C. Y. J. Ying

Poling-inhibited ridge waveguides in lithium niobate crystals

Ultraviolet laser irradiation of a lithium niobate +z polar surface enables the production of ridge waveguides. Ultraviolet laser induced inhibition of poling is used to define an inverted domain pattern which transforms into a ridge structure by differential etching in hydrofluoric acid. The laser irradiation step also induces a refractive index change that provides the vertical confinement within the ridge structure. Furthermore, it was observed that poling-inhibition results in a significant enhancement of the refractive index contrast between the bulk crystal and the ultraviolet irradiated tracks.

Pyroelectric field assisted ion migration induced by ultraviolet laser irradiation and its impact on ferroelectric domain inversion in lithium niobate crystals

The impact of UV laser irradiation on the distribution of lithium ions in ferroelectric lithium niobate single crystals has been numerically modelled. Strongly absorbed UV radiation at wavelengths of 244–305 nm produces steep temperature gradients which cause lithium ions to migrate and result in a local variation of the lithium concentration. In addition to the diffusion, here the pyroelectric effect is also taken into account which predicts a complex distribution of lithium concentration along the c-axis of the crystal: two separated lithium deficient regions on the surface and in depth. The modelling on the local lithium concentration and the subsequent variation of the coercive field are used to explain experimental results on the domain inversion of such UV treated lithium niobate crystals.

Enhanced electro-optic response in domain-engineered LiNbO3 channel waveguides

Substantial enhancement (36.7%) of the intrinsic electro-optic coefficient (r_33) has been observed in lithium niobate channel waveguides, which are made to overlap with a pole-inhibited ferroelectric domain. The waveguide and the overlapping ferroelectric domain are both produced by a single UV irradiation process and are thus self-aligning. The enhancement of the electro-optic coefficient effect is attributed to strain, which is associated with the ferroelectric domain boundaries that contain the channel waveguide.

Ferroelectric domain building blocks for photonic and nonlinear optical microstructures in LiNbO3

The ability to manipulate the size and depth of poling inhibited domains, which are produced by UV laser irradiation of the +z face of lithium niobate crystals followed by electric field poling, is demonstrated. It is shown that complex domain structures, much wider than the irradiating laser spot, can be obtained by partially overlapping the subsequent UV laser irradiated tracks. The result of this stitching process is one uniform domain without any remaining trace of its constituent components thus increasing dramatically the utility of this method for the fabrication of surface microstructures as well as periodic and aperiodic domain lattices for nonlinear optical and surface acoustic wave applications. Finally, the impact of multi exposure on the domain characteristics is also investigated indicating that some control over the domain depth can be attained.

Optical characterization of LiNbO 3 whispering gallery mode micro-resonators fabricated by surface tension reshaping

Lithium niobate (LN) is an attractive material for high-Q whispering gallery mode (WGM) resonators due to its wide optical transparency window, high electro-optic coefficient and nonlinearity. Recently, LN structures suitable for WGM micro-resonators have been fabricated using surface tension reshaping of a previously micro-structured substrates, producing ultra-smooth surfaces while also maintaining the useful crystalline properties of the original material [1]. The method is based on the preferential melting of a surface layer [2] at temperatures close to the melting point for the bulk material. Upon cooling, the melted surface layer re-crystallizes, seeded by the bulk crystal that remains solid during the process, and is reshaped by the surface tension to form ultra-smooth single crystal superstructures. Fig. 1 shows some surface tension reshaped structures which have been obtained by varying the initial microstructure and the thermal treatment conditions, (a): 5µm prolate spheroid, (b): 3µm capsule, (c-e): 7, 30, 80µm diameter pillars. Such structures are suitable for supporting WGMs as the smooth side surface should lead to low scattering loss. Their obtainable small dimensions leads to a small number of supported WGMs and increases the free spectral range (FSR). Both are beneficial for the fabrication of spectral filters and lasers.

UV laser-induced poling inhibited domain building blocks for photonic and nonlinear optical microstructures

Summary form only given. UV laser induced poling inhibition (PI) is a method for domain engineering in ferroelectric lithium niobate crystals. This method is capable of producing localised ferroelectric domains whose shape and orientation is defined by a UV laser pre-irradiated pattern. Poling inhibition is observed at the locations of the +z face of the crystal that have been exposed to UV laser radiation (244 nm-305 nm) prior to uniform domain inversion by electric field poling at room temperature [1].Poling inhibition is attributed to lithium migration under the influence of steep temperature gradients which form due to the strong absorption of UV radiation within a small crystal volume thus, raising the temperature close to the melting point which is ~ 1250°C. Lithium ions migrate away from the hot crystal volume resulting in local variation of lithium concentration, which produces a corresponding local variation of the coercive field [2]. The coercive field variation, which reflects the lithium concentration distribution is responsible for the spatially selective domain inversion when applying a uniform electric field. UV laser induced PI has been demonstrated in both congruent and MgO doped crystals. PI normally requires tight focussing of the UV laser. The width of the resulting PI domains corresponds to the temperature distribution which is induced by the irradiating beam. Consequently, only small (a few μm) size domain widths have been achieved so far [3]. Here we demonstrate that partial overlap of UV-irradiated tracks can produce larger PI domains, which consist of merged individual PI domains as defined by individual UV laser tracks, thus removing the restriction, which was imposed by the limited width of the UV irradiated beam. We show that individual pole inhibited domain tracks can be stitched together to produce larger and more complex structures with precise specifications. The result of such a domain-stitching process is demonstrated in the SEM microscopy images of figure 1 where a set of three ring and three disc structures is shown. These structures are composed of concentric ring shaped domains, which are combined to produce rings of varying width and discs of varying diameter. The composite domain structure has been made visible by wet etching in HF acid solution Measurement of the PI domain depth as a function of the overlapping factor between adjacent UV irradiated track revealed that overlapped irradiation does not influence the PI depth. The depth of the composite PI domains was measured from the SEM images of wedge polished and chemically etched samples.The ability to combine individual PI domains to construct larger and more complex structures increases dramatically the utility of this method for the fabrication of arbitrary surface microstructures as well as periodic and aperiodic nonlinear optical and acoustic lattices.

Electro-optic coefficient enhancement in poled LiNbO 3 waveguides

Lithium niobate crystals (LN) show a significant electro-optic (EO) response which contributes to the fabrication of low-voltage operation, high speed integrated optical modulators routinely used in optical telecommunication and integrated optics [1]. A UV laser direct writing method for the fabrication of optical channel waveguides has been proposed and characterized recently [2–4]. Here we report on the enhancement of the electro-optic response of these UV laser-written LN waveguides as a result of a post-poling process. More specifically we have observed a 26% increase of the r33 coefficient compared to the bulk in LN waveguides, fabricated by direct UV writing, that have been subjected to poling inhibition [5]. Poling inhibition produces inverted ferroelectric domains which are only a few microns deep. These domains are formed exactly in the same place as the UV written tracks which are responsible for the waveguide formation, and they overlap significantly with the propagating waveguide mode as is illustrated schematically in Fig. 1. Due to the polarization-selective transmission in the UV-written waveguides only the r33 coefficient could be investigated.