Erik Busley, Leon Espert Miranda, Christian Kurtscheid, Frederik Wolf, Frank Vewinger, Julian Schmitt, Martin Weitz:
Phys. Rev. A 107, 052204 (2023)
🔓 arXiv:2207.12070 (2022)
The Liouville theorem states that the phase-space volume of an ensemble in a closed system remains constant. While gases of material particles can efficiently be cooled by sympathetic or laser cooling techniques, allowing for large phase-space compression, for light both the absence of an internal structure, as well as the usual non-conservation of particle number upon contact to matter imposes fundamental limits e.g. in fluorescence-based light concentrators in three-dimensional systems. A different physical situation can in principle be expected for dye-solution filled microcavities with a mirror spacing in the wavelength range, where low dimensional photon gases with non-vanishing, freely tunable chemical potential have been experimentally realized. Motivated by the goal to observe phase-space compression of sunlight by cooling the captured radiation to room temperature, we in this work theoretically show that in a lossless system the phase space volume scales as (ΔxΔp/T)d=constant, where Δx and Δp denote the rms position and momentum spread and d the dimensionality of the system (d= 1 or 2). We also experimentally realize a sunlight pumped dye microcavity, and demonstrate thermalization of scattered sunlight to a two-dimensional room temperature ensemble with non-vanishing chemical potential. Prospects of phase space buildup of light by cooling, as can be feasible in systems with a two- or three-dimensional band gap, can range from quantum state preparation in tailored potentials up to technical applications in diffuse sunlight collection.