The stability of spin of macroscopic quantum states to intrinsic noise is studied for non-resonantly pumped optically-trapped polariton condensates. We demonstrate flipping between the two spinpolarised states with >10⁴ slow-down of the flip rate by tuning the optical pump power. Individual spin flips faster than 50 picosecond are time resolved using single-shot streak camera imaging. We reproduce our results within a mean-field model accounting for cross-spin scattering between excitons and polaritons, yielding a ratio of cross- to co-spin scattering of ~0.6, in contrast with previous literature suggestions.
The stability of spin of macroscopic quantum states to intrinsic noise is studied for non-resonantly pumped optically-trapped polariton condensates. We demonstrate flipping between the two spinpolarised states with >10⁴ slow-down of the flip rate by tuning the optical pump power. Individual spin flips faster than 50 picosecond are time resolved using single-shot streak camera imaging. We reproduce our results within a mean-field model accounting for cross-spin scattering between excitons and polaritons, yielding a ratio of cross- to co-spin scattering of ~0.6, in contrast with previous literature suggestions.
The stability of spin of macroscopic quantum states to intrinsic noise is studied for non-resonantly-pumped optically-trapped polariton condensates. We demonstrate flipping between the two spin-polarised states with >10 4 slow-down of the flip rate by tuning the optical pump power. Individual spin flips faster than 50 ps are time resolved using single-shot streak camera imaging. We reproduce our results within a mean-field model accounting for cross-spin scattering between excitons and polaritons, yielding a ratio of cross- to co-spin scattering of ∼0.6, in contrast with previous literature suggestions.
Exciton-polariton Bose-Einstein condensates (BECs) are newly emerged quantum systems that are capable of showing macroscopic quantum phenomena with intrinsic open-dissipative nature. The spatial distribution of the polariton density, without any external potential, can be controlled by the geometric shape of the pumping laser, enabling the investigation of polariton dynamics with topologically non-trivial configurations. Meanwhile, exciton-polaritons have spin degrees of freedom inherited from excitons and photons, making it a candidate for the realization of quantum logic gates. In this thesis, we will investigate theoretically the nonlinear dynamics of exciton-polariton BECs involving both polaritons' spatial degrees of freedom and spin degrees of freedom, and interactions between them. This thesis is organised as follows: In Chapter 1, we will present an overall review of exiton-polariton systems and important properties of polariton BECs and then introduce the dynamical equations with various interactions that will serve as the main theoretical tool for subsequent chapters. Several polariton pumping and trapping techniques appearing in later chapters will also be introduced. In Chapter 2, we will investigate the superfluidity properties of a single-component polariton condensate under an incoherent annular pumping configuration. By studying the stability properties of polariton persistent currents, we find that the persistent currents can exhibit dynamical instability and energetic-like instability according to different parameter region. A stability phase diagram will be given and its relation with the Landau's criterion will be discussed. In Chapter 3, we will investigate the spin dynamics of a two-component polariton condensate under a homogeneous pumping configuration. Owing to the Josephson coupling, there exist multiple steady state solutions that allow of controlled spin state switching. A desynchronized region where there exists no stable steady solution is found. In the desynchronized region, a desynchronized state beating periodically over time can exist, which will serve as a building block of spin waves presented in the next chapter. In Chapter 4, by combining results from the previous two chapters we will investigate generally the nonlinear dynamics of polariton condensates under an annular pumping configuration. The spin-orbit interaction provided by the Josephson coupling supports azimuthon states that have simultaneous modulations in both amplitude and phase. The azimuthon states, when viewed in a different polarization basis, form rotating spin waves that can be referred to as the optical ferris wheel. In Chapter 5, results from previous chapters will be extended to micocavities that support the anisotropic TE-TM splitting interaction. Rotating singularities (small-scale vortices) are found as a result. Their properties and experimental observation techniques will be discussed. Chapter 2-5 provide a theoretical framework for the nonlinear dynamics of polariton condensates. They rely mostly on optical trapping techniques and are ready to be tested in experiments. In Chapter 6, polaritons trapped by an engineered periodic mesa potential will be discussed. We will investigate the band structure of polaritons under the influence of the periodic potential together with discussions on the phase-modulated interference pattern which corresponds to the polariton Talbot patterns observed in experiments.
We resolve single-shot polariton condensate polarization dynamics, revealing a high degree of circular polarization persistent up to T=170 K. The statistical analysis of pulse-to-pulse polariton condensate polarization elucidates the stochastic nature of the polarization pinning process, which is strongly dependent on the pump laser intensity and polarization. Our experiments show that by spatial trapping and isolating condensates from their noisy environment it is possible to form strongly spin-polarized polariton condensates at high temperatures, offering a promising route to the realization of polariton spin lattices for quantum simulations.
Polaritons are quasiparticles resulting from the strong coupling between photons and excitons embedded in a semiconductor microcavity. The exciton component of polaritons provides strong nonlinear interactions, while their photonic counterpart facilitates long range coherence. Moreover, polaritons carry a pseudospin that can be accessed through the polarization state of the emitted light, and brings up spinor related nonlinear effects. This study aims at in-depth investigation of the noise related phenomena in polariton system. First, a series of experiments of polariton bistability and multistability are performed to deepen our knowledge about the effect of the biexciton creation and polariton-polariton interaction on multistable regimes in exciton polaritons. Then, using an external Gaussian noise, the polariton transition rate between two stable states of a polariton bistability is characterized. It is shown that the external noise specifications, intensity and correlation time, can efficiently modify the polariton Kramers time and residence time. We also discuss the performance of the bistable behaviour in steady state regime in terms of experimental acquisition time compared to the noise-assisted residence time. In the next part, taking advantage of polariton bistability and spin-trigger regime, we have evidenced two different types of stochastic resonance. Intensity stochastic resonance and spinor stochastic resonance. We have shown that the synergic interplay between intensity fluctuations and external modulated signal, imprinted on the DC component of the driving field, can enhance the coherent processing of an input signal buried in noise. Moreover, we evidence that, due to the exceptional spin properties of polaritons, a noisy modulated polarized input signal can drive the switching of a fully polarized polariton population. This effect unveils an original field of stochastic resonance which we called as spinor stochastic resonance. All experimental results are well reproduced by a model based on Gross-Pitaevskii equation. At the end, the influence of nonlinear interactions on polariton intensity fluctuations and probability distribution have been investigated.
Ultrafast transitions between opposite-spin states of multicomponent bosonic condensates, proceeding on the timescale comparable to the lifetime of the particles the condensate consists of, are demonstrated in a resonantly excited system of cavity polaritons in a magnetic field. Spin-sensitive pair interactions enable fast and massive spin flips occurring at critical amplitudes due to the interplay between amplitude-dependent shifts of eigenstates and the Zeeman splitting. Dominant spin of a condensate can be toggled back and forth by tuning of the pump intensity only, which was impossible in spin-degenerate cavity-polariton systems, cavities with weak exciton-photon coupling, and other previously known photonic systems and devices.
The spin and intensity of optically trapped polariton condensates are studied under steady-state elliptically polarized nonresonant pumping. Three distinct effects are observed: (1) spin inversion where condensation occurs in the opposite handedness from the pump, (2) spin and intensity hysteresis as the pump power is scanned, and (3) a sharp ``spin collapse'' transition in the condensate spin as a function of the pump ellipticity. We show these effects are strongly dependent on trap size and sample position and are linked to small counterintuitive energy differences between the condensate spin components. Our results, which fail to be fully described within the commonly used nonlinear equations for polariton condensates, show that a more accurate microscopic picture is needed to unify these phenomena in a two-dimensional condensate theory.