Improved NLO crystals for mid-IR laser applications

摘要

In the past two decades mid-IR nonlinear optical crystals have grown from scientific curiosities to practical robust materials generating efficient, multi-watt output in the 3-12um spectral range. Nonetheless, improved NLO crystals are critical for further advancing mid-IR laser development. In particular, mid-IR materials are needed which: 1) efficiently convert cw pump sources; 2) can be pumped with 1-micron (Nd and Yb) lasers; and 3) offer improved performance in the 8-12 micron range and beyond. For such applications a high nonlinear coefficient is desirable, but this is less important than more practical properties such as low absorption loss, high laser damage threshold, low thermal lensing, and non-critical phase matching (NCPM). To meet these demands, the search for improved mid-infrared NLO crystals is proceeding on three fronts. First, work continues to reduce losses in the best existing materials such as ZnGeP_2, AgGaSe_2, and CdGeAs_2. Secondly, new compounds such as AgGaGeS_2 and AgGaGe_5Se_(12) are being evaluated. Thirdly, and most promising, is the work on all-epitaxial processing of orientation-patterned gallium arsenide (OPGaAs) and other binary quasi-phase-matched semiconductors such as ZnSe, GaP, and GaN. This review paper will discuss recent advances on all of these fronts.