Ultrafast Spectroscopy of Materials Driven Far From Equilibrium

The part of the electromagnetic spectrum that lies between microwaves and the mid-infrared is called the terahertz (THz) band. Many materials have characteristic excitations, energy gaps and dielectric properties at these frequencies with the result that low power THz radiation has been widely used and explored in the last two decades for its applications in fundamental science, security, sensing, industrial quality control and medicine [1]. However, only in the last few years have technologies been developed to allow the generation of ultrashort pulses of high power THz radiation that can be used to study the nonlinear properties of materials in the lab. In particular, the study of materials driven far from equilibrium by strong THz frequency fields is a new area of research in which electromagnetic radiation is used to create atomic displacements equivalent to those obtained at GPa pressures, drive electron spin precession using the large magnetic component of the THz field and accelerate electrons to considerable energies on sub-picosecond time scales. Studies under these far from equilibrium conditions provides insight into the physics underpinning the equilibrium properties.
The Ultrafast Science and Terahertz Photonics Group has recently built intense (~ 1 MV/cm peak electric field) THz sources based on laser pumping of nonlinear crystals and photo-ionization of gases and we now seek to apply them to study materials driven into non-equilibrium phases or highly nonlinear states where new physics can be observed and new tests of theories can be made [2]. The time evolution of material properties following excitation by strong THz ‘pumping’ can be monitored in detail using a variety of optical probes. This experimental physics PhD project will mainly be concerned with using such pump-probe techniques to investigate the non-equilibrium dynamics of thin film material systems which are of interest from either a technological or fundamental viewpoint, such as ones displaying metal-insulator transitions, high temperature superconductivity or coupling between electric and magnetic properties. The common theme is that these are systems where the theoretical explanation for the material properties is incomplete or contested and where experimental observations in previously uncharted regions of parameter space could help develop understanding. A secondary aim of the project is to explore enhancing the THz electric field in samples by concentrating incident radiation using arrays of nanometer scale apertures in surface metal films [3]. Electric fields 10-100 times higher than 1 MV/cm would allow the study of even more extreme behaviour such as breakdown of the effective mass approximation and the creation of strongly coupled light-matter states.
You will have access to ultrafast laser based terahertz spectroscopy and advanced nanofabrication facilities. There will be opportunities to benefit from in-house expertise in the underlying theory and the wider interactions and training opportunities offered by the group’s position within the Nanoscience Group in Bath and its affiliation with the Bath-Bristol Centre for Doctoral Training in Condensed Matter Physics. The project might involve occasional travel to perform complementary experiments elsewhere, for example to the free electron laser facility in Nijmegen.

Applications: Applicants should have a strong interest in pursuing experimental condensed matter research and have or expect to gain a First or good Upper Second Class UK Honours degree in physics. Applications from equivalently qualified non-UK students are also welcome.

Contact Dr Steve Andrews ([Email Address Removed]) for informal enquiries.
Group web site: http://people.bath.ac.uk/pyssra/Index.html
Nanofabrication facilities: http://bath.ac.uk/facilities/nanofab/