Interactions of single-crystal lithium niobate surfaces with ultra-violet laser radiation

Abstract

The aim of this thesis is to investigate the influence of ultraviolet laser light on single crystal lithium niobate surfaces with a focus on useful functionalisation. The investigations split naturally into three areas: effects on hydrophilicity by low intensity laser irradiation, effects on ferroelectric domain inversion by focussed laser irradiation, and modelling of the heating and subsequent lithium diffusion during focussed laser irradiation.

It was seen that irradiation of ultraviolet laser light with a photon energy greater than the bad gap of lithium niobate resulted in an increase to the hydrophilicity, or wettability, of the surface. The magnitude of the change was seen to be dependent upon both the conditions of laser intensity and exposure time and upon the environment in which exposures took place with a greater change seen with greater exposure time and/or intensity and with a greater environmental humidity. Under vacuum no change to the hydrophilicity was seen. It was shown that spatial structuring of the surface wettability could be achieved at the sub-micron level.

Irradiation of focused continuous wave ultraviolet laser light was seen to have two contrasting effects upon ferroelectric domain inversion. It was first seen that illumination of focussed laser light of high enough intensity could directly invert the ferroelectric polarity in the exposed region. The effect was characterised by chemical etching in hydrofluoric acid and by piezoresponse force microscopy and was seen to be most effective on the -z face of the crystal. An explanation was proposed whereby photo-excited charges travel within a pyro-electric field, creating a space-charge field which locally inverts the spontaneous polarisation. The anisotropy in behaviour between the +z and -z faces was explained by the different mobilities of electrons and holes. It was also seen that if the crystal was electrically poled after illumination of the +z face, domain inversion was inhibited in the illuminated region. The region of inhibited domain inversion was seen to be greater than the region of direct poling and created domains with high quality surfaces. This was demonstrated to be a highly effective method for topographical surface microstructuring when followed by chemical etching.

The heating of lithium niobate by focussed continuous wave ultraviolet laser light was modelled. It was found that the temperature distributions created were highly localised to the beam spot and were independent of beam scan speeds for practically achievable speeds. The temperature dependence of the thermal diffusivity was included and seen to greatly increase the gradients of the temperature distributions at high temperatures. The diffusion of lithium ions due to the steep temperature gradients and, hence, ionic diffusivity gradient was modelled and it was found that lithium ions will diffuse from the surface, into the bulk of the crystal. This will leave a high refractive index surface layer through the dependence upon lithium concentration. The depth of the modelled lithium concentration profiles after the passage of the beam were found not to be sufficient to cause optical waveguiding.

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ORCID for Robert Eason: orcid.org/0000-0001-9704-2204

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