High refractive index contrast optical waveguides in swift heavy ion irradiated KY(WO4)2

This thesis explores several fabrication technologies to integrate KY(WO4)2 crystalline material with other host materials, as well as fabricate channel waveguide structures with a high refractive index contrast. This high contrast increases the mode confinement and reduces the threshold for effects such as stimulated Raman scattering and lasing from rare-earth ions. Swift heavy carbon ion irradiation of the crystal is used to provide vertical confinement of the light, and to induce a buried damage layer in the crystal susceptible to chemical wet-etching. The damage layer has a lower refractive index than the undamaged crystal, leading to a planar optical waveguide at the surface of the KY(WO4)2 crystal. The structural and optical properties of this layer have been studied in detail, showing an increased stress and deformation of the crystalline structure, but also a close to step-like refractive index profile as function of depth in the crystal, enabling confinement of the light to the high refractive index top layer of the crystal. Additionally, exfoliation of this top layer is demonstrated by bonding the irradiated KY(WO4)2 crystal to glass, and subsequent etching of the damage layer.

The main result of this work is the demonstration of channel waveguides in KY(WO4)2 with a high refractive index contrast, both directly etched into the planar waveguide and by strip-loading the KY(WO4)2 with TiO2. Optical propagation losses as low as 3.2 dB/cm at 980 nm can be achieved in the channel waveguides directly etched into the KY(WO4)2.

By demonstrating highly confined bend structures in KY(WO4)2 as well as the heterogeneous integration of KY(WO4)2 on glass, the work in this thesis contributes towards the realization of active integrated optical devices in KY(WO4)2 and the integration of these devices with other photonic integrated circuitry.