Current Research Activities
III-V Quantum Cascade Lasers
|
Quantum cascade lasers (QCLs) are now established as versatile sources of radiation in the mid- and far-infrared (THz) range. Sophisticated theoretical and computational models of these devices have been developed, incorporating all relevant scattering processes, self-consistency, and electron heating effects, in order to describe electron transport and laser gain. These models are used for modelling the existing GaAs/AlGaAs QCLs, analysing effects of structural parameters on output characteristics, as well as for the design and optimisation of new structures, including those based on the InP material system and on antimonide semiconductors. |
Scanning electron micrograph of a terahertz quantum cascade laser cavity. |
DMS Structures and III-V QCLs in a Magnetic Field
Scanning electron microscope picture of a quantum well infra red photodectector grating |
Magnetic fields have profound effects on quantum nanostructures. A magnetic field splits 2D subbands into sets of Landau levels, effectively performing in-plane electron confinement, and producing a fully discrete spectrum. Additionally, in nanostructures based on diluted magnetic semiconductors (DMS), like CdMnTe or Zn(Cd)MnSe, they lift spin degeneracy. This leads to a number of useful effects, e.g. field dependent absorption wavelength or selective depopulation of states, which result in improved performance of QCLs, or potentially to field-tunable THz photodetectors. Computational models are being developed to describe electron transport in these structures, and to analyse the device performance. |
Quantum Dots
|
Quantum dots provide 3D electron (or hole) confinement, and give a fully discrete energy spectrum. They are based on strained semiconductor overgrowth, e.g. InAs/GaAs, SiGe/Si, etc., and have a pyramidal or cone-like shape and, after completion of growth, are fully embedded in the host material. Stacked, multiple-layer arrays of vertically aligned dots are also feasible. Sophisticated and efficient codes, based on either the effective mass or k.p method, have been developed for calculation of the electronic structure of quantum dots, for electron relaxation processes and for the interaction with light. Prospective quantum dot based devices, that rely on intraband optical transitions, include infrared photodetectors and intersublevel lasers. Their design principles are currently considered. |
|
Si/SiGe Quantum Cascade Emitters
The group leads a major UK collaboration who are working towards achieving THz laser emission in p-type Si/SiGe cascade structures. Initial considerations of n-type Si/SiGe cascade structures are also under way. Such devices would be highly desirable in view of possible monolithic integration with electronic components on chips. An additional benefit is that a Si/SiGe cascade laser of either n- or p-type would, depending on the substrate orientation and transitions involved, allow for either the conventional edge-emitting configuration (with cavity end mirrors, microdisk, or DFB resonator) or the surface-emitting (VCSEL) configuration. However, there are numerous challenges on the route to demonstration of a laser, mostly related to strain.
Complex physical models - based on k.p or empirical pseudopotential bandstructure calculations, and self-consistent rate equation or Monte Carlo simulations of carrier transport - are being used to design cascade structures that are expected to provide significant population inversion. Gain in excess of resonator losses has been predicted in suitably designed cascades. Fabrication of high quality structures is now possible, and THz intersubband electroluminescence has been demonstrated. Intersubband lifetimes have been measured and analysed using our theoretical models.
Experimental work with quantum cascade lasers
|
Experimental research is investigating and optimising new designs of quantum cascade laser. A particular emphasis is placed on far-infrared (terahertz) structures, following on from the initial demonstration in 2002 of lasing at 4.4 THz. Higher operating temperatures, greater powers and the ability to tune the emission frequency are being investigated. In addition, quantum cascade lasers are being used as a source of radiation for probing a range of quantum electronic structures, including quantum well infrared photodetectors and semiconductor nanostructures. |
Optical mode profile from terahertz quantum cascade laser |
III-Nitride semiconductor structures
Quantum wells based on GaN, AlN, InN and their alloys are different from the conventional wells based on other III-Vs, in that they may have a very strong polarization field or no field, depending on the growth direction. In addition, the potential barrier at GaN/AlN is large, which enables large intersubband transition energies, up to the near infrared. Furthermore, optical phonon relaxation in these materials is very fast.
Using these properties we have designed cascade structures for laser operation in the long wavelength range (34 micrometers), and also in the short wavelength range (3 micrometer.) Due to a very fast depopulation of the lower laser states, GaN QCLs are predicted to have considerable values of population inversion, but suffer from large linewidths.