KY(WO 4 ) 2 and other materials of the double tungstate crystal family have been used for decades in active optical applications because of their relatively high refractive index (n≈2-2.04 @ 1550 nm), high transparency window (0.3-5 μm), excellent gain characteristics when doped with rare-earth ions and reasonably high thermal conductivity (~3.3 Wm -1 K -1 ). Low-contrast (Δn<0.02) on-chip amplifiers and lasers in this material with good performance have been shown in recent years. Higher refractive index contrast can further improve this performance, and allow easier integration with other integrated optics platforms due to their smaller footprint. Because double tungstate materials cannot be directly grown on many prospected substrates, other methods to fabricate optical waveguides with a thickness of 1-2 μm need to be investigated. In this work, swift heavy ion irradiation has been used to produce a planar waveguide by introducing a buried layer of lower refractive index in the KY(WO 4 ) 2 at a depth of ~2.5 μm. After the irradiation, an annealing step was introduced to reduce the scattering losses. The refractive index profile, effective refractive indices and absorption spectra of the planar waveguides have been investigated for several annealing temperatures, and end-facet free-space coupling of 1550 nm has been used to measure the losses. For a fluence of 3•10 14 ion/cm 2 of 9 MeV C ions, propagation losses <1.5 dB/cm have been demonstrated at 1550 nm after an annealing step at 350°C.

Rare-earth ion doped KY(WO 4) 2 is a well-known active laser crystal, due to its excellent gain characteristics and its relatively high nonlinear refractive index. As these properties are of great benefit to applications in integrated photonics, a study has been done into the fabrication of high refractive index contrast slab waveguides in KY(WO 4) 2 as a first step towards the fabrication of channel waveguides. When properly choosing the fluence and annealing parameters, ion irradiation with 12 MeV carbon ions produces a step-like damage profile. Confocal Raman microscopy, X-ray diffraction and transmission electron microscopy are used in this work to study the structural damage induced by ion irradiation. The characterization indicates damage to the crystal structure due to the ion irradiation that increases as a function of both depth and ion fluence till the threshold for amorphization is achieved. Successive annealing steps of the irradiated crystals at different temperatures show partial repair of the crystalline structure when the irradiation did not fully amorphize the material. When the threshold of amorphization was reached, annealing further increases the damage induced by the irradiation. By tuning the irradiation fluence, a high-refractive index contrast slab waveguide in KY(WO 4) 2 produced by ion irradiation was demonstrated.

A method for determining the spatially resolved acoustic field inside a water-filled microchannel is presented. The acoustic field, both amplitude and phase, is determined by measuring the change of the index of refraction of the water due to local pressure using stroboscopic illumination. Pressure distributions are measured for the fundamental pressure resonance in the water and two higher harmonic modes. By combining measurement at a range of excitation frequencies, a frequency map of modes is made, from which the spectral line width and Q-factor of individual resonances can be obtained.

KY(WO 4 ) 2 is an attractive material for integrated photonics due to its high refractive index and excellent non-linear and gain characteristics. High refractive index contrast structures increase light-matter interaction, reducing the threshold for lasing and non-linear effects. Furthermore, high refractive index contrast permits dispersion engineering for non-linear optics.In this work, we present a novel fabrication method to realize pedestal microdisk resonators in crystalline KY(WO 4 ) 2 material. The fabrication process includes swift heavy ion irradiation of the KY(WO 4 ) 2 with 9 MeV carbon ions and sufficient fluence (>2.7·10 14 ion/cm 2 ) to create a buried amorphous layer. After annealing at 350°C, microdisks are defined by means of focused ion beam milling. A wet etching step in hydrochloric acid selectively etches the amorphized barrier producing a pedestal structure. The roughness of the bottom surface of the disk is characterized by atomic force microscopy.

Rare-earth ion doped potassium yttrium double tungstate, RE:KY(WO 4 ) 2 , is a promising candidate for small, power-efficient, on-chip lasers and amplifiers. There are two major bottlenecks that complicate the realization of such devices. Firstly, the anisotropic thermal expansion coefficient of KY(WO 4 ) 2 makes it challenging to integrate the crystal on glass substrates. Secondly, the crystal layer has to be, for example, <1 µm to obtain single mode, high refractive index contrast waveguides operating at 1550 nm. In this work, different adhesives and bonding techniques in combination with several types of glass substrates are investigated. An optimal bonding process will enable further processing towards the manufacturing of integrated active optical KY(WO 4 ) 2 devices.