Polariton parametric amplifier pump dynamics in the coherent regime

We studied the pump coherent dynamics in a II-VI microcavity parametric amplifier, using angle-resolved four-wave mixing. The polariton parametric amplification is found to result in a strong quenching and saturation of the pump coherence lifetime above the threshold. For the polariton scattering processes that remain below the amplification threshold, we find an angle-dependent collision broadening associated with the efficiency of the polariton scattering towards the excitonic reservoir.     

Spatial coherence of a polariton condensate

We perform Young's double-slit experiment to study the spatial coherence properties of a two-dimensional dynamic condensate of semiconductor microcavity polaritons. The coherence length of the system is measured as a function of the pump rate, which confirms a spontaneous buildup of macroscopic coherence in the condensed phase. An independent measurement reveals that the position and momentum uncertainty product of the condensate is close to the Heisenberg limit. An experimental realization of such a minimum uncertainty wave packet of the polariton condensate opens a door to coherent matter-wave phenomena such as Josephson oscillation, superfluidity, and solitons in solid state condensate systems.     

Spin dynamics of a vortical condensate of exciton polaritons in a GaAs microcavity

The temporal dynamics of a spinor exciton-polariton condensate in a high-quality anisotropic GaAs microcavity under pulsed resonant excitation with light possessing a nonzero orbital angular momentum is investigated. The phenomenon of spatial separation of the spin components of the polariton condensate upon pumping with a coherent superposition of two beams with opposite circular polarizations and orbital angular momenta is observed. The key factors for the observation of this effect are the lateral anisotropy of the microcavity that causes a splitting between the linear components of the polariton ground state and the occurrence of opposite orbital angular momenta for the two spin components of the condensate. The experimental results are in qualitative agreement with the theoretical model of the phenomenon developed in JETP Lett. 104, 827 (2016).     

Second-order time correlations within a polariton Bose-Einstein condensate in a CdTe microcavity

Second-order time correlations of polaritons have been measured across the condensation threshold in a CdTe microcavity. The onset of Bose-Einstein condensation is marked by the disappearance of photon bunching, demonstrating the transition from a thermal-like state to a coherent state. Coherence is, however, degraded with increasing polariton density, most probably as a result of self-interaction within the condensate and scatterings with noncondensed excitons and polaritons. Such behavior clearly differentiates polariton Bose condensation from photon lasing.     

Bessel型光晶格中自旋-轨道耦合极化激元凝聚的稳态结构

Bessel型光晶格是一种非空间周期性的柱对称的光晶格势场,其兼具无限深势阱和环状势阱的特征,在0阶Bessel光晶格势场中央形成深势阱,而在非0阶Beseel光晶格势场中能形成具有中央势垒的环状浅势阱.极化激元是一种半光半物质的准粒子,该准粒子甚至可以在室温条件下发生玻色-爱因斯坦凝聚相变,形成极化激元凝聚.另外,通过极化激元能级的腔诱导TE-TM分裂能在极化激元凝聚中实现足够强的自旋-轨道耦合作用.极化激元凝聚能在室温条件下实现,在其中又存在自旋-轨道耦合作用,其为量子物理的研究提供了全新的平台.本文把Bessel光晶格势场引入到极化激元凝聚系统,研究了存在自旋-轨道耦合作用下的旋量双组分极化激元凝聚系统的稳态结构.通过求解Gross-Pitaevskii方程给出了极化激元凝聚系统在实验室坐标系和旋转坐标系中极化激元凝聚系统的稳态结构,由于Bessel势场的引入,使得稳态结构更具有多样性.给出了实验室坐标系中在中央深势阱中存在的基础型高斯孤立子、多极孤立子和在环状浅势阱中存在环状孤立子和多极孤立子的稳态结构;给出了旋转坐标系中存在的涡旋环状孤立子,及其由于自旋-轨道相互作用引起的组...     

Interactions in confined polariton condensates

We investigate the effect of interactions in zero-dimensional polariton condensates. The shape of the condensate wave function is shown to be modified by repulsive interactions with the reservoir of uncondensed excitons. In large micropillar cavities, when uncondensed excitons are located at the center, the condensate is ejected toward the pillar edges. The same effect results in the generation of optical traps in wire cavities. Once polariton condensates are spatially separated from the excitonic reservoir, spectral signatures of polariton-polariton interactions within the condensate are evidenced.     

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.     

Room-temperature polariton parametric scattering driven by a one-dimensional polariton condensate

We demonstrate a novel way to realize room-temperature polariton parametric scattering in a one-dimensional ZnO microcavity. The polariton parametric scattering is driven by a polariton condensate, with a balanced polariton pair generated at the adjacent polariton mode. This parametric scattering is experimentally investigated by the angle-resolved photoluminescence spectroscopy technique under different pump powers and it is well described by the rate equation of interacting bosons. The direct relation between the intensity of the scattered polariton signal and that of the polariton reservoir is acquired under nonresonant excitation, exhibiting the explicit nonlinear characteristic of this room-temperature polariton parametric process.     

Soliton excitations in a polariton condensate with defects

To study soliton excitations in a polariton condensate with defects, we use the Gross-Pitaevskii equation and its hydrodynamic form. An extra term is added to take into account the non-equilibrium nature of the polariton condensate and the presence of defects. The reductive perturbation method transforms these hydrodynamic equations into a modified Korteweg-de Vries equation in the long wavelength limit. We linearize this equation and study the soliton linear excitations.We give an analytic expression of traveling excitations using the variation of constants method. In the more general form,we show numerically that the excitations are oscillations, i.e., the amplitude and the width of the dark soliton oscillate simultaneously but in an opposite way.     

Half-quantum vortices in exciton–polariton condensates in applied magnetic field

We study the behavior of half-quantum vortices (HQVs) in exciton–polariton condensates in planar semiconductor microcavities in applied magnetic field. Below the critical magnetic field, that is defined by the polariton–polariton interaction constant, the condensate is elliptically polarized and there are two types of HQVs, deep and shallow. They correspond to singularities in majority and minority circular components of the condensate wave function, respectively. The core radius (healing length) of the deep HQVs decreases and the core radius of the shallow HQVs increases with increase of magnetic field. The shallow HQVs disappear and the deep HQVs transform into the integer vortices in the circularly polarized condensate when the applied magnetic field exceeds the critical one.     

Oblique solitons generated by the flow of a polariton condensate past an obstacle

The formation of oblique solitons in a polariton condensate flowing past an obstacle is considered. Because of the finite lifetime of polaritons, the condensate flow is inhomogeneous, which leads to a significant modification of the conditions necessary for the generation of oblique solitons as compared to the conditions established earlier for the flow of an atomic condensate. In particular, it is established that oblique solitons in the polariton case can be generated by a subsonic flow of the condensate in agreement with the results of recent experiments [9]. The geometric shape and other parameters of oblique solitons are analytically calculated using a model based on the nonlinear Schr?dinger equation with damping, and the analytical results are confirmed by numerical simulations.     

Nonlinear optical spin Hall effect and long-range spin transport in polariton lasers

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.     

Polariton condensates at room temperature

We review the recent developments of the polariton physics in microcavities featuring the exciton–photon strong coupling at room temperature, and leading to the achievement of room-temperature polariton condensates. Such cavities embed active layers with robust excitons that present a large binding energy and a large oscillator strength, i.e. wide bandgap inorganic or organic semiconductors, or organic molecules. These various systems are compared, in terms of figures of merit and of common features related to their strong oscillator strength. The various demonstrations of polariton laser are compared, as well as their condensation phase diagrams. The room-temperature operation indeed allows a detailed investigation of the thermodynamic and out-of-equilibrium regimes of the condensation process. The crucial role of the spatial dynamics of the condensate formation is discussed, as well as the debated issue of the mechanism of stimulated relaxation from the reservoir to the condensate under non-resonant excitation. Finally the prospects of polariton devices are presented.     

Coherence properties and luminescence spectra of condensed polaritons in CdTe microcavities

We analyse the spatial and temporal coherence properties of a two-dimensional and finite sized polariton condensate with parameters tailored to the recent experiments which have shown spontaneous and thermal equilibrium polariton condensation in a CdTe microcavity [J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeambrun, J.M.J. Keeling, F.M. Marchetti, M.H. Szymanska, R. Andre, J.L. Staehli, et al., Nature 443 (7110) (2006) 409]. We obtain a theoretical estimate of the thermal length, the lengthscale over which full coherence effectively exists (and beyond which power-law decay of correlations in a two-dimensional condensate occurs), of the order of 5 μm. In addition, the exponential decay of temporal coherence predicted for a finite size system is consistent with that found in the experiment. From our analysis of the luminescence spectra of the polariton condensate, taking into account pumping and decay, we obtain a dispersionless region at small momenta of the order of 4 degrees. In addition, we determine the polariton linewidth as a function of the pump power. Finally, we discuss how, by increasing the exciton-photon detuning, it is in principle possible to move the threshold for condensation from a region of the phase diagram where polaritons can be described as a weakly interacting Bose gas to a region where instead the composite nature of polaritons becomes important.     

Nonlinear Dynamics of Exciton-Polariton Condensates in a One-Dimensional Lattice

We consider the problem of exciton-polariton-condensate formation in a semiconductor microcavity in the strong coupling regime. The condensate is confined in a one-dimensional periodic potential and coupled to an exciton reservoir that is formed by the external cw pumping. The condensate dynamics is studied in the center and in the edges of the Brillouin zone (BZ). Modeling the formation of the condensate from weak initial noise shows that besides steady states the macroscopic oscillations of polariton density can also occur. Within the framework of the mean field approach, we obtain important analytic relations for the condensate eigenstates using the developed simplified model for three coupled spatial harmonics. A numerical analysis verifies the correctness of the analytic results.     

Coherent response of a semiconductor microcavity in the strong coupling regime

We have studied the coherent dynamics of a semiconductor microcavity by means of interferometric correlation measurements with subpicosecond time resolution in a backscattering geometry. Evidence is brought of the resolution of a homogeneous polariton line in an inhomogeneously broadened exciton system. Surprisingly, photon-like polaritons exhibit an inhomogeneous dephasing. Moreover, we observe an unexpected stationary coherence up to 8 ps for the lower polariton branch close to resonance. All these experimental results are well reproduced within the framework of a linear dispersion theory assuming a coherent superposition of the reflectivity and resonant Rayleigh scattering signals with a well-defined relative phase.     

Investigation of a nonequilibrium polariton condensate in cylindrical micropillars in a strong magnetic field

We analyze the photoluminescence of a nonequilibrium polariton condensate in cylindrical micropillars etched on the surface of a high-Q GaAs microcavity in a wide range of detunings in a magnetic field up to 12 T for various levels of nonresonant laser pumping by nanosecond pulses. With such a method of excitation, a considerable effect of the interaction of the reservoir of photoexcited excitons with the condensate on the Zeeman splitting of the polariton condensate levels can be expected, which can lead to a decrease in its value and even to sign reversal. However, the measurements of photoluminescence in a wide range of optical excitation densities show that Zeeman splitting weakly depends on the optical pumping (its variation does not exceed 15% of the splitting in a field of 12 T). The estimation of the exciton density in the reservoir based on these data gives a value lower than 10⁸ cm^–2. In addition, a noticeable decrease (by a factor of about 1.8) in the polariton condensation threshold in a magnetic field is detected.     

High-temperature Bose-Einstein condensation of photonlike atom-light polaritons

We propose a method for obtaining a Bose-Einstein condensate of an ideal one-dimensional gas of atom-light polaritons of the lower branch in a trap at high temperatures (on the order of room temperature or higher). The system under consideration is an atomic ensemble that interacts with a single-mode quantum electromagnetic field in the presence of optical collisions with a high-pressure buffer gas. The possibility of using biconical waveguides for revealing the predicted phenomenon is discussed. The dynamics of the polariton condensate in the trap is investigated on the basis of the variational approach.     

Bragg polaritons: strong coupling and amplification in an unfolded microcavity

Periodic incorporation of quantum wells inside a one-dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic band gap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification.     

Controlled switching between quantum states in the exciton–polariton condensate

Optically controlled switching between modes of a polariton laser having different symmetries has been demonstrated experimentally. The microscopic shift of the optical excitation spot dramatically changes the shape of the polariton condensate formed in a cylindrical micropillar on the basis of the planar semiconductor microcavity. Switching between the ring and lobed condensate is achieved owing to the violation of the cylindrical symmetry of the effective potential formed by the lateral surface of the pillar and by the cloud of incoherent excitons created by optical pumping.