Observation of light thermalization to negative temperature Rayleigh-Jeans equilibrium states in multimode optical fibers

Abstract

Although the temperature of a thermodynamic system is usually believed to be a positive quantity, under particular conditions, negative temperature equilibrium states are also possible. Negative temperature equilibriums have been observed with spin systems, cold atoms in optical lattices and two-dimensional quantum superfluids. Here we report the observation of Rayleigh-Jeans thermalization of light waves to negative temperature equilibrium states. The optical wave relaxes to the equilibrium state through its propagation in a multimode optical fiber, i.e., in a conservative Hamiltonian system. The bounded energy spectrum of the optical fiber enables negative temperature equilibriums with high energy levels (high order fiber modes) more populated than low energy levels (low order modes). Our experiments show that negative temperature speckle beams are featured, in average, by a non-monotonous radial intensity profile. The experimental results are in quantitative agreement with the Rayleigh-Jeans theory without free parameters. Bringing negative temperatures to the field of optics opens the door to the investigation of fundamental issues of negative temperature states in a flexible experimental environment.

12 pages, 14 figures

References in corpus (10)

• Bose-Einstein condensation of photons in an optical microcavity
• Negative Absolute Temperature for Motional Degrees of Freedom
• Temperature in and out of equilibrium: a review of concepts, tools and attempts
• Thermodynamic theory of highly multimoded nonlinear optical systems
• Gibbs, Boltzmann, and negative temperatures
• Condensation in disordered lasers: theory, 3D+1 simulations and experiments
• Dramatic acceleration of wave condensation mediated by disorder in multimode fibers
• Interplay of thermalization and strong disorder: Wave turbulence theory, numerical simulations, and experiments in multimode optical fibers
• Wave condensation with weak disorder versus beam self-cleaning in multimode fibers
• Fermi-Pasta-Ulam phenomena and persistent breathers in the harmonic trap