Domain engineering techniques and devices in lithium niobate

Abstract

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.

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