Growth and characterization of orientation patterned GaAs crystals for non-linear optical frequency conversion
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Growth and characterization of orientation patterned GaAs crystals for non-linear optical frequency conversion O. Martínez1, M. Avella1, V. Hortelano1, J. Jiménez1, M. Snure2, C.Lynch2, D. Bliss2 1
Optronlab Group, Dpto. Física de la Materia Condensada, Centro I+D, Univ. de Valladolid, Paseo
de Belén 1, 47011 Valladolid, Spain 2
Air Force Research Laboratory, Sensors Directorate, Hanscom AFB, MA 01731, U.S.A
ABSTRACT Orientation patterned (OP)-GaAs crystals have high potential as non linear optical systems. Mid-infrared and terahertz lasers sources can be fabricated with these crystals by frequency conversion from shorter wavelength sources. The optical propagation losses are critical; therefore, the OP-GaAs crystals must have high quality with low incorporation of defects and high homogeneity to reduce the refractive index fluctuations. Defects with electro-optic signature must be characterized in order to reduce their presence. Cathodoluminescence studies of these crystals permit the distribution of the main defects to be established, both extended and point defects. Special attention is paid to the role of the walls between the two domain orientations, and to the incorporation of impurities in Si-doped samples. INTRODUCTION Mid infrared (mid-IR) and terahertz (THz) frequency generation is very important for IR spectroscopy, remote gas sensing, IR countermeasures for aircraft protection, and THz imaging (1). In spite of this, mid-IR sources suitable for these applications are scarce. An attractive approach for these sources is the non linear optical conversion by quasi-phase matching (QPM) with GaAs crystals (2). GaAs presents excellent properties as a mid-IR material, because it has a very large non linear optical coefficient, it is transparent in the 0.9-17 μm spectral window, has excellent thermal conductivity, and also high laser power damage threshold; however, it is isotropic, therefore, one needs to develop strategies allowing the fabrication of structures with periodically modulated non-linear optical coefficient. They can be prepared by epitaxial growth of a grating of GaAs crystals with alternated crystallographic orientation, forming the so-called orientation patterned GaAs crystals (OP-GaAs), which are periodic gratings of domains and inversed domains separated by antiphase boundaries (APBs) (3); the grating period can be varied allowing different ranges of frequency conversion. These crystals are inserted in an optical parametric oscillator (OPO), allowing the fabrication of laser sources with wavelengths ranging from 2 μm to the THz range. The crystal thickness must be > 500 μm in order to accommodate the pumping laser beam. Therefore, fast growth of the periodic [001] / [00-1] oriented GaAs domain structure is required. Hydride vapour phase epitaxy (HVPE) is the technique of choice for growing so thick structures (4). The crucial issue to attain high conversion efficiency is the minimization of the optical losses; for which one needs high quality materials, free of defects; therefore, an exhaust
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