UV laser‐induced poling inhibition in proton exchanged LiNbO3 crystals

distortion of the ideal domain shape and size thus making fabrication of fine domains challenging [3]. Poling inhibition (PI) induced by UV laser irradiation [6, 7] in congruent (CLN) and MgO-doped LN can overcome the limitation of EFP by reducing the aspect ratio of the inverted domains allowing for the fabrication of finer domain structures. This method is capable of producing domains with a moderate depth (a few microns below the surface) that can be controlled, to some extent, by the UV laser exposure [8]. UV laser irradiation of the +z polar surface of the crystal results in a local increase of the coercive field by causing migration of lithium ions due to (1) diffusion in the temperature gradients [9] and (2) drift in the pyro-electric field[10]. Consequently, a uniform electric field applied along the z direction will invert the inherent polarisation of the crystal everywhere apart from the volume that has been affected by the UV laser irradiation, which remains poling inhibited. The depth of the PI domains suggests that such structures will be suitable for optical waveguide systems; therefore, it is important to investigate whether poling inhibition is applicable with established waveguide fabrication methods. In this paper, we are investigating the impact of the UVinduced PI procedure on proton exchanged (PE) waveguides, which are fabricated in z-cut LN substrates. Proton exchange is a well-established method for the fabrication of low-loss waveguides, which are resistive to photorefractive damage [11]. PE waveguides are formed by the replacement of lithium ions with protons from a benzoic acid melt (up to a depth of ∼1 μm), which increases the extraordinary refractive index of the crystal [12, 13]. The main question that this work attempts to address is whether the UV laser irradiation will produce sufficient coercive field contrast for selective poling, as in the case of poling inhibition of undoped crystals [6, 7]. Here, we present Abstract The applicability of the UV laser-induced poling inhibition method for ferroelectric domain engineering in proton exchanged lithium niobate planar waveguides is investigated. Our results indicate that intense UV irradiation of proton exchanged lithium niobate samples can, indeed, produce poling inhibited domains in this material under certain irradiation conditions. However, there is strong indication that the temperature gradient that is formed during UV irradiation modifies the local proton concentration leading to changes in the refractive index profile of the original planar waveguide.