Light thermalized at room temperature in an optically pumped, dye-filled microcavity resembles a model system of noninteracting Bose-Einstein condensation in the presence of dissipation. We have experimentally investigated some of the steady-state properties of this unusual state of light and found features which do not match the available theoretical descriptions. We have seen that the critical pump power for condensation depends on the pump beam geometry, being lower for smaller pump beams. Far below threshold, both intracavity photon number and thermalized photon cloud size depend on pump beam size, with optimal coupling when the pump beam matches the thermalized cloud size. We also note that the critical pump power for condensation depends on the cavity cutoff wavelength and longitudinal mode number, which suggests that energy-dependent thermalization and loss mechanisms are important.The decision to categorize an experimentally observed phenomenon as Bose-Einstein condensation (BEC) goes hand in hand with the consensus microscopic description. For exam ple, the popular definition of BEC by Penrose and Onsager [ 1 ] of extensive or macroscopic occupancy by identical bosons of a single quantum state was chosen to extend the original idea of Bose and Einstein to interacting particles, implicitly assuming homogeneity, in their case superfluid helium.In general, BEC at thermal equilibrium arises because the Bose-Einstein distribution diverges when the chemical potential approaches the energy of the ground state (from below). In dissipative, nonequilibrium condensation of exciton polaritons in semiconductors (e.g., [2-4]) or of polaritons in organic molecules [5,6], the system may be effectively homogeneous, so the Penrose and Onsager definition of BEC applies, but thermal equilibrium is not always strongly established. In these cases, BEC is widely accepted when thermal equilibrium is experimentally demonstrated to be a good description, and a macroscopic population is observed in the lowest-energy state, despite the strong interactions.Photons thermalized in a dye-filled microcavity probably have the weakest interactions of any system to have exhibited BEC, including atoms near Feshbach resonances [7,8]. In this intrinsically inhomogeneous system, thermal equilibrium and macroscopic occupancy of the ground state are the usual criteria for BEC, and both have been observed despite the dissipation [9,10], so BEC is assigned without major controversy [11]. Interactions are so weak, that questions have been asked about the mechanism by which the condensate forms [12], There has been considerable recent activity developing microscopic models of this physical system, but most of the models, e.g., by Kruchkov [13], assume that near-thermal-equilibrium conditions hold.'Correspondence should be addressed to r.nyman@imperial.ac.uk Published by the American Physical Society under the terms o f theCreative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the pu...
We demonstrate runaway evaporative cooling directly with a tightly confining optical-dipole trap and achieve fast production of condensates of 1.5ϫ 10 5 87Rb atoms. Our scheme uses a misaligned crossed-beam far off-resonance optical-dipole trap ͑MACRO-FORT͒. It is characterized by independent control of the trap confinement and depth allowing forced all-optical evaporation in the runaway regime. Although our configuration is particularly well suited to the case of 87 Rb atoms in a 1565 nm optical trap, where an efficient initial loading is possible, our scheme is general and will allow all-optical evaporative cooling at constant stiffness for every optically trappable atomic or even molecular species.
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