Thermalization of coupled atom-light states in the presence of optical collisions

Abstract

The interaction of a two-level atomic ensemble with a quantized single-mode electromagnetic field in the presence of optical collisions is investigated both theoretically and experimentally. The main focus is on achieving thermal equilibrium for coupled atom-light states (in particular dressed states). We propose a model of atomic dressed-state thermalization that accounts for the evolution of the pseudo-spin Bloch vector components and characterize the essential role of the spontaneous emission rate in the thermalization process. Our model shows that the time of thermalization of the coupled atom-light states depends strictly on the ratio of the detuning to the resonant Rabi frequency. The predicted time of thermalization is in the nanosecond domain at full optical power and about 10 times shorter than the natural lifetime in our experiment. Experimentally we investigate the interaction of the optical field with rubidium atoms in an ultrahigh-pressure buffer gas cell under the conditions of large atom-field detuning comparable to the thermal energy in frequency units. In particular, an observed asymmetry of the saturated lineshape is interpreted as evidence of thermal equilibrium of coupled atom-light states.

Laser cooling of dense rubidium-noble gas mixtures via collisional redistribution of radiation

“…Direct measurement of the lifetime of the excited 5P rubidium levels under 200 bar helium buffer gas confirm the natural lifetime of 27 ns within an experimental uncertainty of 5 ns. 26 The here observed shift could be explained by an average decrease of the excited state lifetime of the rubidium 5P state due to quenching by roughly 1 ns. Despite the use of high purity buffer gas (we typically use helium and argon buffer gas of purity 6.0), we cannot exclude that residual impurities, for example by a small nitrogen admixture, result in a corresponding reduction of the lifetime due to the high number of collisions the rubidium atoms undergo, typically several 10000 within a natural lifetime.…”

Lasing and phase transition in atomic system with dressed states

“…Formally, in the limiting case of λ = 0, from equation ( 10) it is easy to recognize the results obtained for DS thermalization in the presence of OC processes in the dense atomic medium in a single pass regime (i.e., without cavity) of atom-light interaction, see [12].…”

“…characterizes a thermodynamic equilibrium value of DS population imbalance in an effective two-level picture, see [12]. In (10b) and thereafter we also assume that Rabi frequency Ω R also includes phase shift η 1 introduced by the collision, i.e.,…”

“…Thermodynamical properties of such processes are being actively investigated recently-see [10,11]. Previously we have proposed a model of atomic dressed states (DS) thermalization that accounts for the evolution of pseudo-spin Bloch vector components and characterizes the essential role of the spontaneous emission rate in the thermalization process [12]. Recently we have shown that in the presence of both photon and polariton trapping in dissipation-free biconical waveguide cavity an appropriate lifetime of polaritons can be obtained; this paving the way to a high temperature BEC observation with low branch (LB) photon-like polaritons-see [13].…”

Light confinement by a cylindrical metallic waveguide in a dense buffer-gas environment

“…We have shown recently that the frequent collisions with the buffer gas atoms can lead to thermal equilibrium of dressed states, i.e. coupled atom-light states [13,14]. The nanosecond lifetimes of alkali excited states in high pressure buffer gas (τ nat ≃ 27 ns for the case of the 5P state of the rubidium atom) is orders of magnitude longer than the picoseconds relaxation times of typical exciton-polariton systems.…”