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 ZnGeP2 (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, CdSiP2 (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 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 ZnGeP2 (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, CdSiP2 (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 ZnGeP2. However, a related NLO crystal, CdSiP2, 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.
Cadmium germanium diarsenide (CdGeAs2,) crystals are very promising for infrared second harmonic generation. However, their use has been limited by optical absorption in the 5 pm region. The role of composition and dopants has been extensively studied, and some point defects have been identified which do affect transparency. While some low absorption material has been produced, it has not been reproducible and variations within boules have been a serious problem. In this paper, which reviews some recent work on this problem, we describe a surprising and complex correlation between optical transparency, dislocations and point defects.
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 (EA = 209 meV) and lithium (EA ~ 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.
Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material useful for frequency conversion applications in the midinfrared. A broad absorption band peaking near 1.2 microns and extending past 2 microns is often observed. To identify the defects responsible for these absorption losses, we have performed an optical absorption investigation from 10 to 296 K on bulk crystals of ZnGeP2 grown by the horizontal gradient-freeze method. Three broad absorption bands in the spectral range from 1 to 4 microns are observed that are due to native defects. Comparison of photoinduced changes in absorption with photoinduced changes in EPR spectra allowed specific defects to be associated with each of the three absorption bands. A band peaking near 1.2 microns and another band peaking near 2.2 microns involve transitions associated with singly ionized zinc vacancies. A third absorption band, peaking near 2.3 microns and extending from 1.5 microns to beyond 4 microns, involves neutral phosphorus vacancies. Absorption bands due to anion-site donor impurities Se and S have also been studied.
CdGeAs2 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.
Cadmium zinc telluride (CdZnTe) is being developed for room- temperature x-ray and gamma ray detectors. Identification and control of point defects and charge compensators are currently important issues. We have used electron paramagnetic resonance (EPR) and photo-induced EPR to evalute shallow-donor defects in CdZnTe crystal grown by two different techniques. Samples grown by the high-pressure Bridgman technique and a crystal grown by horizontal Bridgman at IMARAD and doped with indium were included in this study. Prior to the EPR investigations, we performed liquid-helium photoluminescence (PL) in order to examine the radiation recombination paths and identify the presence of other defects in these crystals. Spectra were obtained showing sharp excitonic lines, shallow and deep DAP emission bands, and a deeper 1.1 eV emission. The PL data help define the optical excitation range used in photo-EPR measurements. The photo-EPR data obtained from our samples is used to determine the concentration of isolated donor centers, while the EPR signal present under no illumination gives a measure of the net compensation. We also report the excitation wavelength dependence of the isotropic EPR signal from the shallow donors.
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 (NdP5O14) 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.
The electrical and optical properties of iodine doped n-type HgCdTe alloys and superlattices grown by metalorganic molecular beam epitaxy using ethyliodide are reviewed. The rationale for the use of iodine rather than indium as the dopant species and the incorporation kinetics of iodine at the growth surface are discussed. The electrical and optical properties of iodine- doped CdTe and HgCdTe (x equals 0.24) are presented for carrier concentrations between 1015 and 1018 cm-3, as determined by Hall effect measurements and low- and room-temperature photoluminescence spectroscopy. These samples show strong room temperature excitonic effects due to free exciton and band to band recombination as determined by photoluminescence excitation spectroscopy. The electrical and optical properties of iodine-doped HgCdTe-CdTe superlattices also are discussed based on magnetoluminescence measurements in tilted magnetic fields of up to 7 Tesla in Voigt and Faraday geometry.
ZnGeP2 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 (VZn-). Photo-induced EPR provides evidence that the dominant compensating donor is a phosphorus vacancy (VP+). Photoluminescence data and optical absorption spectra are interpreted in terms of transitions between these donor-acceptor pairs.
Strain parameters of thin films are needed to numerically simulate the optical gain of light amplification for semiconductor devices. We report a complete quantitative treatment of the stress-strain relations for several common (h,h,k) orientations. We find that earlier reports that attributed the presence of tensile elastic strain to both thermal and lattice-mismatches gave strain estimates that were too low. Using our calculations, we present a model to explain the observed shifts in excitonic recombination energies that have been reported by many groups to date.
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