You are cordially invited to the SCL online seminar of the Center for the Study of Complex Systems, which will be held on Thursday, 23 December 2021 at 14:00 on Zoom. The talk entitled

On the theoretical description of photon Bose-Einstein condensates

will be given by Dr. Axel Pelster (Department of Physics, Technical University of Kaiserslautern, Germany). Abstract of the talk:

Since the advent of experiments with photon Bose-Einstein condensates in dye-filled microcavities in 2010 [1], many investigations have focused upon the emerging effective photon-photon interaction. Despite its smallness, it can be identified to stem from two physically distinct mechanisms [2]. On the one hand, a Kerr nonlinearity of the dye medium yields a photon-photon contact interaction, whose microscopic theoretical description is based on a Lindblad master equation [3]. On the other hand, a heating of the dye medium leads to an additional thermo-optic interaction, which is both delayed and non-local [4]. The latter turns out to represent the leading contribution to the effective interaction for the current 2D experiments.

A new experimental platform, which is currently built up in Kaiserslautern, will be devoted to analyse the dimensional crossover in trapped photon gases from 2D to 1D. As the photon-photon interaction is generically quite weak, they behave nearly as an ideal Bose gas. Moreover, since the current experiments are conducted in a microcavity, the longitudinal motion is frozen out and the photon gas represents effectively a two-dimensional trapped gas of massive bosons, where the anisotropy of the confinement allows for a dimensional crossover. A detailed investigation for such a system allows to determine its effective dimensionality from thermodynamic quantities [5].

Furthermore, we investigate how the effective photon-photon interaction changes when the system dimension is reduced from 2D to 1D [6]. To this end, we consider an anisotropic harmonic trapping potential and determine via a variational approach how the properties of the photon Bose-Einstein condensate in general, and both aforementioned interaction mechanisms in particular, change with increasing anisotropy. We find that the thermo-optic interaction strength increases at first linearly with the trap aspect ratio and later on saturates at a certain value of the trap aspect ratio. Furthermore, in the strong 1D limit the roles of both interactions get reversed as the thermo-optic interaction remains saturated and the contact Kerr interaction becomes the leading interaction mechanism. Finally, we discuss how the predicted effects can potentially be measured experimentally.

[1] J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, Nature 468, 545 (2010).
[2] J. Klaers, J. Schmitt, T. Damm, F. Vewinger, and M. Weitz, Appl. Phys. B 105, 17 (2011).
[3] M. Radonjic, W. Kopylov, A. Balaz, and A. Pelster, New J. Phys. 20, 055014 (2018).
[4] E. Stein, F. Vewinger, and A. Pelster, New J. Phys. 21, 103044 (2019).
[5] E. Stein and A. Pelster, arXiv:2011.06339 (2020).
[6] E. Stein and A. Pelster, arXiv:2109.11211 (2021).

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