Pulsed Laser Deposition (PLD) is a versatile technique for growing thin films and can be applied to a very wide range of materials.
A pulsed laser (usually ultra-violet (UV) wavelength) is used to ablate a target, and a plasma plume is formed by the ejected material, as shown in Fig. 1; the plasma plume then expands away from the target surface and interacts with the chamber atmosphere until it reaches the substrate, where it is deposited as a thin film.
The substrate can be heated, as shown in Fig. 2, to assist with nucleation and allow crystal growth, and a background gas can be used to help control the film composition. The PLD process covers many different areas of physics and chemistry, and an increasing understanding of what drives the mechanisms behind successful depositions is critical for the continuous improvement of future films and devices. Several deposition parameters must be carefully optimised before high quality films can be produced.
A major advantage of the technique is the occurrence of stoichiometric transfer. This allows complex multi-component materials to be grown and has the added benefit that targets can be adapted easily from small pieces of bulk material.
Our work is mainly focussed on the production of high quality optical films for novel laser devices. The thin-film format is ideal for planar waveguide and thin-disk structures, and devices using both of these geometries are currently being pursued.
The group operates a state-of-the-art triple-target deposition chamber as shown in Fig. 3(a) and 3(b), and has several lasers and wavelengths available to use on the three different targets (three 266 nm Nd:YAG lasers and an excimer laser capable of operation at 248 nm and 193 nm). The triple target approach allows for some very complex designer structures to be grown, and challenges other film growth techniques to rival it. Targets can be used sequentially to build up discrete multilayer structures such as superlattices or cladding-pumped designs analogous to cladding pumped fibres. Alternatively, targets can be used concurrently to make custom crystal compositions, hybrid intermediate compositions, or even to grade composition continuously with depth to make custom refractive index or dopant concentration profiles as shown in Fig. 4. The control and automation of this system is critical for its potential to be fully realised.
PLD offers some unique material deposition capabilities, in particular complex composition film growth and crystal film growth. Some of the group's work time is spent supplying special films to other groups within the University and externally.