Terahertz Spectroscopy (Dr Darren Graham)
For many years the terahertz region of the electromagnetic spectrum was inaccessible to science, but now with cutting-edge ultrafast laser-based spectroscopic techniques we are starting to exploit this region. With new tools at our disposal we are gaining insight into the fundamental properties of materials which in turn is enabling us to address the energy challenge. Recently our research has focused on unlocking the potential of catalysts that could reduce our reliance on fossil-fuel energy sources, by enabling more economical energy usage and offering a route for the efficient conversion of solar energy into chemical fuels. The most powerful catalysts at our disposal are nature’s own biological catalysts, enzymes, which can increase the rate of a chemical reaction by up to an incredible 1021 in comparison to uncatalysed reactions. Our ability to tap into their power is, however, hindered by our poor understanding of their function. Recently, it has been proposed that terahertz-frequency vibrations of the enzyme may enhance the reaction rate. This programme of work involves close collaboration between members of the Photon Physics group and colleagues at both the Manchester Institute of Biotechnology and the Institute of Materials Research at Tohoku University, Japan.
Spectroscopy of Semiconductors (Dr Phil Dawson)
“Energy is the single most important problem facing humanity today– not just the U.S., but also worldwide.”, Testimony of Nobel Laureate Richard E. Smalley to the Senate Committee on Energy and Natural Resources —April 27, 2004.
It is anticipated that significant energy savings can be made by the introduction of bright, high efficiency displays and traffic signals but also so-called “white LEDs”. As the white LEDs consist of a phosphor pumped by a nitride LED, it is only possible to produce such devices by using Nitride semiconductors. We are seeking a detailed understanding of the factors that influence the light emission of these LEDs. We are involved in an extensive program of fundamental studies whose long term aim is to facilitate a whole range of optoelectronic devices that will have significant impact in the world wide effort to save energy. This program is a joint activity involving a group at the University of Cambridge.
This collaboration has resulted in a number of world-leading achievements including photoluminescence internal quantum efficiency (IQE) values for quantum wells grown on sapphire, emitting in the green and near ultraviolet, and the particularly noteworthy achievement of an IQE of 70% for quantum wells grown on silicon. Along with these notable performance figures of optical output we are also developing a detailed understanding of the nanoscale localisation of InGaN, the effects of modifications to the internal piezoelectric field in InGaN quantum wells as well as how electrons and holes are spatially distributed in LEDs.
Ultrafast Processes in Colloidal Quantum Dots (Dr Dave Binks)
In conventional single junction solar cells, almost half the energy absorbed from the sun becomes waste heat within a few picoseconds as initially hot photogenerated carriers cool on an ultrafast time-scale. This limits the maximum possible efficiency of a photovoltaic cell and hence reduces the potential economic competitiveness of solar power. However, in colloidal quantum dots another process, multiple exciton generation (MEG), competes with carrier cooling. Here, hot electrons transfer their excess energy to the valence band, creating one or more additional excitons. These increase the photocurrent of the cell, improving its efficiency. Our objective is to optimise the design of colloidal quantum dots to maximise this MEG process. Key to this is ultrafast laser spectroscopy which enables us to study MEG and its competitor processes on a picosecond time-scale.
Surface and nanoparticle spectroscopy (Prof Wendy Flavell)
Part of our research is concerned with the electronic structure and surface properties of complex metal oxides and chalcogenides, including giant magnetoresistive oxides, catalysts and photovoltaics - particularly those of interest in the manufacture of 'next-generation' solar cells. Much of this work is aimed at developing an understanding of the link between the electronic structure of a material and its end application. It is aimed at answering questions such as 'how can we make solar cells cheaper and more efficient?'
Our most recent research has focussed on pump-probe spectroscopic techniques involving photoemission to investigate the fundamental physics of potential new solar nanocell materials. Our research is carried out using laser systems and electron spectrometers in house at the Photon Science Institute at the University of Manchester, but also uses a number of international synchrotron sources to probe valence and shallow core levels using photoemission spectroscopy. Near Edge X-Ray Absorption Spectroscopy (NEXAFS) is utilised to probe the geometry and electronic structure of adsorbed molecules. We have recently worked at ELETTRA in Trieste, Italy, SOLEIL in Paris and MAX-Lab in Lund, Sweden and are now using facilities for ultrafast spectroscopy at SPring8 in Japan.
Ultrafast Energy Processes in Nanoscale Optoelectronic Materials (Dr Patrick Parkinson)
Over the past 60 years, semiconductor materials have become the core component of the majority of technological advances. This is most true in the field of optoelectronics, where the conversion of energy or signals between the optical and electronic domains has led to tools for communication (lasers), lighting (LEDs), power generation (photovoltaics) and sensitive detection (photodetectors). In a constant push towards faster communication, cheaper devices or higher efficiency of operation, many materials have been developed to their limits. Over the past decade, nano- and meso-structured materials consisting of a blend of multiple materials with structure on the same scale as optical wavelengths have been developed, promising a revolutionary approach to materials development.
The study of the fundamental working principles of devices based on nanostructured systems requires both theoretical and experimental knowledge of energy processes on the nanoscale, for instance energy transfer across interfaces or migration with a domain. Because of the small length scales involved, processes often occur very quickly; this means that experiments sensitive to processes on the shortest (femto- to pico-second) are required. We investigate novel nanostructured material systems for optoelectronic applications using time-resolved and pump-probe experiments with femtosecond laser pulses and on microscopic length scales to reveal the fundamental energy processes within an exciting material architecture.