40 research outputs found
Spatial correlation of two-dimensional Bosonic multimode condensates
This research has been supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program),” by Navy/SPAWAR Grant No. N66001-09-1-2024, and by National Science Foundation Grant No. ECCS-09 25549. W.H.N. acknowledges a Gerhard Casper Stanford Graduate Fellowship for support.The Berezinskii-Kosterlitz-Thouless (BKT) theorem predicts that two-dimensional Bosonic condensates exhibit quasi-long-range order which is characterized by a slow decay of the spatial coherence. However previous measurements on exciton-polarition condensates revealed that their spatial coherence can decay faster than allowed under the BKT theory, and different theoretical explanations have already been proposed. Through theoretical and experimental study of exciton-polariton condensates, we show that the fast decay of the coherence can be explained through the simultaneous presence of multiple modes in the condensate.Publisher PDFPeer reviewe
Single vortex-antivortex pair in an exciton polariton condensate
In a homogeneous two-dimensional system at non-zero temperature, although
there can be no ordering of infinite range, a superfluid phase is predicted for
a Bose liquid. The stabilization of phase in this superfluid regime is achieved
by the formation of bound vortex-antivortex pairs. It is believed that several
different systems share this common behaviour, when the parameter describing
their ordered state has two degrees of freedom, and the theory has been tested
for some of them. However, there has been no direct experimental observation of
the phase stabilization mechanism by a bound pair. Here we present an
experimental technique that can identify a single vortex-antivortex pair in a
two-dimensional exciton polariton condensate. The pair is generated by the
inhomogeneous pumping spot profile, and is revealed in the time-integrated
phase maps acquired using Michelson interferometry, which show that the
condensate phase is only locally disturbed. Numerical modelling based on open
dissipative Gross-Pitaevskii equation suggests that the pair evolution is quite
different in this non-equilibrium system compared to atomic condensates. Our
results demonstrate that the exciton polariton condensate is a unique system
for studying two-dimensional superfluidity in a previously inaccessible regime
Signature of the microcavity exciton-polariton relaxation mechanism in the polarization of emitted light
We have performed real and momentum space spin-dependent spectroscopy of
spontaneously formed exciton polariton condensates for a non-resonant pumping
scheme. Under linearly polarized pump, our results can be understood in terms
of spin-dependent Boltzmann equations in a two-state model. This suggests that
relaxation into the ground state occurs after multiple phonon scattering events
and only one polariton-polariton scattering. For the circular pumping case, in
which only excitons of one spin are injected, a bottleneck effect is observed,
implying inefficient relaxation.Comment: 7 pages, 7 figure
Role of supercurrents on vortices formation in polariton condensates
Observation of quantized vortices in non-equilibrium polariton condensates
has been reported either by spontaneous formation and pinning in the presence
of disorder or by imprinting them onto the signal or idler of an optical
parametric oscillator (OPO). Here, we report a detailed analysis of the
creation and annihilation of polariton vortex-antivortex pairs in the signal
state of a polariton OPO by means of a short optical Gaussian pulse at a
certain finite pump wave-vector. A time-resolved, interferometric analysis of
the emission allows us to extract the phase of the perturbed condensate and to
reveal the dynamics of the supercurrents created by the pulsed probe. This flow
is responsible for the appearance of the topological defects when
counter-propagating to the underlying currents of the OPO signal.Comment: 8 pages, 5 figure
Vortices in polariton OPO superfluids
This chapter reviews the occurrence of quantised vortices in polariton
fluids, primarily when polaritons are driven in the optical parametric
oscillator (OPO) regime. We first review the OPO physics, together with both
its analytical and numerical modelling, the latter being necessary for the
description of finite size systems. Pattern formation is typical in systems
driven away from equilibrium. Similarly, we find that uniform OPO solutions can
be unstable to the spontaneous formation of quantised vortices. However,
metastable vortices can only be injected externally into an otherwise stable
symmetric state, and their persistence is due to the OPO superfluid properties.
We discuss how the currents charactering an OPO play a crucial role in the
occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure
Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots
Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent "dark" nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed - the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time