Type-II second-harmonic-generation properties of YCOB and GdCOB single crystals

As excellent nonlinear optical (NLO) crystals, YCa4O(BO3)(3) (YCOB) and GdCa4O(BO3)(3) (GdCOB) have been paid much attention since their first appearance in 1990' s. From that time to now, almost all of related researches and applications have focused on their type-I phase-matching (PM) configurations which possess large effective NLO coefficient (d(eff)). In this paper, type-II second-harmonic-generation (SHG) properties of these two crystals are reported, including PM curve, d(eff), angular acceptance and walk-off angle. Both of the type-II SHG experiments for 1064 and 1320 nm have indicated that the optimum directions which have maximum d(eff) locate in the second octant, i.e. (90 degrees < theta < 180 degrees, 0 degrees < phi < 90 degrees). For a (112 degrees, 81.3 degrees)-cut, 24 mm long YCOB crystal, the largest type-II SHG conversion efficiency of a 1064 nm Nd:YAG pico-second laser is 55%, which reaches the same level of the optimum type-I sample. To our knowledge this is the first time that type-II SHG performance of YCOB and GdCOB crystals is investigated intensively. Our research has shown that the smaller d(eff) of type-II PM can be compensated by its larger angular acceptance and less beam walk-off. The same level SHG conversion efficiency implies for such type crystals the type-II components have the potential to replace type-I ones and obtain important NLO applications in the future. (C) 2015 Optical Society of America