Induction and relaxation of optical second-order nonlinearity in tellurite glasses

Abstract

Second harmonic generation has been examined for 30ZnO⋅70TeO2 glass with a two-step poling procedure in order to understand the poling temperature dependence of second harmonic intensity. When the poling temperature increases, the second harmonic intensity increases, manifests a maximum at the temperature which we call an optimum poling temperature, and decreases drastically just below the glass transition temperature. The glass treated with two-step poling, which includes poling at 300 °C and subsequent poling at the optimum poling temperature, i.e., 280 °C, exhibits much smaller second harmonic intensity and more unambiguous Maker fringe pattern than that poled only at 280 °C. This fact suggests that the decrease in second harmonic intensity with an increase in poling temperature cannot be attributed to a reversible process like a thermal fluctuation of dipoles, but is governed by an irreversible one. Based on a linear relation between the optimum poling temperature and glass transition temperature, the irreversible process is deduced to consist of some oxidation reactions such as a migration of nonbridging oxide ions to and subsequent evaporation of oxygen gas at the anode side. Decay of the second harmonic intensity for 30NaO1/2⋅70TeO2 glass as well as 30ZnO⋅70TeO2 glass has also been examined at room temperature. Whereas the 30ZnO⋅70TeO2 glass does not show a decay, the second harmonic intensity of the 30NaO1/2⋅70TeO2 glass decays rapidly with an average relaxation time of 10 h. This relaxation behavior is explainable in terms of the difference in mobility between Zn2+ and Na+ ions.