Dynamics of exciton-polariton multistability is theoretically investigated. Phase portraits are used as a tool to enlighten the microscopic phenomena which influence spin multistability of a confined polariton field as well as ultrafast reversible spin switching. The formation of a non-radiative reservoir, due to polariton pairing into biexcitons is found to play the lead role in the previously reported spin switching experiments. Ways to tailor this reservoir formation are discussed in order to obtain optimal spin switching reliability.
Dynamics of exciton-polariton multistability is theoretically investigated. Phase portraits are used as a tool to enlighten the microscopic phenomena which influence spin multistability of a confined polariton field as well as ultrafast reversible spin switching. The formation of a non-radiative reservoir, due to polariton pairing into biexcitons is found to play the lead role in the previously reported spin switching experiments. Ways to tailor this reservoir formation are discussed in order to obtain optimal spin switching reliability.
We present a theoretical study of spin multistability and hysteresis in resonantly pumped exciton–polaritons within semiconductor microcavities. This multistable behavior arises from the nonlinear, polarization-dependent anisotropy of polariton–polariton interactions. Unlike conventional optical bistability in spinless nonlinear optical systems, the dynamics are governed by spin-selective coupling. We calculate the steady-state polariton density and degree of circular polarization of the intracavity polariton field, demonstrating that for a fixed pump polarization, wave vector, and detuning, the degree of polarization could take on multiple values depending on the pump strength. This spin-sensitive multistability lays the groundwork for multichannel optical switches governed not just by total pump intensity but also by its pump polarization.
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.
Experimental studies have been performed of the nonequilibrium transitions in multistable cavity-polariton systems under 10 ps long pulsed excitation, i.e., within nearly the shortest time scale necessary for the multistability to be revealed in GaAs-based microcavities. The experiments were aimed to determine the ratio of the strengths of the pair interactions between polaritons with parallel and antiparallel spins, which also was the question raised in a number of recent studies. We show that in contrast to the cases of continuous-wave or nanosecond long pumping conditions, in the fast pulse regime the system is not affected by the excitonic reservoir. The interaction between polaritons with opposite spins is definitely attractive at a nearly zero detuning of exciton and photon modes; as a result, optical response can be essentially multistable at a linearly polarized excitation. With increasing pulse duration up to 100 ps, the reservoir can start manifesting itself in a reduction of the spin multistability.
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.
We investigate the multistability of exciton-polariton condensates excited by a nonresonant pump. An increase in pumping power moves the system away from non-Hermitian spectral degeneracy towards spectrum splitting through an exceptional point, which induces a transition from monostability to multistability. In the region of multistability, the system contains one steady and two metastable states. The analyses of stability show that metastable states maintain a finite lifetime and eventually evolve to steady states. A steady state with multi-peak soliton different from general single-peak soliton is discovered for attractive polariton-polariton interaction. Moreover, we depict the diagram of the multistability in full parameter space to accurately manipulate the multistability. Our results open up exciting possibilities for controlling non-Hermitian quantum multistable states, which may be useful to designing polariton-based devices exploiting optical multistability.
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.