Modelocking in terahertz-frequency quantum cascade lasers

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. Nevertheless, the last twenty years have witnessed a remarkable growth in the field owing to the development of innovative sources, detectors, and imaging systems. Due to these developments 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.
One notable development has been the quantum cascade laser, which comprises of a series of quantum wells formed by thousands of layers of semiconductor material, each controlled to atomic-layer thickness. These devices are now used by research groups around the world as a compact source of high-power terahertz waves. Yet one challenge to the scientific community has been to develop quantum cascade lasers that can generate ultrashort (picosecond-duration) pulses of terahertz radiation. The generation of ultrashort laser pulses has been achieved in the visible and infrared regions of the spectrum using laser modelocking, which has led to a wide range of new measurement techniques and technologies including time-resolved measurements, ultra-high speed communications, and high-precision metrology and spectroscopy. The development of such a laser operating in the terahertz range would open up new opportunities for metrology, coherent imaging and tomography, materials analysis and molecular spectroscopy.
This PhD project aims to study how high-power terahertz pulses interact with the quantum wells in these lasers, and to use this understanding to develop a modelocked quantum cascade laser device that can generate ultrashort terahertz pulses.