Novel crystal growth with multi-beam laser deposition technique
Crystals are excellent materials for laser hosts, due to their good mechanical, optical and thermal properties. Doping with 1% of a lasing ion, leads to familiar lasing materials such as Nd:YAG (yttrium aluminium garnet), or Ti:sapphire, and these can be used for bulk, planar or channel waveguide devices. But what happens if you want something more exotic? A mixed crystal for example that is a combination of YAG and YGG or Al2 O3 (sapphire) and Y2 O3? This is harder to find, if indeed it exists at all. You may need to grow it for yourself.
Pulsed Laser Deposition (PLD) is a relatively simple technique that lets you grow thin films (1-100μm) of crystalline materials via a laser plasma deposition process. A target of the material from which you want to grow the film is ablated inside a vacuum chamber, and the resultant plasma is incident on a heated substrate. If all goes according to plan, a thin film forms with, ideally, the same composition as the original target material. This can then be used for the waveguide or lasing experiments you want to perform. But what happens if you cannot find the correct target material to start with? To grow single crystal YAG (Y3Al5O12) for example, you can also use two separate targets of Al2O3 and Y2O3: all you have to do is generate the two plasmas in the correct ratio, and the resultant mixing can then allow growth of the final material you want.
Extending this principle to three targets however is an even better idea. In just the same way as projectors or displays use red-green-blue (RGB) sources to produce any colour that is needed from the full palette available, we can use three different targets, producing three different plasma sources with any strength, time-sequence, duration, position, etc to grow essentially any structure we wish. This is the theme behind our recently awarded 3 year EPSRC grant called MULTIPLE. Three targets and three lasers are used to grow a range of novel graded, layered, combinatorial films, whose properties can be tuned via the use of these three sources.
One property that is highly desirable to tune is, of course, the refractive index. If the growth allows the gradual mixing of two different plasma sources, then we can grow films with designer index profiles, a pre-requisite for optical waveguide applications, and familiar from the basic concept of core and cladding indices in optical fibres. With MULTIPLE PLD plasmas, we can design, grow, test and then use thin film structures that can have specified refractive index and doping profiles, and the materials grown can be excellent single crystals, ideally of optical quality equivalent to bulk crystal samples.
Shown in the figure below is one of our current results on multilayer growth. The multi-plume system has proved to be every bit as versatile as we had hoped, and growth of such sophisticated layered and graded structures is now being achieved. The example shows a symmetrical 11-layer structure; each layer is single crystal, and has the ratio of x% GGG (gadolinium gallium garnet), and y% YSGG (yttrium scandium gallium garnet), both of which are excellent laser hosts. The resultant crystal film is water-clear, proving that such growth is feasible and of very high optical quality. We are now ready to grow the full range of crystal composites discussed in our MULTIPLE grant.
We are now ready to explore the full range of possibilities from multi-plume geometries. The ability to tailor the local environment of the active ions in graded, Gaussian, or other arbitrary profiles offers unprecedented opportunities for new operating regimes in crystalline based laser sources. We know of no other technique that is this useful, flexible, fast and of extremely high precision.
For more details of this and other PLD based projects, please go to the PLD website at: www.orc.soton.ac.uk/pld.html where there are details of a funded PhD project to start as soon as possible.
Posted by Marketing Officer, on 14 January 2008