Large single crystals of gallium selenide: growing, doping by In and characterization

Gallium selenide is a material with great potential for nonlinear optical applications, due to its many attractive properties, including a broad low-loss transmittance (from 0.65 to 18 μm) and a large nonlinear coefficient. However, its current application is limited due to difficulties with its growth, and poor mechanical properties. Our current research has a threefold purpose. First, this work is intended to investigate the feasability of growing large, optical quality single crystal boules of GaSe; second, to investigate the impact of isovalent indium doping on the mechanical, electrophysical, and optical properties of GaSe, and third, to determine the concentration of In which is optimal for purposes of facilitating the practical implementation of GaSe in nonlinear devices.     

Limited efficiency of a silver selenogallate optical parametric oscillator caused by two-photon absorption

The nonresonant two-photon absorption (TPA) coefficient in silver selenogallate (AgGaSe(2)) crystals was measured for both ordinary and extraordinary polarizations in the 1300-1600-nm wavelength range. We found a cutoff wavelength for the TPA at between 1400 and 1500 nm, which corresponds to half of the bandgap energy of the AgGaSe(2) crystal. Below the cutoff wavelength we measured the TPA coefficient to be approximately 0.035 cm/MW for the extraordinary polarization and two to three times lower for the ordinary polarization. We compared the AgGaSe(2) samples from two manufacturers and observed a factor of 2 difference in the TPA coefficients. Because of the high TPA, the 1.32-mum pumped AgGaSe(2) optical parametric oscillator conversion efficiency was clipped at a low level.     

Morphology control of layer-structured gallium selenide nanowires

Layer-structured group III chalcogenides have highly anisotropic properties and are attractive materials for stable photocathodes and battery electrodes. We report the controlled synthesis and characterization of layer-structured GaSe nanowires via a catalyst-assisted vapor-liquid-solid (VLS) growth mechanism during GaSe powder evaporation. GaSe nanowires consist of Se-Ga-Ga-Se layers stacked together via van der Waals interactions to form belt-shaped nanowires with a growth direction along the [11-20], width along the [1-100], and height along the [0001] direction. Nanobelts exhibit a variety of morphologies including straight, zigzag, and saw-tooth shapes. These morphologies are realized by controlling the growth temperature and time so that the actual catalysts have a chemical composition of Au, Au-Ga alloy, or Ga. The participation of Ga in the VLS catalyst is important for achieving different morphologies of GaSe. In addition, GaSe nanotubes are also prepared by a slow growth process.     

High-efficiency mid-infrared ZnGeP2 optical parametric oscillator in a multimode-pumped tandem optical parametric oscillator

We demonstrate a high-efficiency ZnGeP(2) optical parametric oscillator (OPO) pumped by another KTP OPO in a multimode-pumped tandem OPO configuration. The maximum optical-to-optical and slope efficiencies were 32% and 42.5%, respectively. Our setup also provides tunable multiband radiation in the 2.03-2.32-mum range and the 2.9-6.2-mum range simultaneously.     

Noise-induced parametric oscillations in nonlinear oscillator

The onset of stochastic oscillations in a nonlinear oscillator with a noise-modulated frequency is considered. It is demonstrated that these oscillations are characterized by the existence of an attractor (i.e., are independent of the initial conditions), but do not exhibit the phenomenon of synchronization typical of self-oscillatory systems.     

A parametric quartz crystal oscillator

Parametric oscillators have been well studied but currently are not used often. Nevertheless, they could be a low-phase noise solution, at least outside the frequency bandwidth of the resonant circuit. The theoretical aspect of parametric oscillations is briefly reviewed in this paper. Indeed, the basic theory of a simple resistance-inductor-capacitor (RLC) circuit working in parametric conditions easily can be extended toward a resonant loop that includes a quartz crystal resonator. Then, as an application, this study is transposed to a quartz crystal oscillator that has been modeled and tested as a first prototype. Simulation results are compared with those actually obtained.     

A nanoscale parametric feedback oscillator

We describe and demonstrate a new oscillator topology, the parametric feedback oscillator (PFO). The PFO paradigm is applicable to a wide variety of nanoscale devices and opens the possibility of new classes of oscillators employing innovative frequency-determining elements, such as nanoelectromechanical systems (NEMS), facilitating integration with circuitry and system-size reduction. We show that the PFO topology can also improve nanoscale oscillator performance by circumventing detrimental effects that are otherwise imposed by the strong device nonlinearity in this size regime.     

Long-pulse, amplitude-modulated optical parametric oscillator

A singly resonant optical parametric oscillator (OPO) based on periodically poled LiNbO(3) is pumped by a Nd:YAG-based oscillator-modulator-amplifier source. This pump source, operating at 1.064 μm, provides the ability to control the temporal characteristics of the OPO waveform. We illustrate pulse tailoring by demonstrating three pulse formats: a pulse with a sharply rising edge, a square pulse, and an amplitude-modulated square pulse. The OPO output is tuned over 1.45-1.67-μm (signal) and 2.9-4.0-μm (idler). We demonstrate a 7-μJ, 2-μs square pulse with 5-MHz sinusoidal amplitude modulation.     

Spatial--OH impurity distribution in gallium phosphate crystals

Piezoelectric properties of quartz and quartz-like materials are strongly related to the impurity content in the crystals and, more especially, to their hydroxyl group (–OH) content. This work has been devoted to the determination of the spatial distribution of this impurity in as-grown crystals of gallium phosphate, GaPO₄. The investigation was undertaken by infrared spectroscopy from eight samples with different growth conditions and completed by thermally stimulated current/relaxation map analysis techniques. The results allow the best growth parameters to be defined, leading to crystals with the lowest –OH impurity content.     

Optical absorption and photoconductivity in gamma-irradiated zinc selenide crystals

The possibility of radiation-induced enhancement of photoconductivity (σ_ph) in crystalline zinc selenide (ZnSe) has been studied. The electron concentration in the initial ZnSe:O crystal was N _Eg = 1.26 × 10¹⁶ cm⁻³ as determined from the optical density of the sample at E _g = 2.58 eV. The irradiation of the ZnSe:O crystal by gamma-photons from ⁶⁰Co source to a dose of 10⁶ Gy leads to the creation of five new energy levels (Γ_6v, 5.76 eV; L _1.3v, 4.85 eV; Zn_i, 3.34 eV; O_Se, 3.13 eV; X, 2.72 eV), a decrease in the electron concentration to N _Eg = 0.63 × 10¹⁶ cm⁻³, and a twofold increase in σ_ph. The doping with Te to 0.2 wt % also creates the L _1.3v level and increases N _Eg to 2.02 × 10¹⁶ cm⁻³ and σ_ph to 2.01 × 10⁻¹⁰ Ω⁻¹. The irradiation of Te-doped crystals leads to the creation of an additional resonant level (Zn_i, 3.34 eV) and a two-fold increase in σ_ph. An increase in the content of Te to 0.5 wt % results in a shift of the Zn_i level to 3.39 eV and a growth in the conductivity to σ_ph = 7.64 × 10⁻¹⁰ Ω⁻¹. However, the gamma-irradiation of these crystals leads to decomposition of this center and to a decrease in the photoconductivity.     

Group-velocity-matched optical parametric oscillator in tilted quasi-phase-matched gratings

An achromatic phase-matching scheme is reported for an optical parametric oscillator in tilted quasi-phase-matched gratings. The spectral angular dispersion is introduced in interaction waves such that each wave component satisfies the two-dimensional (noncollinear) quasi-phase matching. This is equivalent to simultaneous quasi-phase matching and group-velocity matching for ultrashort pulses. The phase-matching bandwidth for 10 mm periodically poled KTP increases by a factor of 12 at lambdas = 1.7 microm compared with one-dimensional quasi-phase matching. The effective interaction length will increase as a result of the matching.     

Slip systems in gadolinium gallium garnet single crystals

Gd₃Ga₅O₁₂ single crystals with compression axes of different orientations have been deformed under creep conditions in air at 1450° C and 1550° C (0.86 T _M and 0.92 T _M, respectively, where T _M is the melting temperature). After a few per cent of permanent strain the deformation substructure has been studied by optical and transmission electron microscopy. Etching and bi-refringence patterns indicate that slip on {110}, {112} and {123} can be activated depending on the orientation of the compression load. Dislocations with a/2 111 Burgers vectors have been observed to glide in {110 planes. They exhibit a segmented aspect suggesting their dissociation out of the glide plane.     

Microhardness of gallium crystals grown in a centrifuge

Gallium single crystals with identified faces grown from melt under usual conditions are compared for the first time to those obtained under conditions of centrifugation at 15000g. The crystals grown in a centrifuge exhibit a change in the lattice parameters and possess a significantly higher microhardness. Crystals of both types are characterized by a strong polar anisotropy.     

The non-degenerate optical parametric oscillator in the strong-coupling regime

Abstract

A fully quantum treatment of the non-degenerate optical parametric oscillator in the extreme quantum limit of small photon numbers and very high nonlinear coupling strength is presented. When the nonlinear coupling constant becomes comparable with the cavity decay rate, the sharp threshold that is usually encountered disappears, a situation reminiscent of microlasers. Furthermore the output light exhibits unusual statistical properties such as strong super-Poissonian behaviour of the pump mode for (very) large coupling constants.     

Deformation of single crystals of gallium arsenide

Deformation of single crystals of gallium arsenide is reported in bend and tension up to 1000° C whilst maintaining stoichiometry in an arsenic atmosphere. Surface defects and impurity segregation are shown to be dislocation sources. The dislocation density is low enough, however, to show large yield drops which are analysed in detail. Strains of 39% are possible. The activation energy for dislocation movement is increased by heat-treatment owing to an increase in point defect population.Electron microscopy shows that the predominant slip systems are {111} 110 and the majority of dislocations have b=a/2 110, the axes lying along 110 and 112 directions. Sub-cell formation occurs with sub-boundaries lying along 110 directions.     

Passively Q-switched 1.57-microm intracavity optical parametric oscillator

We demonstrate an eye-safe KTP-based optical parametric oscillator (OPO) driven intracavity by a diode-pumped 1064-nm Nd:YAG laser, passively Q-switched by a Cr4+:YAG crystal. The characteristics of this system, which operates at 1570 nm with a repetition rate as high as 50 Hz, are studied as a function of Cr4+:YAG optical density. Under optimum conditions the OPO generates 1.5-mJ, 3.4 +/- 0.1-ns pulses in a single transverse mode. For a Cr4+:YAG Q-switch element with an optical density of 0.5 the conversion efficiency of the intracavity energy is approximately 45% with the ratio of OPO to Nd:YAG peak-pulse intensity exceeding unity. These and other OPO characteristics compare favorably with a simple rate equation model of the OPO dynamics.     

Optical parametric oscillator pumped by a Bessel beam

We demonstrate operation of a KTP optical parametric oscillator (OPO) pumped by a Bessel beam for the first time to our knowledge. It is shown that the output of the OPO has a transverse profile, which is consistent with noncollinear phase-matching relations defined by a conical pump. The central spot and ring related to the pair of signal and idler beams were generated in the OPO. By adjusting the OPO cavity mirrors, we easily selected the lowest-order mode as well as the higher-order transverse modes in the central spot. Bessel beam pumping was shown to be useful, providing tubular beam coupling into OPO cavity modes. The OPO threshold pump energy was ~100 muJ in a 6-ns pulse.