Polaritons are an odd cross-breed of a particle, half-matter, half-light. They could offer an abundant crop of new and improved optoelectronic devices — a promise already being fulfilled.
When I first encountered the idea of a polariton, while reading the doctoral work of John Hopfield1, its beauty stunned me. These mixed 'quasiparticles' are produced in semiconductor materials when the pairing of an electron and its phantom, a hole (this combination in itself a quasiparticle known as an exciton), couples with the photons of a light field. The result is something that is part matter, part light, and inherits qualities of both. Yet for all that they are useful for those who wish to get to grips with the optical properties of semiconductors, I always had a feeling that polaritons were not actually 'real'. I never expected that they would make their way out of the laboratory; still less that they would ever have any practical application.
History is proving me wrong. On page 372 of this issue, Tsintzos et al.2 detail the latest stage of the long march of polaritons to workaday respectability. They describe how they have produced a gallium arsenide diode that emits light directly from polariton states when they break up, at temperatures of up to 235 kelvin — just 60 kelvin or so below room temperature.
This breakthrough is significant, as my initial reservations about polaritons were founded largely on the fact that they can be stable only at very low temperatures and densities — under normal conditions, they tend to disintegrate, transforming themselves spontaneously into run-of-the-mill photons. In a semiconductor laser, or in a light-emitting diode (LED), temperature and density are both high. Light amplification and emission in these contexts are driven not by polariton interactions, but by the behaviour of an electron–hole plasma driven by the current flowing through the diode. It is a simple and robust mode of operation that has remained essentially unaltered in the near-50-year history of such devices, even in the newest vertical-cavity surface-emitting lasers (VCSELs). Over the years, the only factors that have changed are that the size of devices has decreased, and their efficiency has improved.
This story took a new twist in 1991, when a new type of polariton, the cavity polariton3, came to light. These quasiparticles arise when electrons and holes, in the form of excitons, are confined by the changes in the chemical composition of a substrate to a quantum energy well; simultaneously, photons are localized in the same region using two highly reflecting mirrors. In this way, stable polaritons can be produced. Stable is in this context relative; to a polariton, stability is a lifetime of more than 1 picosecond, a millionth of a millionth of a second.
More pertinently to the case in hand, stability means that cavity polaritons with zero wavevector — a quantity related to their momentum — are naturally the energetic ground state of the cavity system. This ground state will emit light precisely perpendicular to the surface of the confining sample. Subsequent studies have attested that these cavity polaritons have quite a number of useful properties. First, they have large de Broglie wavelengths of around 1 micrometre, meaning that their quantum wavefunctions are large enough to be manipulated easily, making them appealing for applications in quantum optics. Second, they might be stable at room temperature, with obvious advantages. Finally, they are good bosons, meaning that they can be parametrically amplified4 (that is, split up or joined together in units of different energy, a useful technique for signal amplification), and that many of them can pile up in a given quantum state, eventually leading to the formation of the state of matter known as a Bose–Einstein condensate5.
A number of patents have been filed on the strength of these admirable qualities. These home in on possible uses for cavity polaritons in, for instance, single-photon emitters, lasers, light-emitting diodes, photodetectors and optical switches. But at least two questions need to be addressed before such applications become reality: whether polaritons can indeed be made to operate at a sensible temperature; and whether they can be activated directly by electrical means.
Polaritons have already been confined in micrometre-sized cavities6. Tsintzos and colleagues' advance2 builds on that, and fits in with a body of work7,8,9 published in recent months. Two of these papers describe the construction of VCSEL-like cavities whose mirrors are of good enough quality to induce either electroluminescence7 or the emission of laser light8 in the polariton-coupling regime. Both these experiments were performed at temperatures of around 10 kelvin. A third9 details the realization of a polariton LED that works at up to 100 kelvin.
The new polariton LED that operates at 235 kelvin is thus a step further towards fulfilling the promise of room-temperature, electrically driven polariton devices. The nigh-on simultaneous advances in the field2,6,7,8, on the back of the parametric amplification4 and Bose–Einstein condensation5 of cavity polaritons, represent in my view the first glimpses of a grand new vista — of an expansive field that one might term 'polaritronics'. It should not be long before real, practical device applications begin to fill that panorama.
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Tsintzos, S. I., Pelekanos, N. T., Konstantinidis, G., Hatzopoulos, Z. & Savvidis, P. G. Nature 453, 372–375 (2008).
Weisbuch, C., Nishioka, M., Ishikawa, A. & Arakawa, Y. Phys. Rev. Lett. 69, 3314–3317 (1992).
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Khalifa, A. A., Love, A. P. D., Krizhanovskii, D. N., Skolnick, M. S. & Roberts, J. S. Appl. Phys. Lett. 92, 061107 (2008).
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Physical Review B (2015)
Nature Physics (2014)
Physical Review B (2014)
Applied Physics Letters (2012)
Acta Physica Polonica A (2012)