Pulsed laser deposition of doped sesquioxide films for planar waveguide lasers

Parsonage, Tina (2016) Pulsed laser deposition of doped sesquioxide films for planar waveguide lasers. University of Southampton, Faculty of Physical Science and Engineering, Doctoral Thesis, 231pp.

Record type: Thesis (Doctoral)

Abstract

The sesquioxides Y₂O₃, Lu₂O₃ and Sc₂O₃ have been identified as promising laser host materials due to their excellent thermo-mechanical properties and ability to be doped with rare-earth ions. However, crystals of these materials are problematic to grow due to their high melting points of ~2500°C, which means standard methods of crystal growth from a melt need to be modified to deal with such high temperatures and become more expensive. Pulsed laser deposition (PLD) has the capability to grow these materials, without requiring a melt of material to grow from. Sintered ceramic targets of the sesquioxides, often doped with rare-earth ions, are ablated with a UV laser pulse and material is ejected. This plume of material travels through a vacuum chamber to a carefully chosen substrate where material is deposited and forms a crystalline film.

Planar waveguide lasers are structures of interest due to their low lasing thresholds in comparison to bulk material, owing to the greater overlap of the pump and laser modes, and their large aspect ratio that allows efficient heat removal from the laser crystal. This, combined with the high thermal conductivity of the sesquioxides, should allow for power scaling of these waveguide lasers with minimal thermal affects. The laser results in this thesis are for progressively higher powers, and no detrimental heating affects are witnessed.

This thesis reports on the growth of single and multilayer sesquioxide waveguides grown by PLD, including growth optimisation, sample analysis and laser experiments. The first sample of this project to be successfully lased was a single layer Tm:Y₂O₃ waveguide on a YAG substrate, which produced a maximum output power of 35 mW with 9% slope efficiency at a wavelength of 1.95 µm, and, to the best of our knowledge, was the first Tm:Y₂O₃ planar waveguide laser. Next, Yb:Y₂O₃ was the material of choice and a multilayer sample was fabricated, where the Yb:Y₂O₃ core was sandwiched between two undoped Y₂O₃ layers, again on a YAG substrate. Laser experiments with this waveguide gave a maximum output power of 1.2 W at 1030 nm, with a slope efficiency of 20%. The highest laser output power of any of the doped sesquioxide waveguides in this project is 8.5 W, achieved using a Yb:Lu₂O₃ sample.