Photonic microstructures and light-matter interactions
One of the major research themes of the group is concerned with the theory of the properties of photonic microstructures and their use in controlling light and its interaction with matter. Photonic microstructures have been produced in many forms and can have diverse properties and applications, including the ability to: reflect light with low loss (e.g. Bragg mirrors); control the propagation of light (e.g. optical fibres and photonic crystals); and trap light (e.g. Fabry-Perot etalons and other optical cavities). Photonic crystals, which are structures with periodic dielectric properties on a length scale comparable to the wavelength of light, have attracted particular attention because their photonic band structure provides much scope to facilitate the control of optical properties. For example, Bragg mirrors are just multilayer structures or ‘one-dimensional’ photonic crystals that reflect light whose frequency lies in one of their photonic band gaps. Photonic crystals can be fabricated using standard epitaxial and lithographic techniques, and much of the early work was focused on semiconductor structures, but now there is considerable interest in metallic and hybrid dielectric-metallic structures as well. In fact we have made considerable use of metallic structure in our research on terahertz technology in Durham. The nature of many photonic microstructures means that the control of light can be integrated with materials that exhibit interesting and/or useful optically-induced behaviour. For example, light can excite electrons and holes in semiconductors, including excitons (which are bound electron-hole pairs) and also optical nonlinearities occur with high intensity light. In metals, under the right conditions, light can excite surface plasma oscillations.
Of particular interest are systems in which semiconductor excitons and photons can be made to interact strongly – for example a resonant Fabry-Perot-type planar optical cavity with the excitons confined within one or more semiconductor quantum wells near its centre. The result of the strong exciton-photon interaction is hybrid excitations called exciton-polaritons (or simply polaritons for brevity), which currently represent one of the most topical and exciting areas of physics. These are part-light, part-matter quasiparticles which obey bosonic statistics. As such, they can form a Bose-Einstein condensate, and do so in some cases relatively close to room temperature due to their extremely light effective mass. They can also exhibit a number of beautiful physical effects, including superfluidity, superradiance and quantum entanglement. However, as well as being of great fundamental interest, polaritons have considerable potential for the realization of a new generation of optoelectronic devices exploiting collective quantum effects at room temperature. Future polariton devices can be expected to include polariton lasers, optical logic gates, polarization modulators, spin-memory elements and quantum information devices. For the development of such technology an interdisciplinary approach is essential, including the involvement of specialists in fundamental and applied semiconductor physics, photonics, quantum optics, crystal growth, nanotechnology and device fabrication and we are fortunate to be involved in two European collaborative projects which bring together some of the world’s leading experimental and theoretical groups in the field.
Also in Durham, we have pioneered the development of a number of ideas for the application of metal-Bragg reflector structures, which have great potential for controlling and utilizing the light–matter interaction. Such structures can support photonic interface states in which light is confined as a result of a photonic band gap of a Bragg reflector on one side of an interface and by total internal reflection or the presence of a thin metallic layer on the other. For example, in a suitably designed structure supporting cavity polaritons that has a metal film on its surface, the coupling of the polariton and interface states can be used to provide lateral spatial control of polaritons by patterning the metal film. In another scenario, the considerable electric field enhancement that occurs near the surface could be used in sensor applications or in promoting nonlinear phenomena. Also, the interaction via their evanescent fields of two neighbouring interface states associated with different surfaces is predicted to lead to distinctive narrow transmission features, which might find application in laser emission control and terahertz frequency generation.