Patterned Gallium Arsenide Devices for Infrared Countermeasures

Scientists are developing nonlinear frequency conversion devices based upon epitaxially grown gallium arsenide.

October 1, 2006

The US Air Force has a need for improved tunable laser sources—both in the midinfrared region, for developing infrared countermeasure (IRCM) applications, and in the longinfrared region, for addressing an increasing variety of threat sensors. Since few direct lasers exist in these spectral regions, scientists generally use nonlinear frequency conversion techniques to convert the output of available lasers into the desired longer wavelengths.

For many years, AFRL has been instrumental in developing nonlinear frequency conversion devices for IRCM, from the devices of the late 1980s, which employed birefringently phasematched crystals (e.g., silver gallium selenide, zinc germanium diphosphide, and potassium titanyl phosphate), to those emerging during the 1990s, which were based on quasi-phasematched periodically poled lithium niobate (PPLN) and similar poled ferroelectric materials. While all such materials have contributed to advancing the state of the art in nonlinear frequency conversion technology, each possesses inherent limitations. For example, crystals that achieve phase-matching as a result of birefringence often experience beam walk-off, which limits their output and efficiency. They also exhibit other problematic characteristics, including slow and limited-range frequency tuning by means of crystal rotation; inadequate power handling, resulting in thermal lensing and damage; and the presence of impurities, leading to scatter or absorption. Although poled ferroelectric materials use quasi-phasematching to avoid many of these problems, their intrinsic absorption prevents their practical use at wavelengths longer than 4 μm.

To reap the benefits of quasi-phasematching across the 2-5 μm and 8-12 μm wavelength regions, AFRL is therefore developing the next generation of nonlinear optical devices. The new technology employs gallium arsenide (GaAs) and similar zincblende crystal structure semiconductor materials, which have several properties particularly suited to IRCM applications. GaAs, for instance, is transparent across the 2-12 μm range and has a high nonlinear coefficient for efficient frequency conversion. Furthermore, GaAs—like all quasi-phase-matched materials—has the capacity to be engineered within broad limits to obtain a desired output wavelength from an available pump source. Engineers pattern the material so as to reverse the sign of the nonlinear coefficient with a periodicity that compensates for the phase mismatch between the input and output waves. Since GaAs is isotropic in its refractive index, quasi-phasematching is the only way to exploit its nonlinear potential.

The main obstacle to developing GaAs for nonlinear devices has been finding a practical method of patterning the material. GaAs is not ferroelectric; therefore, it is impossible to pattern it by electric field poling. An initial patterning approach, pursued by Stanford University researchers over a decade ago, involved polishing GaAs wafers to the necessary thickness and then cutting, inverting, and manually stacking them.¹ Although this early technique produced samples that successfully doubled carbon dioxide laser output, the fabrication process was not only prohibitively labor intensive, but was clearly incapable of producing the material thicknesses and tolerances necessary for reaching optical parametric oscillator (OPO) thresholds.

Several years later, motivated by the significant potential of GaAs as a nonlinear optical material, other Stanford researchers developed a two-step fabrication technique that could produce device-quality samples.² In this technique, growth of the desired pattern occurs on a GaAs substrate using molecular beam epitaxy (MBE). MBE growth is slow, however, and yields patterned growth just a few microns thick. Reaching thicknesses in the hundreds of microns needed for device demonstration requires the deposit of additional material onto the MBE-grown template. Researchers accomplished this using 1960s-vintage technology: hydride vapor phase epitaxy (HVPE).

While preliminary proof-of-concept experiments proceeded at Stanford using samples fabricated in France, AFRL scientists initiated an in-house, basic research program to study a low-pressure HVPE growth process at the lab's Optoelectronic Technology Branch, Hanscom Air Force Base (AFB), Massachusetts (see Figure 1) . This group has since taken the lead in growing thick-layer, orientation-patterned GaAs (OPGaAs), and its laboratory remains the only US facility able to provide this technology for patterned semiconductor device research.³ To spur the practical development of GaAs nonlinear devices, AFRL secured Air Force Dual-Use Science and Technology program sponsorship, initiated the Compact and Rugged Midinfrared Active (CARMA) sensor program, and executed a pair of technology investment agreements with industrial partners Northrop Grumman and BAE Systems.

The goal of the CARMA program is to leverage AFRL expertise and facilities to transition breakthrough OPGaAs technology from academia to industry. Under the agreements, collaborating scientists send MBEgrown templates to the AFRL HVPE facility for thick-layer growth. AFRL then provides the finished samples to Northrop Grumman, BAE Systems, and in-house researchers at the lab's Electro-Optical Countermeasures Technology Branch for characterization and device demonstration.

Progress to date has been encouraging. Researchers at AFRL's HVPE facility can readily obtain layer growth up to 1 mm thick, producing device-quality samples for a variety of nonlinear interactions. More significantly, CARMA team members have demonstrated OPO performance in OPGaAs devices. Specifically, BAE Systems obtained an output of nearly 0.5 W at 3.4 and 5.2 μm wavelengths with 20% slope efficiency using a thulium- and holium-doped yttrium lithium fluoride laser as the pump source, and successfully tuned this output over a 0.5 μm range by varying the OPGaAs crystal temperature.⁴ In addition, the Stanford group demonstrated a tuning range of several microns by pumping with a tunable PPLN OPO in addition to temperature tuning.⁵ AFRL has made progress in transitioning the technology to industry. When the CARMA program started, Stanford was the only source for MBE-grown templates; now, BAE Systems successfully grows these templates, has increased template diameter from 2 to 3 in., and plans to launch an industrial-grade HVPE facility in the near future.

To fully realize the potential of OPGaAs-based nonlinear optical materials, researchers must still address some significant challenges. Although 1 mm thick HVPE-grown GaAs is now possible, the material's periodic pattern is often lost after the first few hundred microns, as the alternating regions grow together (see Figure 2). This limits the area in which nonlinear conversion can occur. A second issue with these materials is optical loss resulting from both scatter and absorption. This loss is generally higher in patterned regions than in unpatterned regions (suggesting that the boundaries between alternating domains may play a role), but the specific causes and mitigation strategies remain unclear. A loss coefficient of 0.01 cm-1 is generally considered favorable for practical devices, but this represents an order of magnitude better performance than most current samples attain. Finally, researchers must improve HVPE thick-layer growth methods to achieve a reliable, safe, production-oriented process.

Through the CARMA program and related efforts, AFRL sensors scientists are working with industry partners to address these challenges, as well as to extend the orientation patterning technique to other promising materials. Future plans include both a follow-on effort to build upon CARMA's accomplishments and a collaboration with AFRL's Survivability and Sensor Materials Division scientists to leverage their expertise and improve fabrication processes for a broad spectrum of candidate materials.

Dr. Rita Peterson, of the Air Force Research Laboratory's Sensors Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203- 6451 or place a request at http://www . afrl.af.mil/techconn_index.asp. Reference document SN-H-06-02.

References

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Eyres, L. A., et al. "All-Epitaxial Fabrication of Thick, Orientation-Patterned GaAs Films for Nonlinear Optical Frequency Conversion." Applied Physics Letters, vol 79, no 7 (Aug 13, 2001): 904.
Bliss, D. F., et al. "Epitaxial Growth of Thick GaAs on Orientation-Patterned Wafers for Nonlinear Optical Applications." Journal of Crystal Growth, vol 287 (2006): 673-678.
Schunemann, P. G., et al. "2.05 μm Laser- Pumped Orientation-Patterned Gallium Arsenide (OPGaAs) OPO." Conference on Lasers and Electro-Optics (CLEO) (2005): 1835-1837.
Vodopyanov, K. L., et al. "Optical Parametric Oscillation in Quasi-Phase- Matched GaAs." Optics Letters, vol 29, no 16 (Aug 15, 2004): 1912-1914.