Formation of langasite-type crystals in corresponding glasses and their second-order optical nonlinearities

Second-order optical nonlinearities for the langasite-type phases obtained by crystallization of corresponding glasses are clarified for the first time. It is confirmed that most of langasite-type crystalline phases can be formed by crystallization in the corresponding glasses without introducting of any impurity phases, e.g., the glasses with the stoichiometric composition of Ba₃Ga₂Ge₄O₁₄, BaGa₄La₂Ge₂O₁₄, Pb₃Ga₂Ge₄O₁₄, etc. We find that Pb₃Ga₂Ge₄O₁₄ and Pb₃AlGaGe₄O₁₄ phases exhibited strong second-harmonic intensities of approximately 100 times as large as α-quartz by means of a powder technique, demonstrating that those langasite-type crystals are excellent nonlinear optical materials. It is also suggested that the octahedral site is one of keys for second-order optical nonlinearity in langasite-type crystals.