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Register a new userEffect of Interactions on Vortices in a Non-equilibrium Polariton Condensate
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Added by
Dmitriy Krizhanovskii Dr
Publication date
2013
fields
Physics
and research's language is
English
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We demonstrate the creation of vortices in a macroscopically occupied polariton state formed in a semiconductor microcavity. A weak external laser beam carrying orbital angular momentum (OAM) is used to imprint a vortex on the condensate arising from the polariton optical parametric oscillator (OPO). The vortex core radius is found to decrease with increasing pump power, and is determined by polariton-polariton interactions. As a result of OAM conservation in the parametric scattering process, the excitation consists of a vortex in the signal and a corresponding anti-vortex in the idler of the OPO. The experimental results are in good agreement with a theoretical model of a vortex in the polariton OPO.
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Polariton condensates are investigated in periodical potentials created by surface acoustic waves using both resonant and non-resonant optical excitation. Under resonant pumping condensates are formed due to polariton parametric scattering from the pump. In this case the single particle dispersion in the presence of the condensate shows a strong reduction of the energy gap arising from the acoustic modulation, indicating efficient screening of the surface acoustic wave potential by spatial modulation of the polariton density. The observed results are in good agreement with a model based on generalised Gross-Pitaveskii equations with account taken of the spatial dependence of the exciton energy landscape. In the case of incoherent, non-resonant pumping coexisting non-equilibrium condensates with s- and p- type wavefunctions are observed, which have different energies, symmetry and spatial coherence. The energy splitting between these condensate states is also reduced with respect to the gap of the one particle spectrum below threshold, but the screening effect is less pronounced than in the case of resonantly pumped system due to weaker modulation of the pump state.
One-dimensional polariton condensates (PoCos) in a photonic wire are generated through non-resonant laser excitation, by which also a reservoir of background carriers is created. Interaction with this reservoir may affect the coherence of the PoCo, which is studied here by injecting a condensate locally and monitoring the coherence along the wire. While the incoherent reservoir is mostly present within the excitation laser spot, the condensate can propagate ballistically through the wire. Photon correlation measurements show that far from the laser spot the second order correlation function approaches unity value, as expected for the coherent condensed state. When approaching the spot, however, the correlation function increases up to values of 1.2 showing the addition of noise to the emission due to interaction with the reservoir. This finding is substantiated by measuring the first order coherence by a double slit experiment, which shows a reduced visibility of interference at the excitation laser spot.
The suppression of Zeeman energy splitting due to spin-dependent interactions within a Bose-Einstein condensate (the spin Meissner effect) was predicted to occur up to a certain value of magnetic field strength. We report a clear observation of this effect in semimagnetic microcavities which exhibit the giant Zeeman energy splitting between two spin-polarised polariton states as high as 2 meV, and demonstrate that partial suppression of energy difference occurs already in the uncondensed phase in a striking similarity to the up-critical superconductors in the fluctuation dominated regime. These observations are explained quantitatively by a kinetic model accounting for both the condensed and uncondensed polaritons and taking into account the non-equilibrium character of the system.
Quantum vortices, the quantized version of classical vortices, play a prominent role in superfluid and superconductor phase transitions. However, their exploration at a particle level in open quantum systems has gained considerable attention only recently. Here we study vortex pair interactions in a resonant polariton fluid created in a solid-state microcavity. By tracking the vortices on picosecond time scales, we reveal the role of nonlinearity, as well as of density and phase gradients, in driving their rotational dynamics. Such effects are also responsible for the split of composite spin-vortex molecules into elementary half-vortices, when seeding opposite vorticity between the two spinorial components. Remarkably, we also observe that vortices placed in close proximity experience a pull-push scenario leading to unusual scattering-like events that can be described by a tunable effective potential. Understanding vortex interactions can be useful in quantum hydrodynamics and in the development of vortex-based lattices, gyroscopes, and logic devices.
Singly quantized vortices have been already observed in many systems including the superfluid helium, Bose Einstein condensates of dilute atomic gases, and condensates of exciton polaritons in the solid state. Two dimensional superfluids carrying spin are expected to demonstrate a different type of elementary excitations referred to as half quantum vortices characterized by a pi rotation of the phase and a pi rotation of the polarization vector when circumventing the vortex core. We detect half quantum vortices in an exciton-polariton condensate by means of polarization resolved interferometry, real space spectroscopy and phase imaging. Half quantum vortices coexist with single quantum vortices in our sample.
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