Large aperture single crystal ZnGeP2 for high-energy applications

Zinc germanium phosphide (ZGP), ZnGeP2, is the non-linear optical crystal of choice for laser frequency conversion in the 2-8 mu m spectral range by virtue of its high non-linear coefficient (d(14) = 75 pm/V) and thermal conductivity (0.35 W/(cm K)) as well as reductions in near-infrared absorption achieved in crystals grown by the horizontal gradient freeze technique. Recently, the growth of high optical quality, single crystal ZGP boules with dimensions of 27 x 39 x 140 mm(3) has been demonstrated. A low-loss (10 0)-oriented ZGP sample measuring 30 x 30 x 22 mm(3) was fabricated from this boule size. By adjusting the seeding orientation in single crystal boules with dimensions of 17 x 30 x 140 mm(3), low-loss ZGP optical parametric oscillator (OPO) samples of 20 x 20 x 16 mm(3) have been produced. Previously, typical ZGP OPO samples were 6 x 6 x 15 mm(3). Besides allowing for larger aperture samples, the larger ZGP single crystals have the added benefit of lower absorption at both 2 mu m (alpha(2 mu m)<0.05cm(-1)) and 1 mu m (alpha(1 mu m)similar to 1.0 cm(-1)) than the smaller scale crystals. Improvements to the fabrication and polishing of ZGP samples resulted in an increased laser-induced damage threshold (LIDT). The LIDT of anti-reflection-coated samples at 2.05 mu m and 10 kHz pulse rate frequency was increased to 2 J/cm(2), which was double the previously measured value of 1 J/cm(2). The combination of increased aperture, lower absorption, and improved LIDT of ZGP has resulted in material better suited to high-energy applications. (C) 2007 Elsevier B.V. All rights reserved.