Abstract

The mechanism by which a semipermanent effective χ⁽²⁾ is encoded in centrosymmetric glass materials has remained a mystery for nearly a decade. It has been determined that the observed high conversion efficiencies result from a permanent, quasi-phase matching χ⁽²⁾ grating established by a χ⁽³⁾ mediated interaction with a frozen-in alternating dc field. We conducted thermal erasure studies of χ⁽²⁾ gratings optically encoded in bulk samples of a high-barium low-alkali borosilicate glass (SK5, Schott Corp.) to determine the energy of trap sites responsible for the large fields (106 V/m). Q-switched mode-locked pulses at 1.064 μm and 532 nm were used to create effective χ⁽²⁾ values of ~10^–16 V/m in 1 cm thick samples. The thermal erasure rate of the χ⁽²⁾ gratings, heated at 510,530,540, and 556 K, were dependent upon the initial grating amplitude indicative of a non-ohmic relaxation. In addition, the thermal decay of the gratings proved to be nonexponential and led to a value of a 1.9 ± 0.2 eV activation energy for the process at short times. Since only trace amounts of alkali metal ions are in the glass, the decay of the internal fields is likely to be due to electrons and or holes rather than ion migration. The nonexponential erasure behavior is addressed in terms of a dispersive transport mechanism for the thermally activated carriers.

© 1992 Optical Society of America