Qiming Liu

Three-dimensional photoprecipitation of oriented LiNbO 3 -like crystals in silica-based glass with femtosecond laser irradiation

We demonstrate crystals (LiNbO(3)-like) that were space-selectively nucleated and grown in the bulk of silica-based glass by femtosecond laser irradiation at a high repetition rate (typ. 300 kHz). Oriented crystals with their polar axis mostly aligned with or perpendicular to the laser scanning direction have been fabricated by manipulation of the temperature gradient in adjusting the laser parameters. The mechanism for the orientation of femtosecond laser-induced crystallization is briefly discussed.

Size-controlled oriented crystallization in SiO 2 -based glasses by femtosecond laser irradiation

3D nano-crystallization has been investigated with a femtosecond laser beam in 33Li2O-33Nb2O5-34SiO2 glass by changing the pulse energy and polarization direction. Electron backscattered diffraction was used to characterize the size and the orientation of crystals in cross sections of laser tracks, while second harmonic generation was employed to investigate the nonlinear optical properties of laser lines macroscopically. We are the first, to the best of our knowledge, to reveal that crystals of nano-size were precipitated both at low pulse energy with laser polarization parallel to the laser motion direction and at high pulse energy in the perpendicular case, but they preferred to orientate perpendicularly to the written lines in the former case.

Stability of electron-beam poling in N or Ge-doped H:SiO2 films

We study the thermal stability of the second-harmonic signal in electron-beam poled silica based thin films developed for this application. Effects of N or Ge concentration as dopant, on one hand, and hydroxyl species, on the other hand, are tested. We conclude that water molecules inducing an ionic conductivity are detrimental to the e-beam poling efficiency and have to be avoided. Nevertheless, despite such precaution, the stability of the second-harmonic signal in an e-beam poled sample is worse than in a thermally poled sample.

One-step photoinscription of asymmetrically oriented fresnoite-type crystals in glass by ultrafast laser.

Oriented fresnoite-type crystals (Sr(2)TiSi(2)O(8)) were photoinduced directly in bulk silica-based glass by femtosecond laser irradiation at high repetition rate (typ. 300 kHz). Unlike related results obtained from other researchers, asymmetrical polar-axis orientation of those nonlinear crystals in transverse direction of the cross section has been demonstrated by electron backscattered diffraction and micro-second-harmonic generation (SHG). The nonlinear optical property of laser lines has been further characterized by SHG measurement. We found that the preferential directions of the polar axis were in the laser motion direction with a small dispersion in part of the heated volume. The other part of the crystallized volume shows an axis perpendicular to the writing direction. The mechanism of asymmetric orientation of femtosecond-laser-induced crystallization also is discussed.

Origin of thermally induced second harmonic generation in PbO–B 2 O 3 glasses

Optical second harmonic generation (SHG) with second-order nonlinearity χ((2)) as high as 2.1 pm/V has been achieved in water quenched PbO-B(2)O(3) glasses. No nonlinear depletion layer or microcrystals were observed in these glasses and the mechanism for nonlinearity has been explored in this Letter. Our results show that the possible mechanism for SHG in these glasses can be attributed to their low thermal conductivity that led to a large surface stress gradient, which broke the inversion symmetry of the glasses and subsequently induced the nonlinear effect. These findings suggest that low thermal conductivity induced high stress gradients to lead to large SHG.