Dr Paul Dean

Research interests

My research interests encompass many aspects of terahertz (THz) opto-electronics with a particular focus on quantum cascade laser (QCL) technology.  Specific research interests include:

• Terahertz imaging and sensing
• Self-mixing phenomena in QCLs
• Non-linear effects and mode-locking in QCLs
• Injection-locking of QCLs
• Frequency-tunable and high-performance QCLs
• THz generation based on 1550nm technology
• High-field THz science

The quantum cascade laser (QCL) is a compact source of terahertz-frequency radiation based on a semiconductor heterostructure - a series of hundreds of thin layers of alternating semiconductor materials, in which electrons are confined into discrete energy subbands. These layers form periodic modules in which electrons can ‘cascade’ from period-to-period, emitting a low-energy (THz-frequency) photon at each step. In these devices, the photon energy is not determined by the bandgap of the material, but may be tuned over a wide range by engineering the widths of the semiconductor layers.

The compact nature of these laser devices, coupled with their ability to generate THz radiation with high power and high spectral purity, make them ideally suited not only to the study of fundamental science but also for real-world applications including biomedical imaging, non-invasive inspection and atmospheric science.

One aspect of my research focuses on 'self-mixing' effects in these lasers, in which optical feedback to the laser cavity mixes with the intracavity field, producing measurable perturbations to the laser operating parameters.  Crucially, this phenomenon allows a single QCL device to be employed as both a source of THz radiation as well as a coherent detector, which has enabled its wide application for three-dimensional imaging, vibration-sensing, materials analysis and high-resolution microscopy.  I am currently also exploring its application to biomedical imaging, specifically for the screening and diagnosis of skin cancers.  Central to these research directions is the investigation and modelling of the fundamental mechanisms and dynamics related to optical feedback in QCLs.

Other research interests include: the development of mode-locked QCLs, using nonlinear effects in these devices, for the generation of ultra-short THz pulses and wideband THz frequency combs; the investigation of injection-locking and gain dynamics in THz QCLs; the development of frequency-tunable QCLs; and the development of THz local oscillators based on QCLs for applications in space and atmospheric science.

• Acoustic control of quantum cascade heterostructures: the THz "S-LASER"

Research groups and institutes

• Pollard Institute
• Terahertz electronics and photonics

Current postgraduate researchers

• Imran Rahman