Photonic Crystals in 3-D – The Physics Congress 2003

Telecoms systems contain an awkward mixture of optics and electronics. A purely optical system would permit the very high data rates needed by the Internet, but at the moment the switching and routing, as well as the “last mile” to the customer, still depend on slower electronic components. Speaking at the Institute of Physics Congress on Monday 24 March, Professor Robert Denning from Oxford University will explain how his novel holographic approach to making 3-dimensional photonics crystals could allow optical components to be built that remove this bottleneck.

Professor Denning said: “By analogy with electronics the complex optical systems required are called ‘photonics’, because they use photons in place of electrons but, because it is much harder to control the flow of light than an electrical current, their development has been slow.” The outlook has recently been transformed by the invention of a new type of device, the photonic crystal. This acts like an optical insulator, and allows the components that handle optical data to be reduced to microscopic sizes. They can then be densely packaged like electronic circuits in a silicon chip.

Unfortunately, the methods used for making electronic chips are not well suited to these new devices. Current manufacturing techniques can only create devices featuring 2-dimensional photonic crystals. However Professors Denning and Professor Turberfield, from the Chemistry and Physics Departments in Oxford, have now found a simple way of using a laser to make the perfectly regular microscopic patterns that are required for 3-dimensional photonic crystals. Their method creates a promising route towards the photonic systems of the future.

Professor Denning said: “2-dimensional photonic crystal structures are easier to make, but diffraction at the edges of the holes that form the pattern leads to the loss of some light. Although this can be made quite small, it cannot be removed completely. In a 3-dimensional structure, confinement of the light is omnidirectional, so no losses can occur. Defining waveguides and cavities within a 3-dimensional structure makes much larger component densities possible, just like the advantage of a multilayer circuitboard over a single layer one.”

The team uses holographic lithography to make the 3-dimensional photonic crystals. Professor Denning said: “Holograms are usually made by making two beams of light interfere with each other and then storing the resultant intensity pattern via a light induced chemical change in some medium, photographic film for example. The holographic lithography is just a fancy name for defining the pattern of the photonic crystal via the intensity variations caused when four laser beams interfere. The trick is to find the right chemical reactions to make this possible.”