Micro-Raman study of UV-written channel LiNbO₃ waveguides

Kostritskii, S., Gallo, K., Gawith, C.B.E., Smith, P.G.R., Mailis, S. and Eason, R.W. (2005) Micro-Raman study of UV-written channel LiNbO₃ waveguides. International Symposium "Micro- and Nano-scale Domain Structuring in Ferroelectrics" (ISDS’05), Ekaterinburg, Russia. 15 - 19 Nov 2005.

Abstract

A positive refractive index change associated to UV light irradiation has recently been observed in congruent LiNbO₃, (LN) crystals and in annealed proton-exchanged (APE) and reverse proton exchanged (RPE) LN layers. Although the physical mechanisms underlying such phenomenon are still not completely clear, such an effect has already been exploited to fabricate channel waveguides at visible and infrared wavelengths [1,2];

In order to elucidate the structural changes associated to UV irradiation in congruent LN, APE:LN and RPE:LN, we carried out systematic micro-Raman analyses on UV-irradiated channels (λ = 244 nm) written under various conditions (UV powers = 40-80 mW, writing speeds = 10-1000 mm/min). In the micro-Raman experiments, light from a He-Ne laser was focused by a microscopic objective x100 on the crystal. A back-scattering scheme was used to extract the light scattered from an extremely small region (width ~ 1.6 µm, depth ~ 3.5 µm) on the surface.

The spectra of the UV-irradiated channels show: (1) Numerous extra lines for some intrinsic damaged areas of the channels; (2) significant decreases of Raman intensities and line broadenings for all the bands related to the LN phonon spectrum.

In particular, the Raman spectra for channels exposed to higher UV powers (70 and 80 mW) exhibit typical features of amorphous LiNbO3 (reported previously for Kr⁺ and He⁺ irradiated LN). New extra lines specific tor the LiNb₃O₈ crystalline phase are apparent. They are reminiscent of the extra Raman lines observed in congruent LN crystals after thermally-induced decomposition [3]. This points out to the formation of micrograins of LiNb3O8 in the UV-irradiated areas due to lithium out-diffusion or/and inter-diffusion caused by a local sharp increase of the temperature.

Current work is under way to estimate the electro-optical coefficients and the spontaneous polarization from the data on the phonon frequencies and Raman scattering efficiencies [4].

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Contributors

Author: C.B.E. Gawith

Author: P.G.R. Smith

Author: S. Mailis

Author: R.W. Eason

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