D. Bliss

Terahertz-wave generation in quasi-phase-matched GaAs

The authors demonstrate an efficient room temperature source of terahertz radiation using femtosecond laser pulses as a pump and GaAs structures with periodically inverted crystalline orientation, such as diffusion-bonded stacked GaAs and epitaxially grown orientation-patterned GaAs, as a nonlinear optical medium. By changing the GaAs orientation-reversal period (504–1277μm), or the pump wavelength (2–4.4μm), we were able to generate narrow-bandwidth (∼100GHz) terahertz wave packets, tunable between 0.9 and 3THz, with the optical-to-terahertz photon conversion efficiency of 3.3%.

On the nitrogen vacancy in GaN

The dominant electrically active defect produced by 0.42 MeV electron irradiation in GaN is a 70 meV donor. Since only N-sublattice displacements can be produced at this energy, and since theory predicts that the N interstitial is a deep acceptor in n-type GaN, we argue that the 70 meV donor is most likely the isolated N vacancy. The background shallow donors, in the 24–26 meV range, actually decrease in concentration, probably due to interactions with mobile N interstitials that are produced by the irradiation. Thus, the recent assignment of a photoluminescence (PL) line as an exciton bound to a 25 meV N-vacancy donor is incompatible with our results. Moreover, we do not observe that PL line in our sample.

High-power source of THz radiation based on orientation-patterned GaAs pumped by a fiber laser.

We demonstrate a new source of frequency-tunable THz wave packets based on parametric down-conversion process in orientation-patterned GaAs (OP-GaAs) that produces muW-level THz average powers at the repetition rate of 100 MHz. The OP-GaAs crystal is pumped by a compact all-fiber femtosecond laser operating at the wavelength of 2 mum. Such combination of fiber laser and OP-GaAs technologies promises a practical source of THz radiation which should be suitable for many applications including imaging and spectroscopy.

Optical parametric generation of a mid-infrared continuum in orientation-patterned GaAs

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 microm, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OP-GaAs sample with a 166.6-microm quasi-phase-matching period was pumped with 3.1-3.3 microm wavelength, 1 ps pulses up to 2 microJ in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%.

Terahertz Sources Based on Intracavity Parametric Down-Conversion in Quasi-Phase-Matched Gallium Arsenide

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of microstructured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. The OPO used type-II phase-matched periodically poled lithium niobate as a gain medium and was synchronously pumped by a mode-locked laser at 1064 nm (7 ps and 200 nJ at 50 MHz). With center frequencies spanning 0.4-3.5 THz, 250-GHz bandwidth radiation was generated. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21times over the singly resonant oscillator performance with an optical-to-THz efficiency of 10-4 and average THz power of 1 mW. The GaAs stabilized the DRO by a thermooptic feedback mechanism that created a quasi- continuous-wave train of THz pulses.

GaAs optical parametric oscillator with circularly polarized and depolarized pump.

We demonstrate an optical parametric oscillator (OPO) based on GaAs pumped with linearly polarized and circularly polarized light and show that the relative OPO thresholds agree with theoretical expectations. For the circularly polarized pump, the threshold was as low as for the [111]-linearly polarized pump case. The pump was also passed through a Lyot depolarizer to produce pseudo-depolarized light, and the OPO threshold in this case was only 22% higher than that for [001]-linearly polarized pump.

Modeling of high pressure, liquid-encapsulated Czochralski growth of InP crystals

A high resolution numerical scheme based on multizone adaptive grid generation and curvilinear finite volume discretization has been implemented to simulate the high pressure, liquid-encapsulated Czochralski (HPLEC) growth of InP crystals and study the effect of gas recirculation on melt flow and crystal/melt interface shape. The model incorporates flows induced by buoyancy and capillary forces and by crystal and crucible rotations, as well as the radiation heat loss from the melt and the crystal surfaces. It is demonstrated that the thermal interaction between the gas and the melt must be accounted for to predict the interface shape and dynamics accurately. The numerical results demonstrate that the transport phenomena in a high pressure growth system is very complex. The temperature distribution in the crystal and the shape of the melt/crystal interface also change significantly with the size of the crystal. This has a strong influence on dislocations in the crystal as shown by the experiments of Kohiro et al. [J. Crystal Growth 158 (1996) 197] and Jordan et al. [J. Crystal Growth 70 (1984) 555].

Thermal characterization of the high pressure crystal growth system for in-situ synthesis and growth of InP crystals

Abstract Indium phosphide (InP) is an important substrate material for light-wave communications, opto-electronics and radiation-resistant solar cells. However, the high cost and low productivity of the current two-step InP crystal growth process remains a severe drawback to its commercial applications. This has motivated many researchers to propose and investigate an innovative scheme of one-step synthesis (by injecting phosphorus vapor into the indium melt) and growth of InP crystals by the liquid-encapsulated Czochralski or Kyropoulos technique. For this one-step process to succeed and produce single crystals of uniform quality, it is important to develop a basic understanding of the mechanisms of energy transport and gas flow in a high-pressure crystal growth (HPCG) system. A series of experiments is conducted to characterize the thermal coupling between the melt and the phosphorus injector and to develop an understanding of the buoyancy-induced flow in a HPCG furnace. The gas flow in a high pressure furnace is turbulent and oscillatory, but radiation dominates the heat transfer. Thermal response of the system is therefore quite stable and predictable. The correlation between temperatures at various locations of the phosphorus injector and the melt is very interesting. The heat of reaction also affects the melt temperature. The phase change phenomenon at the bottom of the phosphorus injector seems to be oscillatory in nature. Theoretical estimates of the strength of gas convection and radiation loss by the melt surface are also presented.

Electrical properties of the hydrogen defect in InP and the microscopic structure of the 2316 cm−1 hydrogen related line

We have studied the microscopic structure of a hydrogen related defect by measuringits vibrational IR absorption at 2315.6 cm−1 in bulk InP crystals doped with deuterium. In contrast to the spectrum observed in nominally undoped samples (with only hydrogen present), the 2315.6 cm−1 line in these samples containing both hydrogen and deuterium is split into at least three components approximately 0.5 cm−1 apart. This can be explained if the defect contains more than one hydrogen atom; the additional lines are caused by mixed vibrational modes containing various combinations of hydrogen and deuterium. We present evidence that the formation of defect-hydrogen complexes leads to creation of a shallow intrinsic donor which can be annihilated under certain annealing conditions.

Vertical gradient freezing of doped gallium–antimonide semiconductor crystals using submerged heater growth and electromagnetic stirring

Abstract An investigation of the melt growth of uniformly doped gallium–antimonide (GaSb) semiconductor crystals as well as other III–V alloy crystals with uniform composition are underway at the US Air Force Research Laboratory at Hanscom Air Force Base by the vertical gradient freeze (VGF) method utilizing a submerged heater. Stirring can be induced in the GaSb melt just above the crystal growth interface by applying a small radial electric current in the liquid together with an axial magnetic field. The transport of any dopant and/or alloy component by the stirring can promote better melt homogeneity and allow for more rapid growth rates before the onset of constitutional supercooling. This paper presents a numerical model for the unsteady transport of a dopant during the VGF process by submerged heater growth with a steady axial magnetic field and a steady radial electric current. As the strength of the electromagnetic (EM) stirring increases, the convective dopant transport increases, the dopant transport in the melt reaches a steady state at an earlier time during growth, and the top of the crystal which has solidified after a steady state has been achieved exhibits axial dopant homogeneity. For crystal growth with stronger EM stirring, the crystal exhibits less radial segregation and the axially homogeneous section of the crystal is longer. Dopant distributions in the crystal and in the melt at several different stages during growth are presented.