Bulk and near-surface second-order nonlinearities generated in a BK7 soft glass by thermal poling

Bulk second-order nonlinearity was generated in BK7 glass at a higher temperature and with a longer poling time than near-surface second-order nonlinearity. The temporal decay of the bulk second-order nonlinearity was slower than that of the near-surface second-order nonlinearity. The thickness of the near-surface nonlinear layer increased with poling time. Poled BK7 glass was also measured by x-ray photoelectron spectroscopy. Depletion of Na at the anodic surface and its accumulation at the cathodic surface was observed. At the cathodic surface, a higher-energy peak near O (1s) appeared, which shows peroxy-radical defects. At the anodic surface, a lower-energy peak near Si (2p) appeared, which may be attributed to E′ centers or to two-coordinated Si defects. The mechanisms of generation of these defects and of the second-order nonlinearities are discussed.