(c) Dr Paul Kinsler. [Acknowledgements & Feedback]

Logo (c) Paul Kinsler Logo (c) Paul Kinsler

Sheffield 1994-1996

Funding: EPSRC

Exciton-Photon Coupling in Semiconductor Microcavities

In 1996 I was offered a postdoc in the U.Shef Physics logoDepartment of Physics and Astronomy at the U.Shef logoUniversity of Sheffield, working in the Low Dimensional Structures & Devices Group with Dr David Whittaker and Dr Tracey Fisher. So I left Queensland to join this widely respected group, with extensive experimental expertise and strong links to the III-V Central Facility which is based in Electronic and Electrical Engineering at Sheffield. Here they had observed unexpectedly narrow spectral lines while looking at exciton-polaritons coupled to the optical mode inside a semiconductor microcavity. This was an (In)GaAs-GaAs-(Al)GaAs system, with Bragg-reflector mirrors surrounding a triple quantum well at the cavity antinode.

I found semiconductors very hard work to start with. Not only are they much more complex than typical optical systems, the semiconductor community has it's own culture of presenting work -- and I needed to work out what that was before finding their papers and presentations easy to understand. Basically, it is like this: a quantum optician will start by defining a model of the system, and continue from that, whereas semiconductor people tend to skip over that and start on with the calculations and results. If it's an experimentalist, they might just even show you some PL spectra, point to various peaks, and go from there. Of course once I got that hang of it things were much easier.

An exciton is a bound electron--hole pair, so an exciton-polariton is the coherent combination of an exciton trapped in the quantum well in the structure with a photon trapped in the microcavity. We worked out how to combine the effects of the microcavity spectrum, which was Lorentzian, with the exciton lineshape, a broadened gaussian caused by interface roughness and alloy fluctuations, then added the dispersion. This combination of factors enabled us to explain the experimantal results.

At Sheffield I started using latex for documents, and started the very useful habit of typing my rough calculations using it. This makes updating and extending them much easier than using paper records. I also started on a HTML directory skeleton for my computer filesystem, but whilst in principle this makes it easier to navigate, I don't really need it so it is rarely up to date. I also started these web pages, which have slowly grown over the years into this current state. Also I ditched the last vestiges of Windows usage and swapped over to using Slackware linux on the computer I run on my desktop. From this point, although the administration of the computer is sometimes frustrating, the effectiveness of the computer as a tool became far greater, and now I would regard having to use a Windows machine as an annoying hindrance.

While at Sheffield, I published a paper based on some work in my PhD thesis: "Testing quantum mechanics using third-order correlations". It was was easy enough to write, but I had a struggle getting it past the referee chosen by Phys. Rev. A. This was largely because I couldn't quite work out what the referee's objection was. Finally, after a third submission, it finally clicked and I added the magic sentence. If only the referee (are you listening, Bill? Eh?) had said, "I think you need to add a sentence saying this ..." it would have saved me (and him) a lot of time. There was also another which I started to write ("Limits to phase information in the parametric oscillator"), but I was never very happy with it, as it seemed rather clunky, and I didn't make much progress. I tried a submission to one journal, but the referee had many criticisms, some I could address, and some were largely a matter of opinion. I didn't think that particular referee was ever going to like it, though, so the paper went back into my files.

Before I left Sheffield, David Whittaker had moved on to Toshiba in Cambridge, leaving the semiconductor group wholely experimental; and indeed shortly after I left, Tracey Fisher returned to her native Australia for a Fellowship at UWA. Toward the end of my contract, I had finished a calculation motivated by some "polariton bottleneck" experiments, which applied quantum Monte Carlo methods to polariton scattering, and predicted a new stimulated scattering process. This was partly also a response to the semiconductor habit of starting a calculation by writing down some rate equations based (it always seemed to me) on a combination of experience and guesswork. With my bias towards rigour, I prefered to start from scratch with a model Hamiltonian, make the necessarily enthusiastic approximations, and derived the rate eqations via the quantum Monte Carlo method. Anyway, I had nearly finished the paper based on the calculation ("Quantum stochastic theory of phonon scattering between polaritons"), but with no interest in what I was doing from the Sheffield group, and with no chance of a continuation at Sheffield, things drifted to a stop.

This postdoc was funded by EPSRC, and only contained enough money for two years work. So with no chance of a continuation at Sheffield, it was time for me to move on again. Since I need to eat, it was time for me to move on again. The next research job I got was another postdoc, still in the north of England, but this time in Leeds.

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Date=0320 16 20020106 20000223 0316 0304 19990121 Author=P.Kinsler Created=1999