Sodium borosilicate [1,2] glasses (NBS) with constant Na2O:B2O3 ratio of 0.2 and various SiO2 contents were thermally poled under argon and the SHG signal was measured for different poling parameters. Significant structural changes were observed after poling by infrared reflectance and Raman spectroscopy. Besides qualitative and quantitative evaluation of such structural changes, micro-Raman scattering and micro-infrared reflectance spectroscopy allowed the assessment of the spatial boundaries of the poling-induced rearrangements on the surface, as well as an analysis as a function of depth of the sample.
It was found that under poling Na ions migrate from the anode to the cathode side. This displacement of cations from the anode region extends to a depth of a couple of ?m and is compensated by structural rearrangements of the glassy network, which lead eventually to the release of oxygen from the anode side. Both effects are facilitated by the conversion of borate tetrahedra into neutral trigonal borate units. The reverse transformation occurs at the cathode side, where borate tetrahedra are formed together with electron centers, also known as oxygen vacancies. The presence of such vacancies, B and Si bonded electron centers (EC), was revealed by optical and EPR spectroscopy.
The oxygen depletion layer which is generated on the anode side establishes the space charge that results in the NLO response of the glass. Other mechanisms which result in the formation of an oxygen depletion layer include the generation of bridging from non-bridging oxygen atoms and the conversion of corner-sharing into edge-sharing polyhedra [3]:
[1] ''Thermal history of a low alkali borosilicate glass probed by infrared and Raman spectroscopy'', D. Moncke, D. Ehrt, C.P.E. Varsamis, E.I. Kamitsos and A. Kalampounias, Glass Tech.: Eur. J. Glass Sci. Technol. A 47, 133 (2006).
[2] ''Thermal poling induced structural changes in sodium borosilicate glasses'', D. Moncke, M. Dussauze, E.I. Kamitsos, C.P.Å. Varsamis and D. Ehrt, Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B 50, 229 (2009).
[3] ''Structural rearrangements and second order optical properties in the space charge layer of thermally poled sodium- niobium borophosphate glasses'', M. Dussauze, E.I. Kamitsos, E. Fargin and V. Rodriguez, J. Phys. Chem. C 111, 14560 (2007).
Spatial Distribution of Structural Changes in Poled Sodium Borosilicate Glasses
Spectral mapping by micro-infrared reflectance spectroscopy reveals the localization of structural rearrangements in a thermally poled sodium borosilicate glass (NBS). The spectra were recorded along a line which crosses the sample and includes a section that was in contact with the anode electrode (see label). For comparison, the figure includes also a spectrum measured outside the poled area designated by the grey filling (1) and the spectrum measured right in the center of the poled area (d=5 mm) which is marked in red (2).
Values of the measured reflectance in the spectra shown in (a) are plotted as a function of distance for bands at ca. 1100, 900 and 1375 cm-1 (b). The section of the sample probed under the anodic electrode is confined within the vertical broken lines.
The intensity of the main absorption band centered around 1100 cm-1 decreases significantly in the poled area. Several different structural entities including the silicate network and the mixed B-O-Si bonds contribute to this band. Critical to the discussion of poling-induced structural changes are also the weaker bands at 900 cm-1 and 1375 cm-1 which arise from tetrahedral borate units, [BØ4]-, and neutral trigonal borate units, BØ3, respectively. The intensity variation of the 900 and 1375 cm-1 bands reveals the transformation of [BØ4]- into BØ3 groups as Na+ ions migrate from the anode to the cathode side of the glass.
Depth Profiling in Poled Sodium Borosilicate Glasses by Micro-Raman Spectroscopy
Micro-Raman spectra of poled NBS glass sample reveal how deep the structural rearrangements penetrate into the bulk sample after thermal poling. The spectra are measured along the cross section of a cut sample from the anodic surface to various distances under the anode. The poling-affected layer which is responsible for the NLO response of glass is found to have a thickness of about 2 ìm.
