We synthesise and characterise novel opaline systems made from SiO^₂ (as in natural opals) and also a range of polymer materials including polyethylene and polymethyl methacrylate. Natural opals consist of silica spheres packed in a highly regular manner as a result of natural, self-assembly processes. The diameters of the spheres are such that the regular arrays diffract and reflect light in ways, which suggest a high degree of control in terms of direction of propagation and photon energy. It is these effects when they occur in the visible part of the spectrum that give rise to the pleasing opalescence that the gemstones possess.

These materials may be fabricated in the laboratory by a variety of means, once the particles themselves have been synthesised. The group makes many different types of particle and attempts to assemble these into a variety of interesting systems. We are particularly interested in the many ways in which the self-assembly process can be facilitated. These range from simple sedimentation methods, where a suspension of particles simply settles out under gravity, through highly controlled evaporation to perhaps the most novel approach- one which the group at Tyndall has helped to pioneer, self assembly via Langmuir-Blodgett film formation. This approach, promises to produce high quality, large area samples that could then be used in the fabrication of many types of optical elements- both passive (e.g. filters, mirrors) and active (e.g. waveguiding systems, optical circuits).

As a link between this area and the CVD growth activities, CVD growth has been used to infill the pores present in the opaline systems so as to provide greater refractive index contrast and hence better photonic band gap properties. In this way the group has produced the world’s first ever inverse opals made from GaP and InP.

The name inverse opal refers to a system where an opal is made and then the air voids between the self-assembled spheres are filled with a second material by CVD or another suitable infilling process. The original host spheres are then removed by selective chemical etching, to leave a photonic system made from the infill material itself.
It is also possible to fabricate particles with added functionality and then allow these to self-assemble into ordered structures. Amongst those currently under examination, metallodielctric particle systems are of particular interest since they enable the researcher to build into a photonic material, some of the optical properties associated with metals such as gold and silver.