Fabrication and characterisation of thin film optical waveguides by pulsed laser deposition
This thesis describes 3 years research on the fabrication and characterisation of thin film optical waveguides using the pulsed laser deposition technique. Optical waveguides have many advantages over their bulk counterparts due to the high intensity-length product that can be achieved by tight beam confinement in a waveguide mode. These waveguides can also form the key elements necessary for assembling a complete thin-film optical integrated circuit. The thin films were deposited in a vacuum chamber that was designed and built in the first 6 months of the research program, at the Department of Physics, Southampton University. The chamber was equipped with several novel designs such as a target manipulation mechanism that enabled many films to be grown from a single target, and a CO2 laser for substrate heating which proved intrinsically cleaner and more efficient than conventional heating methods. The crystal structure of the films was investigated using x-ray diffraction techniques and the chemical composition was determined by Rutherford backscattering spectroscopy and energy-dispersive x-ray analysis. Crystalline and stoichiometric KNbO3 thin film optical waveguides were fabricated onto heated MgO substrates, for the first time, using pulsed laser deposition. The chemical composition and deposition rate of the films were measured as a function of the oxygen partial pressure, excimer laser fluence, substrate temperature and target-substrate distance. For this material only, it was necessary to use a potassium enriched ceramic target to compensate for the potassium deficiency in the films. Amorphous thin films of the chalcogenide glass, Ga-La-S were deposited onto CaF2 and glass substrates at room temperature. The refractive indices of the films were measured using the dark-mode prism coupling technique and the values were dependent on the excimer laser fluence. The photosensitive properties of the films were investigated and shown to induce a permanent refractive index change as large as -1%. On the basis of these results, grating structures were fabricated on the surface of the films using both laser and e-beam addressing. Finally, a crystalline layer of neodymium doped Gd3Ga5O12 was deposited onto a heated Y3Al5012, substrate. The waveguide lased at ~1.06µm when pumped with light from a Ti:sapphire laser at 808nm. This is the first ever lasing result from a waveguide structure that has been fabricated by pulsed laser deposition.