Crystallization and second harmonic generation in thermally poled niobium borophosphate glasses

Crystallization of glasses with compositions (1-x)(0.95 NaPO(3) + 0.05 Na(2)B(4)O(7)) + xNb(2)O(5), x = 0.4, 0.43, 0.45, 0.48 was investigated by differential scanning calorimetry and X-ray powder diffraction. Crystallization of two phases was observed in the glasses with x = 0.43-0.48. First phase is a sodium niobate with the structure of tetragonal tungsten bronze (T(c)similar to 720-760 degrees C) and second phase is Na(4)Nb(8)P(4)O(32) (T(c)similar to 830-850 degrees C). The crystallization of sodium niobate is correlated with increasing of nonlinear optical efficiency reported for thermally poled glasses with x > 0.4. The results of Raman spectroscopy show the formation of three-dimensional (3D) niobium oxide framework in the glasses with increase of niobium concentration. This framework is supposed to have tetragonal tungsten bronze structure and to be responsible for nonlinear optical properties of the glass. Second harmonic generation signals of as prepared and crystallized glass after thermal poling are compared. The nucleation and crystallization do not improve the NLO properties of the glasses under study. (c) 2005 Elsevier Inc. All rights reserved.