Theoretical Physics

7 October 2014
Time: 14:00 to 15:00
Location: Roger Stevens Lecture Theatre 06

Bose-Einstein Condensates of Photons in a Dye-Filled Microcavity

Rob Nyman, Imperial College London

Photons are bosons, but don't normally make Bose-Einstein condensates because their number is not conserved in thermal equilibrium. However, light pumped into a fluorescent dye can exchange energy with the dye-solvent mixture, coming to thermal equilibrium without destroying the photons. The electronic structure of the dye sets a minimum energy, preventing photons from being destroyed altogether. Placing this system between two curved, high-reflectivity mirrors, the light is confined for a nanosecond, much longer than the few picoseconds it takes for thermalisation. Therefore thermal equilibrium can be achieved at room temperature with photon number determined by pump intensity. At fixed temperature, and sufficient density, a Bose-Einstein condensate (BEC) will form, as first achieved in 2010 [1]. We have recently become only the second laboratory to create this quantum-fluid state of light.

There are many recent theoretical works describing photon BEC, several of which take into account the dynamical equilibrium between pumping and decay, some in a fully quantised model of interactions between light-and-molecule and molecule-and-solvent-bath, some using rate-equation models. Our current experimental efforts are aimed at testing the validity of these various theories. For example, we observe that the critical pump power for condensation depends on the size of the pump beam, which contradicts simple equilibrium theories, as well as the more sophisticated theories that assume homogeneous pumping. The energy-dependence of critical pump power the we see indicates that energy-dependent loss mechanisms must also be taken into account by any microscopic description.

The interactions between photons in the dye-filled microcavity are poorly characterised, and we have theoretically analysed an experimental method for inferring the strength of interactions in photon BEC, by observing these excitations using angle-resolved spectroscopy of the light that leaks through the mirrors~[2]. Very weak interactions should be measurable this way. The project's long-term aims include measuring the interaction strength, characterising coherence properties of the condensate, and fabricating mirror shapes to allow observation of 1D gases of photons as well as photon BECs with mesoscopic numbers.

[1] J. Klaers et al, Nature, vol. 468, p545 (2010) [2] R.A. Nyman and M.H. Szymanska, Phys. Rev. A, vol 89, 033844 (2014)



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