Large aperture single crystal ZnGeP2 for high-energy applications

Abstract

Zinc germanium phosphide (ZGP), ZnGeP2, is the non-linear optical crystal of choice for laser frequency conversion in the 2–8 μm spectral range by virtue of its high non-linear coefficient (d14=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 27×39×140 mm3 has been demonstrated. A low-loss (1 0 0)-oriented ZGP sample measuring 30×30×22 mm3 was fabricated from this boule size. By adjusting the seeding orientation in single crystal boules with dimensions of 17×30×140 mm3, low-loss ZGP optical parametric oscillator (OPO) samples of 20×20×16 mm3 have been produced. Previously, typical ZGP OPO samples were 6×6×15 mm3. Besides allowing for larger aperture samples, the larger ZGP single crystals have the added benefit of lower absorption at both 2 μm (α2 μm<0.05 cm−1) and 1 μm (α1 μm∼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 μm and 10 kHz pulse rate frequency was increased to 2 J/cm2, which was double the previously measured value of 1 J/cm2. The combination of increased aperture, lower absorption, and improved LIDT of ZGP has resulted in material better suited to high-energy applications.