Domain engineering techniques and devices in lithium niobate

This thesis presents the results from investigations directed at novel approaches to domain engineering single-crystal congruent lithium niobate at the micron/sub-micron scale for practical device applications. Experimental etch-rate measurements from a parametric study of etch-rates and etch-quality of single-crystal lithium niobate z-faces, as a function of specific ratios for mixtures of HF/HNO3, to ascertain whether the widely-employed 1:2 mixture was in fact optimum for achieving the largest differential etch-rates between lithium niobate z-faces, revealed that pure HF produced an etch-rate that is a factor of two higher than that for the more frequently used 1:2 mixture. The observed etch-quality as compared to the 1:2 ratio was also improved for either pure HF or HF/HNO3 in a 1:4 ratio. A discussion of the etch-chemistry involved, and an explanation of the observed difference in etch-rates between the +z and -z faces has been proposed. The experimental results are also suggestive of a second differential etch-rate between virgin and newly poled z-faces. The observed variation in the differential etch-rate as a function of time-delay following poling, was suggestive of small atomic displacements following poling, and was quantified by the evidenced shifts in six major Raman spectral peaks. The noticeable modifications in the etch-behaviour of undoped congruent z-cut lithium niobate by pre-illumination with sub-picosecond UV-laser radiation of 248 nm wavelength at energy fluences below the ablation threshold, demonstrates the potential applicability of this technique for µm-scale surface structuring of lithium niobate. An innovative technique for surface domain-inversion, based on the conventional e-field poling, but involving an intentional over-poling step, was employed to fabricate 1D and 2D periodic structures with good domain uniformity. Domain periods as short as ~1µm have been achieved, and the technique shows full compatibility with standard waveguide fabrication techniques in lithium niobate. Quasi-phase matched harmonic generation at the fundamental wavelength of 1.064 µm, by means of the first-order (G10) reciprocal lattice vector, from a surface hexagonally poled planar annealed proton exchanged waveguide, with domain period of 6.7 µm, was demonstrated. First-order quasi-phase matched blue light generation with reasonable efficiencies at 413.17 nm, with domain periods of 2.47 µm from a surface poled Ti-indiffused channel waveguide was also demonstrated. Finally a novel route, sequentially employing techniques such as photolithographic patterning, e-field poling, direct-bonding and domain-sensitive differential wet etching for the fabrication of free-standing piezoelectric micro-cantilevers in single-crystal lithium niobate, with MEMS/MOEMS end-applications, was demonstrated

Determination of refractive indices from the mode profiles of UV-written channel waveguides in LiNbO₃-crystals for optimization of writing conditions

We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in LiNbO3 for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of ~ 0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide