Pulsed laser deposition for the fabrication of all-crystalline resonant reflectors

Abstract

The focus of this project was the development of all-crystalline dielectric resonant reflectors. The final goal to demonstrate the fabricability of single crystal high-reflectivity mirrors, which would potentially remove the limitations of the current state-of-the-art in terms of laser-induced damage threshold (LIDT). Two types of reflectors have been investigated: grating waveguide structures (GWS) and quarter-wave Bragg stacks. The novelty of the work presented in this thesis is the use of epitaxially grown crystalline films to improve the mechanical and thermal properties of the reflectors.
The deposition method of choice for this work was Pulsed Laser Deposition (PLD), owing to its versality and proven efficiency for the epitaxial growth of crystalline films. The project focussed on the optimisation of the growth of sesquioxides on sapphire (0001), in particular Sc2O3, Lu2O3 and Y2O3., and their characterisation in terms of crystallinity, scattering, LIDT and dispersion. Ion probe measurements demonstrated how the chamber background pressure and the target ablation fluence affected the energy of the plasma constituents, and hence the layer growth mechanism. Studying the effect of the pulse repetition rate revealed that with a decreasing repetition rate, the lattice constant of the Sc2O3 film decreased and the film became more textured.
The fabrication of the GWSs involved the epitaxial growth of Sc2O3 waveguides on etched sapphire substrates. Atomic force microscopy (AFM) measurements showed that the top surface of the waveguide followed the shape of structured substrate, due the high directivity of the PLD plasma. The final GWSs exhibited more than 85% reflectivity at 1030 nm and 70% reflectivity at 1970 nm, for both TE and TM polarisations. The main conclusion was that the uniformity of the waveguide thickness was critical to ensure a high-quality resonance. Bragg mirrors were also grown via PLD, with alternating quarter-wave layers of Sc2O3 and Y2O3. The reflectivity of the 101-layer sample reached more than 77% at the resonant wavelength, which was tuneable by scanning the surface of the sample. These results validate the possibility of epitaxially growing all-crystalline sesquioxide reflectors for photonics applications.

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ORCID for Jacob Mackenzie: orcid.org/0000-0002-3355-6051

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