Laser sources operating near a wavelength of four microns are important for a broad range of applications that require power scaling beyond the state-of-the-art. The highest power demonstrated in the spectral region from a solid-state laser source is based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP<sub>2</sub> (ZGP). High-power operation in ZGP is known to be limited by thermal lensing. By comparing the figure of merit for thermal lensing in ZGP with other NLO crystal candidates, CdSiP<sub>2</sub> (CSP) particularly offers significant advantages. However as was the case with ZGP during its early development, the physics of observed crystal defects, and their relevance to power scaling, was not at first sufficiently understood to improve the crystal’s characteristics as a NLO wavelength conversion element. During the past decade, significant progress has been made (1) with the first reported growth of a large CSP crystals, (2) in understanding the crystal’s characteristics and its native defects, (3) in improving growth and processing techniques for producing large, low-loss crystals, and (4) in demonstrating CSP’s potential for generating high-power mid-infrared laser light. The paper will summarize this progress.
Laser sources operating near a wavelength of four microns are important for a broad range of space and airborne applications. Efficient solid-state laser sources, demonstrating the highest output power, are based upon nonlinear optical (NLO) conversion using the NLO crystal ZnGeP<sub>2</sub>. However, a related NLO crystal, CdSiP<sub>2</sub>, is now under investigation by several groups around the world. A comparison of its figure of merit for high-power handling with other NLO candidates indicates its potential for higher performance. In addition, the crystal’s characteristics as well as efforts to understand the crystal’s defects that presently limit NLO performance are briefly discussed.
Photoluminescence (PL) and electron paramagnetic resonance (EPR) are high-resolution techniques used to study donors and acceptors in optoelectronic materials. Zinc oxide (ZnO), with a room-temperature band gap of 3.37 eV, has significant potential for applications ranging from light emission to sensors and detectors. The low-temperature near-edge PL of ZnO is rich in detail, with sharp-line emissions from bound excitons related to various donors and acceptors. Strong phonon couplings in this material produce a series of LO and TO phonon sidebands at slightly lower energies. Donor-acceptor pair and electron-acceptor recombinations related to nitrogen (E<sub>A</sub> = 209 meV) and lithium (E<sub>A</sub> ~ 0.6 eV) are detected. Copper and iron impurities show characteristic luminescence spectra in the visible. Thermal anneals in air induce significant changes in the PL spectra. Complementary information can be obtained from EPR and photoinduced EPR experiments performed at low temperature. In ZnO, EPR spectra have been observed from neutral nitrogen acceptors, neutral copper acceptors, neutral lithium acceptors, hydrogenic shallow donors, as well as deeper donors such as nickel and iron. In previous work, EPR spectra have been assigned to singly ionized oxygen vacancies and singly ionized zinc vacancies in electron-irradiated crystals.
CdGeAs<sub>2</sub> is an important nonlinear optical infrared material. Room-temperature absorption and temperature-dependent photoluminescence (PL) of as-grown p-type bulk crystals and crystals doped with indium and tellurium have been measured. The intensity of an intervalence band absorption near 5.5 microns (0.225 eV) is correlated with the intensity of a PL band near 0.55 eV. Both of these optical features indicate the presence of a native shallow acceptor level at 120 meV above the top valence band. The 0.55-eV PL band is donor-acceptor-pair recombination between shallow donors and the shallow acceptor level. A second PL band peaking near 0.35 eV is donor-acceptor-pair recombination between shallow donors and a deeper acceptor at 300 meV above the top valence band. Doping with indium and tellurium produces n-type material. The intervalence band absorption at 5.5 microns is completely eliminated in the n-type samples. Indium donors are incorporated on the Cd site and Te donors are incorporated on the As site.
Electron paramagnetic resonance (EPR) has been used to identify and characterize point defects in lithium triborate (LiB<sub>3</sub>O<sub>5</sub>) crystals grown for nonlinear optical applications. As-grown crystals contain oxygen vacancies and lithium vacancies (as well as trace amounts of transition-metal ions in a few samples). Exposing a crystal to ionizing radiation at 77 K produces “free” electrons and holes. These electrons are trapped at the pre-existing oxygen vacancies and give rise to an EPR signal with a large hyperfine from one <sup>11</sup>B nucleus. The corresponding holes become self-trapped on oxygen ions as a result of the significant lattice relaxation of a nearest-neighbor fourfold-bonded boron ion. This gives rise to an EPR signal with a smaller <sup>11</sup>B hyperfine pattern due to the oxygen’s threefold bonded boron neighbor. Warming the crystal to approximately 130 K destroys the self-trapped hole centers that were initially formed, and allows a second holelike signal to be observed (which in turn decays between 150 and 200 K). The structure of the second hole center is very similar to the self-trapped hole center and a neighboring lithium vacancy makes this latter center more thermally stable. The EPR spectra from Ni<sup>+</sup> and Cu<sup>2+</sup> ions are also reported.
Point defects are known to limit the performance of many nonlinear optical materials. For example, gray tracks form along the beam path in some KTP crystals when they are subjected to lasers operating at high peak powers. Transient absorption bands in the visible and ultraviolet can be induced in KDP and BBO crystals by intense pulsed laser beams. Chalcopyrite crystals such as ZnGeP<sub>2 </sub>and AgGaS<sub>2</sub> can have unwanted absorption bands which overlap their desirable 2-μm OPO pump region. All of these device-limiting extrinsic absorption bands are associated with point defects in the bulk of the crystals. Among the responsible defects are transition-metal-ion impurities, other nonmagnetic impurities, cation vacancies, anion vacancies, and antisites. In some materials, the point defects present in the as-grown crystals may already exhibit absorption bands, while in other cases, the existing point defects may need to trap an electron or hole during device operation before the absorption band is activated. Eliminating the optically active point defects during crystal growth will lead to materials with higher damage thresholds. With this as a goal, numerous investigators have used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to identify and characterize point defects which give rise to the unwanted absorption bands. EPR can detect concentrations of defects at levels as low as tens of parts per billion. Furthermore, each defect has a unique g-value signature which allows a variety of defects to be monitored simultaneously. Superhyperfine interactions with surrounding nuclei permits a "mapping" of the wave function for each defect and this results in a detailed model of the defect. The following review provides examples of how EPR and ENDOR are used to characterize commercially available nonlinear optical materials.
Two types of solid-state lasers have served as key elements in the development of laser fusion: tunable lasers, such as Ti:sapphire, and lasers with discrete emissions based on neodymium. These lasers have been utilized for research, diagnostics, and as oscillators (i.e., Nd:YLF) in the first stage. Crystal-line phosphates were studied in depth many years ago for laser applications, but these crystals generally fell into disfavor when they could not be easily commercialized. A class of self-activated materials, referred to as stoichiometric phosphates, were particularly interesting, since they could operate efficiently at high active ion concentrations without fluorescence quenching. Neodymium pentaphosphate (NdP<SUB>5</SUB>O<SUB>14</SUB>) initiated this interest, but the potential for rare-earth orthophosphate (REOP) crystals was not seriously considered at that time. Extrinsic effects observed during some fundamental studies of REOP crystal properties, such as by electron paramagnetic resonance (EPR), may heighten the interest in using these latter materials for far-ranging laser applications, including laser fusion.
ZnGeP<SUB>2</SUB> is a candidate material for tunable mid-infrared optical parametric oscillators pumped by a 2-micron laser. Performance, thus far, has been limited by appreciable optical absorption extending from the band edge near 0.7 micrometers to beyond 2 micrometers . In the present investigation, electron paramagenetic resonance (EPR) and electron- nuclear double resonance (ENDOR) is used to establish the specific identities of the defects responsible for the optical absorption in crystals grown by the horizontal gradient freeze technique. The dominant acceptor, observed by EPR in as-grown crystals, is a singly ionized zinc vacancy (V<SUB>Zn</SUB><SUP>-</SUP>). Photo-induced EPR provides evidence that the dominant compensating donor is a phosphorus vacancy (V<SUB>P</SUB><SUP>+</SUP>). Photoluminescence data and optical absorption spectra are interpreted in terms of transitions between these donor-acceptor pairs.
The formation of gray tracks in KTP is initiated when a Nd:YAG laser produces above-band-gap photons which, in turn, create electron-hole pairs along the beam path through the crystal. These photons result from nonlinear processes (e.g., tripling of the fundamental beam, sum- frequency generation with a fundamental and a doubled beam, etc.). These processes will create 355-nm photons which, coincidentally, nearly match KTP's room-temperature band edge at 350 nm. Many of these electrons and holes produced by above-band-gap photons will recombine; however, a portion of them will be trapped at stabilizing entities such as vacancies or impurities and form 'stable' gray tracks. In the present work, x-rays are used to simulate the effects of an intense laser beam, and thus increase the total number of centers within the sample that are available for study with electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. In flux-grown KTP crystals, holes are trapped at trivalent iron (Fe<SUP>3+</SUP>) ions, thus forming Fe<SUP>4+</SUP> centers, and electrons are trapped on titanium ions having an adjacent oxygen vacancy, thus forming Ti<SUP>3+</SUP>-V<SUB>O</SUB> centers. At room temperature, the decay of these electron and hole traps has a half-life of approximately 2 hours. Optical absorption bands associated with these electron and hole traps give rise to the gray- track color.
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.