My group usually consists of two or three Part II students, some doctoral students and several post-doctoral assistants. All our work is centred round the optical properties of condensed matter, and makes use of laser spectroscopy as well as measurements of non-linear optical parameters. There is considerable interaction with other ICL groups interested in the solid state. We are at the physical end of the spectrum of ICL activities, but usually do some straightforward synthesis, some spectroscopy at liquid helium temperatures and some interpretation. We grow crystals from solution and from melts, make glasses and polymer films, all containing species with important optical properties. Our recent industrial collaborators include British Telecom and British Nuclear Fuels Ltd.
Our apparatus is centred round three Neodymium-YAG lasers, one generating 40 ps pulses, another with higher power 10 ns pulses that drives a dye laser with a tuneable output covering most of the visible, UV and near-IR regions of the spectrum, and a third used for the measurement of non-linear optical properties. We have a number of optical cryostats including one with a base temperature of 0.3°K, and another containing a superconducting magnet with a field of 5 Tesla.
At the moment the following subject areas are being pursued:
1. Two-photon Spectroscopy of Inorganic Chromophores: This technique has selection rules, in centrosymmetric environments, which are complementary to those operating in ordinary one-photon spectroscopy. For example the pure electronic transitions within the d-shell of transition metals, as well as the f-shell of lanthanides and actinides, which are forbidden by the one-photon electric-dipole mechanism, can be clearly indentified in this way. Currently we are using two-photon data to investigate how the crystalline field in lanthanides modifies the electron-correlation interaction. This objective is aided by extensive model calculations.
2. Materials for Non-Linear Optic Applicatons: These have applications in switching and routing at telecommunications nodes, as well as in all-optical computing. We are studying a variety of compounds with large third-order non-linear optic coefficients, using a technique known as Degenerate Four-Wave Mixing. These include metal-dithiolene complexes, and conjugated porphyrin polymers. The latter, made by Dr Anderson’s group in the Department, have coefficients amongst the largest ever measured in non-absorbing materials. We have also studied the second-order coefficients of mixed-valence metal compounds, which have the highly direction charge transfer transitions. For this purpose we have constructed the only UK instrument capable of measuring Hyper-Rayleigh Scattering.
3. Photonic Bandgap Devices: These rely on an entirely new class of materials intended to control and manipulate photons in a manner analogous those used to control electrons in semiconductors. Currently the properties of these devices are only understood in theory, amongst the most striking being the suppression of spontaneous emission, and the prospect of lasers that operate with very low thresholds. However a satisfactory fabrication method has not yet been found. In a collaboration with the Clarendon Laboratory we have developed a simple three-dimensional holographic technique for the fabrication of these devices, which we have patented. Numerous chemical challenges occur in optimising this method.