The terahertz frequency range sits between the microwave and mid-infrared regions of the electromagnetic spectrum, but has long resisted exploitation owing to difficulties in fabricating convenient sources and detectors; terahertz radiation is too high in frequency to be generated by the electronic techniques used in mobile telephones, but too low in frequency to be produced by the optical techniques exploited in, for example, CD player lasers.

However, the last twenty years have witnessed a remarkable growth in the field owing to the development of innovative sources, detectors, and imaging systems-and in particular, the quantum cascade laser. These developments have enabled a wide range of imaging and spectroscopy studies in which the selective absorption or transmission of terahertz radiation has provided unique and fundamental information about the physical and chemical properties of materials in this relatively unexplored region of the spectrum. Recent commercial application of terahertz instrumentation is now finding application in the pharmaceutical and automotive industries, and in high-resolution fault isolation in semiconductor devices and 3D imaging of integrated circuits, inter alia.

This PhD project will address this shortfall by combining two exciting technologies: scattering tip near-field imaging in an atomic force microscope (AFM), and self-mixing interferometry with terahertz quantum cascade lasers. Self-mixing interferometry is a technique that we have developed in which the emitting laser cavity itself is used as an exceptionally sensitive coherent self-detector of scattered/reflected radiation. Signals scattered from remote samples induce perturbations in the intra-laser electric field, and are manifest by measurable fluctuations in the laser voltage. We will combine this with a near-field imaging approach in which the incident terahertz radiation is focused locally on the sample by an AFM needle. By combining the nanometer-scale resolution of the AFM with the high sensitivity and compact footprint of the self-mixing detection technique, we will develop a terahertz nanoscale microscope and apply it to the investigation of a range of nanostructures and 2D materials in the terahertz range.