«It is clear that Cavity Polaritons are a system of choice to explore Boson interactions under quantum conditions.»
— Weisbuch in 2005.
Microcavities in École Polytechnique Fédérale de Lausanne, École Polytechnique (France) and elsewhere: past, present and future. C. Weisbuch and H. Benisty in Phys. Stat. Sol. B 242:2345 (2005). What the paper says!?
This is an account of the «past, present and future of microcavities» for a symposium held in honour of Marc Ilegems, where it is consigned that the «name cavity polaritons was given at the Erice summer school during some heated sessions (involving in particular the two Elis, Eli Burstein and Eli Yablonovitch) on the nature of these excitations». This makes the volume itself of prime interest for historical purposes.[1]
The paper appears to be essentially taken from another one, written at the same time by the same Authors,[2] to which Weisbuch added personal recollections (switching abruptly and in the most bizarre way to first-person narrative).
There is a great introduction on excitons and polaritons (including in bulk). The most interesting is, however, Section 3 «Looking backwards: a short history of microcavities in solids». There, Weisbuch (in the two-author paper), takes the pen and write: «Let me describe the sequence of events that led to the discovery of CPs» (CP stands for Cavity Polaritons).
I was at the University of Tokyo for four months in the summer of 1991, hosted by Professor Arakawa, and I was looking for some experiments to do. After some wonderings about QWs in high magnetic fields and QD excitation spectroscopy, two experiments that did not appear practical in the short time I was there, I decided to look at QWs in microcavities. Arakawa had some samples from a previous experiment [42], and I was thinking that their properties could not be as simple as previously reported. I then saw some strange effects when moving the sample around the resonance (mainly due to the imbalance between front and back mirror, but at that time I did not see this point). I therefore asked Arakawa for better samples (with only one type of wells, as the old ones were two-well samples) and M. Nishioka grew three samples with variable number of wells, all of them excellent, a remarkable feat as he had not grown such complex, demanding samples for two years. I then tried to observe the increase in luminescence intensity that should occur when excitons and microcavities are resonant. [...] I decided then that luminescence could not tell me the resonance, as luminescence is a complex phenomenon, but that reflectivity would. I thus mounted a reflectivity measurement, but then I would never observe a resonant peak, but a doublet whenever I would be ‘around’ resonance (as displayed by some resonance in PL intensity). After many observations of the double peak behaviour of the resonance crossing, and the absence of any single peak no matter how hard I looked, I then decided that this was a true physical effect, and had to decide which. Having worked some 15 years before on excitonic polaritons in the bulk [44–47] and having in particular determined their dispersion curve directly by resonant Brillouin scattering [44], I was familiar with such strong coupling excitations. In particular, I knew their dynamics, in particular that the scattering times are much longer than for free carriers, due to their neutral type. Therefore, I was not afraid to state from a ‘back-of-envelope’ calculation that indeed the coupling strength of cavity photons with excitons was stronger than any scattering mechanism, at variance with ‘common knowledge’ which predicted that the strong coupling just previously observed in atomic physics would never occur in solid-state physics due to excitation damping [...] I had difficulties convincing some colleagues that indeed one was observing strong coupling... As for publication, one referee of Physical Review Letters (PRL) observed that everything was fine with the experiment and the model, but as such phenomena had already been observed in atomic physics, it did not warrant publication in PRL. The second referee said exactly the same thing on experiment, model and atomic physics, but reached the opposite conclusion in that it was so surprising that it would occur in the solid state that it certainly was PRL material. After another two months, the third referee that I had requested just came up with the remark: ‘That’s what we like in physics: always good for surprises’ and it was published [28]. The name cavity polaritons was given at the Erice summer school [22] during some heated sessions (involving in particular the two Elis, Eli Burstein and Eli Yablonovitch) on the nature of these excitations.
The rest describes the rapid growth of polariton research at EPFL (and early EU research projects, SMILES and SMILED, on polaritons).
There is an interesting discussion of light-vs-matter quantum features:
It might prove useful to wonder why it appears that ones gets more leverage when quantizing photons as compared to electron states, or at least why it seems a more useable approach to new phenomena and applications (I know that I will raise some controversy here).
Nice conclusion:
It should be stressed, however, that, as usual for novel fields, the more interesting results and applications will come from concepts that we do not foresee today!