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

TL;DR

This paper reports the experimental observation of light waves reaching negative temperature equilibrium states in multimode optical fibers, demonstrating Rayleigh-Jeans thermalization and expanding the understanding of negative temperature phenomena in optics.

Contribution

First experimental demonstration of negative temperature equilibrium states for light waves in a conservative optical system using multimode fibers.

Findings

Negative temperature states feature non-monotonous radial intensity profiles.

Experimental results align with Rayleigh-Jeans theory without free parameters.

High energy modes are more populated than low energy modes in negative temperature states.

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.