable output in the mid-infrared. As expected, the performance of the OPOs is adversely affected by residual optical absorption from native defects that are unintentionally present in the crystals. Electron paramagnetic resonance (EPR) identifies these native defects. Singly ionized silicon vacancies (V-Si) are responsible for broad optical absorption bands peaking near 800, 1033, and 1907 nm. A fourth absorption band, peaking near 630 nm, does not involve silicon vacancies. Exposure to 1064 nm light when the temperature of the CdSiP2 crystal is near 80K converts V-Si acceptors to their neutral and doubly ionized charge states (V0-Si and V2-Si , respectively) and greatly reduces the intensities of the three absorption bands. Subsequent warming to room temperature restores the singly ionized charge state of the silicon vacancies and brings back the absorption bands. Transitions responsible for the absorption bands are identified, and a mechanism that allows 1064 nm light to remove the singly ionized charge state of the silicon vacancies is proposed

White Matter Hyperintensities in Vascular Contributions to Cognitive Impairment and Dementia (VCID): Knowledge Gaps and Opportunities

White matter hyperintensities (WMHs) are frequently seen on brain magnetic resonance imaging scans of older people. Usually interpreted clinically as a surrogate for cerebral small vessel disease, WMHs are associated with increased likelihood of cognitive impairment and dementia (including Alzheimer\u27s disease [AD]). WMHs are also seen in cognitively healthy people. In this collaboration of academic, clinical, and pharmaceutical industry perspectives, we identify outstanding questions about WMHs and their relation to cognition, dementia, and AD. What molecular and cellular changes underlie WMHs? What are the neuropathological correlates of WMHs? To what extent are demyelination and inflammation present? Is it helpful to subdivide into periventricular and subcortical WMHs? What do WMHs signify in people diagnosed with AD? What are the risk factors for developing WMHs? What preventive and therapeutic strategies target WMHs? Answering these questions will improve prevention and treatment of WMHs and dementia

Rapamycin Pharmacokinetic and Pharmacodynamic Relationships in Osteosarcoma: A Comparative Oncology Study in Dogs

Signaling through the mTOR pathway contributes to growth, progression and chemoresistance of several cancers. Accordingly, inhibitors have been developed as potentially valuable therapeutics. Their optimal development requires consideration of dose, regimen, biomarkers and a rationale for their use in combination with other agents. Using the infrastructure of the Comparative Oncology Trials Consortium many of these complex questions were asked within a relevant population of dogs with osteosarcoma to inform the development of mTOR inhibitors for future use in pediatric osteosarcoma patients.This prospective dose escalation study of a parenteral formulation of rapamycin sought to define a safe, pharmacokinetically relevant, and pharmacodynamically active dose of rapamycin in dogs with appendicular osteosarcoma. Dogs entered into dose cohorts consisting of 3 dogs/cohort. Dogs underwent a pre-treatment tumor biopsy and collection of baseline PBMC. Dogs received a single intramuscular dose of rapamycin and underwent 48-hour whole blood pharmacokinetic sampling. Additionally, daily intramuscular doses of rapamycin were administered for 7 days with blood rapamycin trough levels collected on Day 8, 9 and 15. At Day 8 post-treatment collection of tumor and PBMC were obtained. No maximally tolerated dose of rapamycin was attained through escalation to the maximal planned dose of 0.08 mg/kg (2.5 mg/30 kg dog). Pharmacokinetic analysis revealed a dose-dependent exposure. In all cohorts modulation of the mTOR pathway in tumor and PBMC (pS6RP/S6RP) was demonstrated. No change in pAKT/AKT was seen in tumor samples following rapamycin therapy.Rapamycin may be safely administered to dogs and can yield therapeutic exposures. Modulation pS6RP/S6RP in tumor tissue and PBMCs was not dependent on dose. Results from this study confirm that the dog may be included in the translational development of rapamycin and potentially other mTOR inhibitors. Ongoing studies of rapamycin in dogs will define optimal schedules for their use in cancer and evaluate the role of rapamycin use in the setting of minimal residual disease