Photonic microstructures are systems in which there is a periodic or aperiodic variation in refractive index. These structures can be experimentally fabricated using, for example, standard semiconductor lithographic techniques or, more recently, holographic means. Such structures have the potential to control the flow of, or even to some extent localise, light and are potentially useful for a variety of applications to control the frequency, routing and transmission of optical signals. In the case of a periodic photonic crystal the "photonic band structure" can be calculated by using techniques very similar to those used in standard electronic structure calculations, such as the plane-wave approach. The simplest, most desirable feature of such structures is the existence of a complete "photonic band gap" (PBG) , a frequency range in which the propagation of electromagnetic waves is suppressed. Although such PBG features are sometimes seen in periodic systems we also study the properties of aperiodic so-called photonic quasicrystals by investigating the light transmission characteristics of such 2-D quasicrystals; the structures themselves are generated using a Penrose tiling scheme. The structures, have a higher "quasi-symmetry" than the maximum 6-fold symmetry allowed in a conventional 2-D periodic system and, in some cases, may exhibit more desirable PBG properties than those of periodic structures.
We have also developed an interest in the theory of quantum microcavity structures, comprised of an optical cavity as well as a carrier-confining heterostructure. Such structures are particularly exciting as they provide a means to control both excitons and photons in the same semiconductor structure, and to vary the strength of the exciton-photon interaction in a controlled way. We have performed extensive studies of microcavities in the form of a multilayered cylindrical Bragg-type reflector. When such a microcavity has an array of quantum dots, or a quantum wire, on its cylindrical axis there are excitonic and photonic modes of the structure, whose coupling can be varied by changing the parameters of the structure and this leads to some interesting results. So-called zero dimensional photonic states, involving full 3-D localisation of light, when coupled to the electronic states of a quantum dot can be considered as promising candidates for the realization of the elementary units of a quantum computer.
Some of the theoretical work carried out by the group is in direct collaboration with our experimental colleagues at Durham. For example, we have been involved in modelling the behaviour of double waveguide structures in support of the activities carried out by the Photonics, Sensors and Materials Group. Also, we have recently been successful in obtaining funding with Professor Chamberlain in Physics and Professors Wood and Petty in the School of Engineering for a project entitled "Artificial Materials for Terahertz Frequency Applications". Terahertz imaging and spectroscopy are currently of great potential interest for medical scans, security and other applications.
In carrying out the work described above we make use of the traditional plane-wave band structure approach, the transfer matrix scheme and also employ analytical studies in collaboration with a number of experimental and theoretical colleagues in the UK, Russia, Germany and France.