Non-linear terahertz spectroscopy of quantum materials

The terahertz (THz) frequency range, lying between the well-developed electronics and optical regions, has been under-exploited to date, particularly in the areas of nonlinear phenomena, coherent control and non-equilibrium systems. These fields, which have provided so much fascinating physics in the visible and microwave regimes, have barely been touched upon in the THz region. By contrast, there are a wealth of exciting systems and materials that possess fundamental transitions or resonances at this energy range: atoms-in-solids, low-dimensional electron systems, Josephson plasma waves in superconductors, vibrational modes in crystalline materials and bio-molecules. While there has been increasing interest in recent years of using linear THz spectroscopy to study the spectral absorption of these materials, the more exciting prospect is making use of intense pulses of THz radiation to coherently control the state of a quantum system

The lack of exploitation to-date of THz nonlinear phenomena principally arises from the difficulty in designing and fabricating high-performance THz sources and detectors. In recent years there has been an intense research effort to close the so-called 'THz gap' and there are now several schemes for the generation and detection of THz radiation, arguably the most successful of which has been THz time-domain spectroscopy (TDS). These systems are widely used for spectroscopy and have found application in a diverse range of systems, including investigations of electron dynamics in semiconductors; determining vibrational modes in molecular crystals; explosives detection; biomolecule identification; biomedical imaging; and metamaterial studies.
The THz region, however, still lags in the areas of non-linear spectroscopy and optical control, where the light-matter interaction goes beyond weak absorption and electromagnetic radiation is used to perturb the sample. To do this controllably and accurately, intense narrowband pulses of THz radiation are required. Currently, the only available THz source for providing this type of radiation is the free-electron laser (FEL) which are both intense and can be tuned over a very wide frequency range, from THz to UV. The major drawback of these systems, however, is their large size, cost, and the infrastructure required to support them. Despite this, FELs are in high demand in many areas of scientific research, such as studies of superconductivity, graphene science, physical chemistry and biology. In addition, the demand for FELs, or a suitable alternative, is likely to grow as there are proposals for intense THz radiation to be used for control of enzyme catalysed reactions and for non-linear spectroscopy of biological molecules.

The project will develop high-power, high-precision terahertz sources to interrogate a range of quantum materials that are active in the terahertz range. The project will make use of the new EPSRC-funded amplified laser facility based in the electronic and electrical engineering, to generate high-field terahertz pulses with bandwidths tailored to the material under investigation.

Aims and objectives:
Generation of high-field and high-peak-power terahertz pulses from amplified laser pulses with a range of bandwidths. Bandwidths below 100 GHz will be required for some materials and novel techniques will be required for these.
Perform linear spectroscopy of quantum materials with narrow transitions in the terahertz range. These include atom-in-solid materials such as impurities in semiconductors, and electronic confined semiconductor nanostructures such as quantum dots and quantum wells.
Non-linear spectroscopy and coherent manipulation of the above materials using high-field terahertz pulses.