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Register a new userFormation Dynamics and Evaporative Cooling of a Dark Exciton Condensate
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We study the formation of an exciton condensate in GaAs coupled quantum wells at low temperatures. We show that the condensate consists of dark excitons, and extends over hundreds of {mu}m, limited only by the boundaries of the mesa. We find that the condensate density is determined by spin flipping collisions among the condensate excitons and with the thermal bath. We show that these processes, which convert dark excitons to bright, evaporatively cool the system to temperatures that are much lower than the bath temperature. We present a rate equations model, which explains the temperature and power dependence of the exciton density, and in particular - the large density buildup at low temperatures. We confirm the validity of the model by reproducing the unique behavior observed when a magnetic field is applied in a direction parallel to the layers.
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We present an experimental study on the ignition and decay of a polariton optical parametric oscillator (OPO) in a semiconductor microcavity pillar. The combination of a continuous wave laser pump, under quasi-phase matching conditions, and a non-resonant, 2 ps-long pulse probe allows us to obtain the full dynamics of the system. The arrival of the probe induces a blue-shift in the polariton emission, bringing the OPO process into resonance with the pump, which triggers the OPO-process. We time-resolve the polariton OPO signal emission for more than 1 nanosecond in both real and momentum-space. We fully characterize the emission of the OPO signal with spectral tomography techniques. Our interpretations are backed up by theoretical simulations based on the 2D coupled Gross-Pitaevskii equation for excitons and photons.
We calculate the evaporative cooling dynamics of trapped one-dimensional Bose-Einstein condensates for parameters leading to a range of condensates and quasicondensates in the final equilibrium state. We confirm that solitons are created during the evaporation process, but always eventually dissipate during thermalisation. The distance between solitons at the end of the evaporation ramp matches the coherence length in the final thermal state. Calculations were made using the classical fields method. They bridge the gap between the phase defect picture of the Kibble-Zurek mechanism and the long-wavelength phase fluctuations in the thermal state.
We report on our recent progress in the manipulation and cooling of a magnetically guided, high flux beam of $^{87}{rm Rb}$ atoms. Typically $7times 10^9$ atoms per second propagate in a magnetic guide providing a transverse gradient of 800 G/cm, with a temperature $sim550$ $mu$K, at an initial velocity of 90 cm/s. The atoms are subsequently slowed down to $sim 60$ cm/s using an upward slope. The relatively high collision rate (5 s$^{-1}$) allows us to start forced evaporative cooling of the beam, leading to a reduction of the beam temperature by a factor of ~4, and a ten-fold increase of the on-axis phase-space density.
Coherent coupling between excitons is at the heart of many-body interactions with transition metal dichalcogenide (TMD) heterostructures as an emergent platform for the investigation of these interactions. We employ multi-dimensional coherent spectroscopy on monolayer MoSetextsubscript{2}/WSetextsubscript{2} heterostructures and observe coherent coupling between excitons spatially localized in monolayer MoSe$_2$ and WSe$_2$. Through many-body spectroscopy, we further observe the absorption state arising from free interlayer electron-hole pairs. This observation yields a spectroscopic measurement of the interlayer exciton binding energy of about 250 meV.
Interacting Bosons, loaded in artificial lattices, have emerged as a modern platform to explore collective manybody phenomena, quantum phase transitions and exotic phases of matter as well as to enable advanced on chip simulators. Such experiments strongly rely on well-defined shaping the potential landscape of the Bosons, respectively Bosonic quasi-particles, and have been restricted to cryogenic, or even ultra-cold temperatures. On chip, the GaAs-based exciton-polariton platform emerged as a promising system to implement and study bosonic non-linear systems in lattices, yet demanding cryogenic temperatures. In our work, we discuss the first experiment conducted on a polaritonic lattice at ambient conditions: We utilize fluorescent proteins as an excitonic gain material, providing ultra-stable Frenkel excitons. We directly take advantage of their soft nature by mechanically shaping them in the photonic one-dimensional lattice. We demonstrate controlled loading of the condensate in distinct orbital lattice modes of different symmetries, and finally explore, as an illustrative example, the formation of a gap solitonic mode, driven by the interplay of effective interaction and negative effective mass in our lattice. The observed phenomena in our open dissipative system are comprehensively scrutinized by a nonequilibrium model of polariton condensation. We believe, that this work is establishing the organic polariton platform as a serious contender to the well-established GaAs platform for a wide range of applications relying on coherent Bosons in lattices, given its unprecedented flexibility, cost effectiveness and operation temperature.
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