Much of the research I have been involved with involves nano-structures. One nano meter is one thousand millionth of a meter, and loosely speaking structures up to a millionth of a meter are still classed as nano - this the regime in which I work; big nano-structures. This size scale is interesting because it is similar to that of the wavelength of visible light, and so producing many strange optical effects. The butterfly's wing and the piece of opal are great examples of how structuring on this size scale leads to strong optical effects. The butterfly's wing acts as an optical filter, only reflecting blue light, producing the vibrant colour. It is important to note that the better the ordering of the structure (shown in the insets) the stronger the optical effect will be. The opal acts in much the same way as the wing, but has a more random structure, leading to the reflection of different colours from different places. Thus to study these effect it is useful to have as close to perfect ordering - repeating of a simple shape - as possible.
There are many ways to produce structures the nano scale; however in the case of the structures I studied for my PhD the surfaces were produced using one of the fundamental properties of spheres - if you put them in a box they will fall into a neat hexagonal pattern. In reality at the small scale it is far more challenging than this, but the chemists on the project have perfected the art and can make perfect arrays of nano spheres over large areas. These arrays are interesting in themselves, however wishing for more of a challenge, gold is electroplate through the template of spheres to create a honeycomb structure. Following this, the spheres are removed so as to be left with their cast. In fact, this structure is still hard to fully understand from an optical point of view, so the structures are limited to the use of a single layer of voids on the surface. The picture shows an electron micrograph of such a gold surface.
These dishes are 600 nm in diameter (the same size as red light) and pictures have been taken for different thickness' of gold - this alters the amount of the sphere that is cast, producing structures ranging from shallow dishes to encapsulated voids.
It is found that even the simple act of placing a droplet of water on such a surface becomes a complex scientific problem. For example, altering the dish shape by changing the thickness causes a water droplet to stand proud of the surface as the structuring modifies the surface tension, as shown.
However, the research in my PhD has been primarily focused on an entity known as a Surface Plasmon Polariton
, a strange type of wave that resides on these nano-structured metal surfaces.
Papers relating to this work:
Wetting of Regularly Structured Gold Surfaces
M.E. Abdelsalam, P.N. Bartlett, T.A. Kelf, and J.J. Baumberg.
, 1753 (2005).
Thesis: Light-Matter Interactions on Nano-Structured Metallic Films (14MB)
Work carried out at the University of Southampton under the supervison of Prof. J.J. Baumberg