Cadmium silicon phosphide, CdSiP2 (CSP), crystals have good nonlinear optical properties resulting in their use in optical parametric generation (OPO and OPA) of mid-infrared light. One common limitation on the performance of OPO materials is residual optical absorption which often results from point defects formed during crystal growth. Electron paramagnetic resonance (EPR) is a powerful technique for identifying and tracking point defects in materials. By correlating behaviors of native point defects exposed to 1064 nm light using EPR with changes in optical absorption bands, models are proposed for three of the observed broad optical absorption bands.
Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.
CdSiP2 (CSP) is a nonlinear optical material used for mid-infrared generation. For nonlinear optical materials, absorption bands associated with point defects often limit output power. We use electron paramagnetic resonance (EPR) to monitor paramagnetic charge states of defects. In CSP crystals, EPR shows singly ionized silicon vacancies (VSi-) initially present are eliminated by exposure to 1064 nm light. Our results suggest that 1064 nm light converts VSi- acceptors to nonparamagnetic doubly ionized (VSi2-) and neutral (VSi0) charge states. A thermal activation energy of 0.23 eV describes the recovery of the VSi- signal including at room temperature.
CdSiP2 (CSP) is a non-linear optical material for mid-infrared optical parametric oscillators. Previous work showed that an intrinsic acceptor (Si vacancy) produced unwanted absorption in the near-IR. The VSi concentrations are much reduced in recent growths. Other compensating defects now play an important role: iron impurities, an intrinsic donor (Si-on-Cd antisite), and a second intrinsic acceptor (Cd vacancy). We present photoinduced electron paramagnetic resonance (EPR) spectra to identify these defects. Illumination using light sources (lasers, LEDs) in the 500nm to 1064nm range can “reveal” these defects by converting them to their paramagnetic charge states. We present the wavelength dependence and thermal stability of these defects. Thermal decay data allow us to determine activation energies for various defect charge state transitions which allows us to predict decay times at room temperature of defect charge states and related absorption bands that can impact laser devices.
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 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 active defects in BaGa4Se7 crystals grown at BAE Systems. Five distinct defects, each representing an electron trapped at a selenium vacancy, are observed with EPR (there are seven crystallographically inequivalent selenium sites in this monoclinic crystal). One defect is seen at room temperature before illumination. The other four are seen at lower temperature after exposure to 532 nm laser light. Each singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site, which indicates a significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion. Optical absorption bands peaking between 430 and 750 nm are produced by the 532 nm light. These photoinduced bands are assigned to the selenium vacancies.
BaGa4Se7 is a promising new nonlinear optical material with a reported bandgap of 2.64 eV, and a broad spectral range out to 18 microns. Our experimental investigations use a variety of light sources from 325 – 442 nm below 160 K revealing the presence of an electron paramagnetic resonance (EPR) spectrum that is tentatively assigned to singly ionized selenium vacancies in the bulk single crystals, while 633 nm light is shown to remove the photoinduced signal. We correlate EPR results with optical data obtained using temperature-dependent absorption, thermoluminescence, and photoluminescence to investigate and characterize a broad absorption band resulting from illumination.
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