Pulsed laser deposition of chalcogenide glass materials for potential waveguide laser applications

Abstract

There are many applications for small scale, solid state lasers in the near infrared, where conversely there are very few such devices. A lasing device in a rare earth doped gallium-lanthanum-sulphide thin film is attractive due to emission at wavelengths in the 2 to 5 µm region where many gasses and liquids have fundamental vibrations and overtones and so are detectable. This region also covers the 3 to 5 µm atmospheric 'windows'. Some examples of such detection is presented in this thesis. 
Using Pulsed Laser Deposition, a relatively new deposition technique, we are able to grow thin films of the chalcogenide glass; gallium-lanthanum-sulphide, gallium-sodium-sulphide and variations of oxysulphides, on a variety of substrates. EXAFS measurements have shown that some of the elements in the glass structure change their bonding arrangement when grown at different energy density producing 'wrong bonds'. This points to the origin of the increased absorption and shift of the optical bandgap which is seen in the materials. It is this tail absorption which ultimately prevented the production of an actual solid state, rare earth laser device.

These amorphous semiconductors have a transmission range from the visible through to the mid infrared part of the spectrum. Chalcogenides can be photomodified. i.e. they have an ability to change refractive index when illuminated with photons whose energies lie in the optical bandgap of the material. This process can be reversible or irreversible depending on post deposition treatment and so gives them potential applications such as optical memory, holographic recording media, lithographically written waveguide structures and potentially laser mediums. For such uses a detailed knowledge of the chalcogenide materials optical parameters is needed.

A novel technique for the optical characterisation of the thin films has been developed which has is able to measure differences in refractive index to an accuracy of 8.5 x 10⁵. We are able to map refractive index changes across an entire surface and more uniquely whilst they are occurring during, and after, photomodification or heating.

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