Candace Lynch

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%.

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.

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.

Wafer-fused orientation-patterned GaAs

The fabrication of thick orientation-patterned GaAs (OP-GaAs) films is reported using a two-step process where an OP-GaAs template with the desired crystal domain pattern was prepared by wafer fusion bonding and then a thick film was grown over the template by low pressure hydride vapor phase epitaxy (HVPE). The OP template was fabricated using molecular beam epitaxy (MBE) followed by thermocompression wafer fusion, substrate removal, and lithographic patterning. On-axis (100) GaAs substrates were utilized for fabricating the template. An approximately 350 μm thick OP-GaAs film was grown on the template at an average rate of ~70 μm/hr by HVPE. The antiphase domain boundaries were observed to propagate vertically and with no defects visible by Nomarski microscopy in stain-etched cross sections. The optical loss at ~2 μm wavelength over an 8 mm long OP-GaAs grating was measured to be no more than that of the semi-insulating GaAs substrate. This template fabrication process can provide more flexibility in arranging the orientation of the crystal domains compared to the Ge growth process and is scalable to quasi-phase-matching (QPM) devices operating from the IR to terahertz frequencies utilizing existing industrial foundries.

Terahertz wave generation in orientation-patterned GaAs using resonantly enhanced schemes

Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient. Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses. Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.

Progress in orientation-patterned GaAs for next-generation nonlinear optical devices

Orientation-patterned GaAs (OPGaAs) shows great promise as a nonlinear optical material for frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in each of the three main areas of OPGaAs development: fabrication of patterned templates using a combination of wafer bonding and MBE techniques; thick-layer HVPE growth; and material and OPO device characterization. This work has led to significant improvements in material quality, specifically reduced optical loss, increased sample thickness, improved patterned domain fidelity, and greater material uniformity. Advances in material quality have in turn enabled demonstration of OPO devices operating in the 3-5 μm spectral region. Optical loss and OPO performance measurements on a series of OPGaAs samples are presented, with the goal of understanding how these properties are influenced by growth conditions, and how OPO performance may be improved. Research continues on understanding loss mechanisms, correlating performance with material properties, transitioning the technology into an industrial process, and extending it to additional materials.

Improved material quality and OPO performance in orientation-patterned GaAs

The U.S. Air Force is developing orientation-patterned GaAs (OPGaAs) for nonlinear frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in OPO device performance which reflects continued improvement in material quality. Seven new OPGaAs samples, representing four distinct growth regimes, were evaluated in terms of threshold, slope efficiency, and output spectral content using a Q-switched Tm,Ho:YLF laser operating at 2 μm as the pump source. The samples were identical in base template, grating period, length, and AR coating, permitting direct comparison of results to identify favorable growth conditions. As anticipated, performance varied significantly among the sample set, with slope efficiencies from 12% to 35% and thresholds from 9 μJ to 40 μJ. Less anticipated was the low level of uniformity across each sample and between samples grown in the same growth run. Significant variations in slope efficiency and threshold were not uncommon. Still, most of the samples performed manifestly better than previously grown material, indicating an overall improvement in OPGaAs quality. Research continues on understanding growth processes, optical loss mechanisms, and how these translate into device performance.

Toward hot-hole THz lasers in homoepitaxial Si and GaAs with layered doping

A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.

Characterization of HVPE-grown thick GaAs structures for IR and THz generation

In a previous paper, we described a method for growing thick epitaxial layers of GaAs on orientation-patterned wafers by low pressure hydride vapor phase epitaxy. The low pressure method allows for rapid growth at rates well above 100 mum/hr and layers up to 1 mm thick have been successfully produced. In this paper we present characterization of these layers by optical microscopy, Hall measurement, and cathodoluminescence imaging. We demonstrate growth of low free carrier concentration, mm-thick orientation-patterned GaAs for efficient nonlinear optical conversion

THz source based on resonantly-enhanced difference frequency generation in periodically-inverted GaAs

We report mW-average-power widely tunable (0.5-3.5 THz) monochromatic THz source based on frequency mixing in periodically-inverted GaAs, between the two closely spaced dasiasignalpsila and dasiaidlerpsila waves, inside the resonant cavity of an optical parametric oscillator.