Investigation of optical waveguides in lithium niobate produced by etching during the in-diffusion of titanium

摘要

A previous investigation, discovered a new technique to surface machine lithium niobate crystals, named as `Etching During the Indiffusion of Titanium' (EDIT). In this technique, an unprocessed `top' lithium niobate wafer, placed on top of a `bottom' Ti patterned lithium niobate wafer and annealed in a wet oxygen atmosphere, is etched corresponding to the Ti pattern on the `bottom' wafer. The resulting etched surfaces were found to be very smooth in nature. Furthermore, the bulk material properties are maintained under the etched region, with no trace of impurities. However, unexpectedly, it was observed that trenches fabricated by this technique appeared to support optical waveguiding. This thesis investigates the optical waveguiding effect observed under EDIT etched trenches. It is demonstrated that the region under an EDIT etched trench is indeed an optical waveguide and supports optical modes. The mode behaviour depends on the trench depth and polarisation of the light input. Evidence suggests that the optical waveguiding is due to a stress induced refractive index change, under an EDIT etched trench. Single mode waveguides at 633 nm wavelength are demonstrated on x, y and z-cut congruent lithium niobate and z-cut 5 mol% Mg-doped congruent lithium niobate. It is shown that in the EDIT technique, when the `bottom' wafer is maintained constant as x-cut congruent lithium niobate, a consistent etch depth is obtained irrespective of the crystal orientation of the lithium niobate `top' wafer. However, in the case of stoichiometric lithium niobate a significant reduction in the etch depth is observed, indicating that vacant lithium sites in the crystal lattice are of significance during the EDIT process. Evidence suggests that the observed waveguiding under EDIT etched trenches occurs due to a combination of a topography change and surface tension reshaping. This is evidenced by the observation of optical waveguiding under trenches produced using hydrofluoric acid etching and reshaping such trenches, by annealing them in the same environmental conditions of the EDIT process. When a lithium niobate crystal is annealed, the surface reshapes and smoothens to minimise surface energy. When cooled to room temperature, the smoothened surface is not in equilibrium with the underlying substrate. To obtain an energetically favourable condition there is induced stress between the reformed surface and the underlying substrate, thereby, causing an increase in the refractive index via the photo-elastic effect. The EDIT technique is used to fabricate single mode optical waveguides at 1550 nm wavelength in z-cut 5 mol% Mg-doped congruent lithium niobate. Furthermore, the electric field poling technique is applied to EDIT processed crystals to fabricate periodically domain inverted structures on EDIT etched trenches, which offers great potential for applications in nonlinear integrated optics. In summary, the region under the EDIT etched trench is an optical waveguide. It is proposed that the physical mechanism behind the formation of EDIT waveguides is a combination of change in surface topography and surface tension reshaping. The resulting waveguides are impurity free and maintain the bulk material properties. Such waveguides show promise for the development of nonlinear integrated optical devices.