A Sunlight-pumped Two-dimensional Thermalized Photon Gas

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 $(\Delta x \Delta p / T)^d = \mathrm{constant}$, where $\Delta x$ and $\Delta p$ 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.