New Light from Gallium Arsenide: Micro-Structured GaAs for Mid-IR and THz-Wave Generation K. L. Vodopyanov, J.E. Schaar, P. S. Kuo, M. M. Fejer, X. Yu, J. S. Harris Stanford University, Stanford, California 94305, USA vodopyan@stanford.edu V. Kozlov, W. C. Hurlbut Microtech Instruments, Inc., Eugene, OR 97403, USA Y.-S. Lee Oregon State University, Corvallis, OR 97331, USA C. Lynch, D. Bliss Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts 01731, USA Abstract: We review nonlinear-optical applications of micro-structured quasi-phasematched GaAs. These include new sources of mid-IR coherent radiation, pump-polarization-insensitive devices, as well as sources of tunable terahertz radiation, based on optical rectification or intracavity frequency mixing. © 2006 Optical Society of America OCIS codes: (190.2620) Frequency Conversion; (160.6000) Semiconductors, including MQW Because of a number of appealing properties of GaAs, namely its large nonlinear-optical coefficient (both in the mid-IR and terahertz region), small IR and terahertz absorption, and high thermal conductivity, it appears to be a very promising material for parametric frequency down-conversion into these poorly explored regions of electromagnetic spectrum. Lack of birefringence in GaAs requires use of quasi-phasematching (QPM) to achieve efficient frequency conversion. Orientation-patterned GaAs (OP-GaAs) is a type of QPM GaAs where the domain inversions are lithographically defined in an all-epitaxial fabrication process based on molecular beam epitaxy and hydride vapor phase epitaxy [1]. Few examples of mid-IR applications of the OP-GaAs include efficient second-harmonic and difference- frequency mid-IR generation, an optical parametric oscillator (OPO), which is continuously tunable from 2 to 11 μm [2], as high-average-power (0.45 W) OPO at 3-5 μm, which was pumped by a high-repetition rate (20 kHz) 2.05- μm-laser [3], as well as mid-IR supercontinuum source (4.5–10.7 μm), based on optical parametric generator, which takes advantage of the broadband parametric gain near the zero-group-velocity dispersion region [4]. The high symmetry of the nonlinear optical susceptibility tensor of GaAs (there are only three equal nonzero elements) and also its isotropic refractive index, offer a variety of possibilities for parametric amplification with respect to the polarizations of the participating waves. For example an optical parametric amplifier (OPA), based on the OP-GaAs, nonzero gain exists for any pump polarization, for an appropriately polarized signal-idler pair. This unusual pump polarization diversity of the OP-GaAs results in the possibility of its pumping with circular or even unpolarized laser radiation. In the terahertz (THz) domain, in addition to its low absorption, GaAs has small mismatch between the optical group velocity and terahertz phase velocities which increases coherent interaction between optical and THz beam in the optical rectification process. Tunable THz output was achieved using quasi-phasematched GaAs, via optical rectification, using fs laser pulses in the wavelength range (2–4.4 μm). Both, epitaxially-grown OP-GaAs (Fig. 1), as well as diffusion-bonded GaAs (DB-GaAs) [5] were used in these experiments. By changing the GaAs QPM period, or the pump wavelength, terahertz-wave tunability in the whole range 0.5-3.5 THz was achieved, with quantum conversion efficiency reaching 3.3% [6]. It is noteworthy that this conversion efficiency was obtained with the pump-pulse energy of only 2.3 μJ. When a compact Tm-based fs fiber laser with the wavelength near 2 μm and 100 MHz repetition rate was used as a pump source, 3 μW of average THz power was generated in the OP-GaAs crystal at 1.8 and 2.5 THz [7]. CMJ1.pdf