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.
We study the polarisation dependence of the homogeneously broadened nuclear spin resonance in a crystal. We employ a combinatorial method to restrict the nuclear states to a fixed polarisation and show that the centre of the resonance is shifted line
arly with the nuclear polarisation by up to the zero polarisation line width. The width shrinks from its maximum value at zero polarisation to zero at full polarisation. This suggests to use the line shape as a direct measure of nuclear polarisation reached under dynamical pumping. In the limit of single quantum of excitation above the fully ferromagnetic state, we provide an explicit solution to the problem of nuclear spin dynamics which links a bound on the fastest decay rate to the observable width of the resonance line.
Several mechanisms are discussed which could determine the spatial coherence of a polariton condensate confined to a one dimensional wire. The mechanisms considered are polariton-polariton interactions, disorder scattering and non-equilibrium occupat
ion of finite momentum modes. For each case, the shape of the resulting spatial coherence function g1(x) is analysed. The results are compared with the experimental data on a polariton condensate in an acoustic lattice from [E. A. Cerda-Mendez et al, Phys. Rev. Lett. 105, 116402 (2010)]. It is concluded that the shape of g1(x) can only be explained by non-equilibrium effects, and that ~10 modes are occupied in the experimental system.
