(c) Dr Paul Kinsler. [Acknowledgements & Feedback]
Optically Pumped Terahertz Inter-subband Emitters
After Sheffield, I was offered my next postdoc in the Institute of Microwaves and Photonics at the University of Leeds (Leeds, UK). This new, research focussed Institute was grown out of the old Department of Electronic and Electrical Engineering during a University reorganisation. Most of the work in the Institute is engineering based, but since modern engineering is not dissimilar to applied physics, particularly in the quantum electronics field, they do do physics research here. I worked with Dr Paul Harrison and Dr Rob Kelsall, and Kate Donovan (a PhD student also working in this area).
The research here was motivated by race to make a far--infrared laser in semiconductors. The THz frequency region had potential for use in short range communications such as wirless comuter networks, and in micro- and medical imaging technologies. However, making such a device was tricky. In order to get a population inversion between THz-separated two subbands in a prototype quantum well device, we had to know a lot about how the electrons move around the structure, how they get in, get out, and how they hop between their quantum energy levels. Unlike the mid-infrared region, here the useful radiative process are completely swamped by electrons interacting with the sea of acoustic and LO phonons that make up the vibrations in the crystal lattice of the semiconductor. Worse, the electrons repel and scatter off each other, a problem that gets worse as we increase the number of them when tring to increase the laser output.
The only way around this was to get clever, and study in detail this myriad of scattering processes. By tailoring the design, we can alter the wavefunctions, the subband spacings, and the phonons -- thus altering the scattering processes. At the IMP we studied sets of prototype designs using theory and comprehensive computer simulations to understand how to make a THz laser work.
While at Leeds I continued typing up notes and calculations using latex and also wrote more comprehensive documentation for the scattering rate and Monte Carlo computer codes that I wrote and used. These were compiled along with seminar slides and other material into a report on the research which I gave to Paul and Rob. The benefit of this was mainly that it helped me develop good documentation habits -- because as as far as I know, both the code and documentation have never been subsequently used. However, as an aid to understanding I rederived some phonon scattering rates from scratch, parts of which Paul Harrison borrowed for use in the text book he was writing at the time.
Interestingly, when doing one of the phonon scattering calculations, I found a step of dubious mathematical validity that seemed to pass entirely without comment in the derivations I had seen. It was not even clear what the physical justifications for the step were, although there must be some, since the calculations using the results seem good enough. The step turns a convolution integral into a product, which could be valid under some circumstances, but is certainly not true in general! I have always meant to test out calculations done with and without this dubious step, but never found the time.
While at Leeds I scanned in a large number of old seminar slides and poster presentations, and incorporated them into these web pages. From then on, I made a point of contining to add any new material as I write it.
This postdoc was again funded by EPSRC, and only contained enough money for two years work. My next employer was in Delft, although to start with I remained at a desk in Leeds as a visiting researcher. Having said that, the administration at Leeds seemed very keen to delete my existence, as the link to my web pages disappeared from the otherwise rather sterile database-driven departmental site even before I left. Of course there are good reasons to maintain a professional departmental image, but it has always seemed to me that the useful things about departmental web sites, beyond contact information and administration details is those maintained by individuals on their own projects. By all means have a well presented and consistent core, but hiding the contributions of the real people working in the department is shooting yourself in the foot. But it's their foot to shoot, I suppose.
Date=16 20020106 1003 20000223 0818 0316 0304 19990121 Author=P.Kinsler Created=1999