Rita D. Peterson

Thermal effects in Cr/sup 2+/:ZnSe thin disk lasers

We report the modeling and experimental characterization of thermal lensing in Cr/sup 2+/:ZnSe face cooled laser disks using the phase shift interferometry technique. The thermal lens powers of the 1 mm and 0.5 mm thick disks were strong (23 and 7 diopters at 8 W pumping). The thermal lens power scaled with disk thickness and pump power, and temperatures were reached in the disks such that nonradiative relaxation was significant. Laser output greater than 4 W average power was achieved using face cooled thin disks of Cr/sup 2+/:ZnSe.

Longwave-IR optical parametric oscillator in orientation-patterned GaAs pumped by a 2 µm Tm,Ho:YLF laser

We demonstrate longwave infrared (LWIR) generation with an optical parametric oscillator (OPO) based on quasi-phasematched orientation-patterned gallium arsenide (OPGaAs). The OPGaAs OPO was directly pumped with a Q-switched λ = 2.054 μm Tm,Ho:YLF laser. OPGaAs samples representing three different grating periods were used to explore the LWIR OPO performance yielding outputs ranging from λ = 2.5-2.7 μm (signal) and λ = 8.8-11.5 μm (idler). Slope efficiencies for the combined signal and idler outputs reach as high as 26% while slope efficiencies for only the idler reached 8%. Spectral measurements of OPO output confirm good agreement with theoretical calculations.

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.

Growth and study of nonlinear optical materials for frequency conversion devices with applications in defence and security

A series of nonlinear materials including GaAs, GaP, and ZnSe have been examined to determine their suitability for non-linear frequency conversion devices (FCD) and more specifically their use as high power, compact and broadly tunable IR and THz sources for defense and security applications. The more mature GaAs was investigated to reveal the causes for the optical losses that restrict achievement of higher conversion efficiency in quasi-phasematched FCD, while the efforts with GaP were oriented in developing simple, cost effective techniques for fabrication of orientation patterned (OP) templates and optimizing the subsequent thick HVPE growth on these templates. Thus, average growth rates of 50- 70 μm/h were achieved in up to 8-hour long experiments. High optical layer quality was achieved by suitable control of the process parameters. The optimal orientation of the pattern was determined and used as essential feedback aiming to improve the template preparation. This led to the production of the first 300-400 μm thick device quality OPGaP. Efforts to suppress the parasitic nucleation during growths with longer duration or to achieve thicker layers by a 2 step growth process were also made. The main challenge with the newer candidate, OPZnSe, was to establish suitable regimes for hydrothermal growth on plain (001) ZnSe seeds grown by chemical vapor deposition. Two different temperature ranges, 330-350 °C and 290-330 °C, were investigated. The mineralized concentration was also manipulated to accelerate the growth in (111) direction and, thus, to improve the growth in (001) direction. The next material in the line is GaN. The traditional HVPE approach will be combined with a growth at low reactor pressure. Growths will be performed in the next sequence: growth on thin GaN layers grown by MOCVD on sapphire wafers, growth on half-patterned GaN templates with different orientations and, finally, growth on OPGaN templates.

Heteroepitaxial growth of OPGaP on OPGaAs for frequency conversion in the IR and THz

For the first time thick orientation-patterned GaP (OPGaP) was repeatedly grown heteroepitaxially on OPGaAs templates as a quasi-phase matched medium for frequency conversion in the mid and longwave IR, and THz regions. The OP templates were fabricated by wafer-bonding and in a MBE-assisted polarity inversion process. Standard low-pressure hydride vapor phase epitaxy (LP-HVPE) was used for one-step growth of up to 400 µm thick device quality OPGaP with excellent domain fidelity. The presented results can be viewed as the missing link between a well-developed technique for preparation of OP templates, using one robust nonlinear optical material (GaAs), and the subsequent thick epitaxial growth on them of another material (GaP). The reason for these efforts is that the second material has some indisputable advantages in point of view of thermal and optical properties but the preparation of native templates encounters challenges, which makes it difficult to obtain high quality homoepitaxial growth at an affordable price. Successful heteroepitaxial growth at such a relatively high lattice mismatch (- 3.6%) in a close to equilibrium growth process such as HVPE is noteworthy, especially when previously reported attempts, for example, growth of OPZnSe on OPGaAs templates at about 10 times smaller lattice mismatch ( + 0.3%) have produced only limited results. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, is a solution that will accelerate the development of high power, tunable laser sources for the IR and THz region, which are in great demand on the market.

Development of orientation-patterned GaP grown on foreign substrates for QPM frequency conversion devices

Thick hydride vapor phase epitaxially grown orientation-patterned gallium phosphide (OPGaP) is a leading material for quasi-phase matching (QPM) frequency conversion in the mid- and longwave infrared (IR). This is due to its negligible two-photon absorption (2PA) in the convenient pumping range 1 – 1.7 μm, compared with the 2PA of some traditional QPM materials, such as GaAs. In this paper, we describe homo- and heteroepitaxial growth techniques aimed to produce hundreds of microns thick OPGaP on: 1) OPGaAs templates fabricated using an improved wafer-fusion process; 2) OPGaAs templates fabricated by using a molecular beam epitaxy (MBE) for sublattice polarity inversion, but one with and one without MBE regrowth after the inversion. Some of the advantages of the heteroepitaxial growth of OPGaP on OPGaAs templates include: 1) achieving good domain fidelity as a result of the significantly higher OPGaAs template quality; 2) eliminating the needs of using the poor quality commercially available GaP in the production of thick OPGaP material, and 3) suppression of the additional absorption band between 2 – 4 μm (which is due to incorporation of n-type impurities) and, in general, improvement of the IR transmittance in the entire IR region. Combining the advantages of the two most promising nonlinear materials, GaAs and GaP, will accelerate the development of high power, broadly tunable laser sources in the IR which, in addition, will be offered with higher device quality and at a reasonably lower unit cost.

Homo and heteroepitaxial growth and study of orientation-patterned GaP for nonlinear frequency conversion devices

Frequency conversion in orientation-patterned quasi-phase matched materials is a leading approach for generating tunable mid- and long-wave coherent IR radiation for a wide variety of applications. A number of nonlinear optical materials are currently under intensive investigation. Due to their unique properties, chiefly wide IR transparency and high nonlinear susceptibility, GaAs and GaP are among the most promising. Compared to GaAs, GaP has the advantage of having higher thermal conductivity and significantly lower 2PA in the convenient pumping range of 1– 1.7 μm. HVPE growth of OPGaP, however, has encountered certain challenges: low quality and high price of commercially available GaP wafers; and strong parasitic nucleation during HVPE growth that reduces growth rate and aggravates layer quality, often leading to pattern overgrowth. Lessons learned from growing OPGaAs were not entirely helpful, leaving us to alternative solutions for both homoepitaxial growth and template preparation. We report repeatable one-step HVPE growth of up to 400 μm thick OPGaP with excellent domain fidelity deposited for first time on OPGaAs templates. The templates were prepared by wafer fusion bonding or MBE assisted polarity inversion technique. A close to equilibrium growth at such a large lattice mismatch (-3.6%) is itself noteworthy, especially when previously reported attempts (growth of OPZnSe on OPGaAs templates) at much smaller mismatch (+0.3%) have produced limited results. Combining the advantages of the two most promising materials, GaAs and GaP, is a solution that will accelerate the development of high power, tunable laser sources for the mid- and long-wave IR, and THz region.

Cascaded OPGaAs OPO for increased longwave efficiency

Optical parametric oscillators (OPOs) producing longwave output from a much shorter pump wavelength suffer from low conversion efficiency into the idler due to the large quantum defect compared with similar devices operating in the 3 – 5 μm regime. One method to increase pump to idler conversion efficiency is to recycle the undesired and higher energy signal photons into additional idler photons in a second nonlinear stage. We present numerical simulation results showing the improvement in efficiency that can be obtained in a linear, two stage, cascaded orientation patterned gallium arsenide (OPGaAs) nanosecond OPO. It includes diffraction, crystal loss, phase mismatch, pump depletion, and back conversion; and it assumes monochromatic waves but it neglects group velocity dispersion. For a singly resonant oscillator (SRO) pumped by a 2.054 μm Tm:Ho,YLF laser with 45 ns pulse widths, the addition of the second crystal in the cavity increases idler generation by overall factor of two and exceeds the quantum defect limit. The model has been validated by comparison with SNLO for the case of a single-stage OPO, and suggests crystal and resonator parameters that will lead to an optimized cascaded OPO.

Recent progress in heteroepitaxy of nonlinear optical materials for frequency conversion devices (Conference Presentation)

Frequency conversion in orientation-patterned (OP) materials is a leading approach for generating mid- and long-wave IR radiation. Although several phase-matching and quasi-phase-matching (QPM) materials have been investigated to date none of these have met all requirements for power, tunability and frequency range of the pursuit applications. We present an original approach that successfully combines in a QPM heterostructure two of the most promising materials, GaP—a material with lower two and three-photon absorption than GaAs, and GaAs—a material with a mature process for fabrication of high quality OP templates. Up to 300 µm thick OPGaP with excellent domain fidelity has been repeatedly grown with 100 µm/h by hydride vapor phase epitaxy on the robust and high quality OPGaAs templates. Some simplifications of both template fabrication and growth process are also reported. The samples, characterized by AFM, SEM, XRD, EDS and TEM, showed smooth surface morphology and high crystalline quality. Special attention was paid to the interface and especially to the mechanism of forming an intermediate ternary transition layer. This led to determining certain criteria that indicate, which other heteroepitaxial cases would be also successful. Thick growths of GaAsP and GaP on other alternative substrate materials by combining a-close-to-equilibrium with a-far-from-equilibrium processes were also performed. Efforts to develop heterostructures in horizontal and vertical direction have been also made. The success with one less favorable (in point of view of lattice mismatch) case, presented here, indicates that we should have even better results in other cases with closer lattice matches.