Induced defects and increase of second-order nonlinearity in ultraviolet-poled GeO2-SiO2 glass

We report on enhancement by thermal defects control of the second-order optical nonlinearity in GeO2-SiO2 glass induced by ultraviolet (UV) poling. The present work is based on the previous results which suggest GeE' centers as the most relevant ingredient to produce the induced nonlinearity, and heat treatment in a vacuum before UV poling has been found to increase the induced second-harmonic generation (SHG). The result is thought to be related to an increase of oxygen deficient defects, which function as precursors of the photoinduced GeE' centers. To obtain larger d coefficient, optimization of pre-heat treatment conditions, temperature and time duration, have been investigated. The largest value of d33 coefficient thus far obtained is 5.1±0.2 pm/V in UV-poled GeO2-SiO2 glass of 15 mole% GeO2 content with the pre-heat treatment at 1200 °C for 5 h, which exceeds d13, the second largest coefficient of LiNbO3.