Quantum degenerate exciton-polaritons in thermal equilibrium

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice.     

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.     

Spatial condensation of trapped polaritons in graphene and semiconductor structures

The theoretical and experimental status of the Bose–Einstein Condensation (BEC) of trapped quantum well (QW) polaritons in a microcavity is presented. The results of recent experiments that have shown the possibility to create an in-plane harmonic potential trap for a two-dimensional (2D) exciton polaritons in a cavity are discussed. We report the theory of BEC and of the trapped QW exciton polaritons in a microcavity. In addition, we study the BEC of trapped magnetoexciton polaritons in a graphene layer (GL) embedded in an optical microcavity in high magnetic field. In both cases the polaritons are considered to be in a harmonic potential trap. We compare the theoretical results with the existing experiments and discuss the experimental observation of predicted phenomena.     

Fast polariton relaxation in strongly coupled organic microcavities

We consider the regime of strong light-matter coupling in an organic microcavity, where large Rabi splitting can be achieved. As has been shown, the excitation spectrum of such a structure, besides coherent polaritonic states, contains a number of strongly spatially localized incoherent excited states. These states form the majority of the excited states of the microcavity and are supposed to play the decisive role in the relaxation dynamics of the excitations in the microcavity. We consider the non-radiative transition from an incoherent excited state into one of the coherent states of the lower polaritonic branch accompanied by emission of a high-energy intramolecular phonon. It is shown that this process may determine the lifetime of incoherent excited states in the microcavity. This observation may be important in the discussion of pump–probe experiments with short pulses. This process may also play an important role for the population of the lowest energy states in organic microcavities, and hence in the problem of condensation of cavity polaritons.     

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.     

On the type of a phase transition in the system of exciton polaritons in an optical microcavity

The type of a phase transition in the quasi-equilibrium system of exciton polaritons in a two-dimensional optical microcavity has been analyzed. It has been shown that, although the system contains two types of bosons undergoing mutual transformations into each other, only one phase transition to the superfluid state with the quasilong-range order occurs in the two-dimensional system. This phase transition is a Kosterlitz-Thouless phase transition. A new physical implementation—excitons in a photon crystal—has been proposed for the Bose condensation of exciton polaritons. The superfluid properties of the ordered phase are discussed, and the superfluid density and Kosterlitz-Thouless transition temperature have been calculated in the low-density approximation.     

Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.     

Dynamics of a parametric exciton-polariton oscillator in a microcavity

We study the dynamics of parametric oscillations of polaritons in a microcavity that consists of a periodic conversion of a pair of pump polaritons into polaritons of signal and idle modes and vice versa. The period and amplitude of oscillations considerably depend on the initial polariton density, the initial phase difference, and the resonance detuning. We show that there is a possibility of phase controlling the polariton dynamics in the microcavity.     

Spontaneous coherent phase transition of polaritons in CdTe microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in k space, exhibiting enhanced spatial and angular coherence at the stimulation onset.     

Bose-einstein condensation of exciton polaritons in high-<Emphasis Type="Italic">Q</Emphasis> planar microcavities with GaAs quantum wells

Condensation of exciton polaritons in planar microcavities with GaAs/AlAs quantum wells in the active area has been studied. It has been found that an increase in the lifetime of polaritons up to ∼10–15 ps when the Q factor of a microcavity exceeds 7000 makes it possible to detect Bose-Einstein condensation of polaritons with a dominant (>90%) photon component. Condensation occurs under thermodynamically nonequilibrium conditions in lateral traps with diameters ∼10 μm formed due to long-range fluctuations of the polariton potential. The violet shift of the polariton emission line at the condensation threshold significantly exceeds the energy of the repulsive interaction between polaritons in the condensate. It has been shown that the shift is mainly due to a decrease in the oscillator strength of bright excitons in lateral traps, caused by the localization of photoexcited long-living dark excitons.     

表面等离子体-微腔激元对顶入射有机薄膜太阳能电池光吸收效率的增强

为了提高顶入射有机薄膜太阳能电池(TOSCs)的光吸收效率,我们将周期性矩形光栅结构引入到TOSCs中,分析了具有光栅结构的空气/Ag_1/有源层/Ag_2/空气(IMIMI)结构理想模型中复合表面等离子激元(SPPs)与微腔模式的耦合机制。通过调节光栅周期和有源层厚度,实现了复合SPPs、微腔模式以及有机材料本征吸收3个区域的重合。由于复合SPPs与微腔模式的反交叉耦合作用形成了表面等离子体-微腔激元,其局域场增强作用有效地提高了有源层的光吸收效率,提高了近19%。     

Formation of an exciton polariton condensate: thermodynamic versus kinetic regimes

We measure the polariton distribution function and the condensation threshold versus the photon-exciton detuning and the lattice temperature in a CdTe microcavity under nonresonant pumping. The results are reproduced by simulations using semiclassical Boltzmann equations. At negative detuning we find a kinetic condensation regime: the distribution is not thermal and the threshold is governed by the relaxation kinetics. At positive detuning, the distribution becomes thermal and the threshold is governed by the thermodynamic parameters of the system. Both regimes are a manifestation of polariton lasing, whereas only the latter is related to Bose-Einstein condensation defined as an equilibrium phase transition.     

Statistical physics of Bose-Einstein-condensed light in a dye microcavity

We theoretically analyze the temperature behavior of paraxial light in thermal equilibrium with a dye-filled optical microcavity. At low temperatures the photon gas undergoes Bose-Einstein condensation, and the photon number in the cavity ground state becomes macroscopic with respect to the total photon number. Owing to a grand-canonical excitation exchange between the photon gas and the dye molecule reservoir, a regime with unusually large fluctuations of the condensate number is predicted for this system that is not observed in present atomic physics Bose-Einstein condensation experiments.     

Characteristics of exciton polaritons in a ZnO microcavity

We have investigated the characteristics of exciton polaritons in a ZnO microcavity with HfO₂/SiO₂ distributed Bragg reflectors. The results of the angle-resolved reflectance spectra were analyzed by calculating the cavity polariton dispersions with a phenomenological Hamiltonian for the coupling between the cavity photon and three kinds of excitons labeled A, B, and C peculiar to ZnO. The vacuum-Rabi-splitting energy is estimated to be ∼80 meV. The reflectance dips originating from the cavity polaritons shift to lower energy side with an increase in temperature. We discuss the temperature dependence of the cavity-polariton energies of the ZnO microcavity on the basis of the phenomenological Hamiltonian taking account of the temperature dependence of the exciton energies with Varshni’s law.     

Micropillar resonator in a magnetic field: Zero and finite temperature cases

In this work, we present a theoretical study of a quantum dot–microcavity system which includes a constant magnetic field in the growth direction of the micropillar. First, we study the zero temperature case by means of a self-consistent procedure with a trial function composed of a coherent photon field and a BCS function for the electron–hole pairs. The dependence of the ground state energy on the magnetic field and the number of polaritons is found. We show that the magnetic field can be used as a control parameter for the photon number, and we make explicit the scaling of the total energy with the number of polaritons. Next, we study this problem at finite temperatures and obtain the scaling of the critical temperature with the number of polaritons.     

Thermodynamics and excitations of condensed polaritons in disordered microcavities

We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.     

Bose-Einstein condensation of stationary-light polaritons

We propose and analyze a mechanism for Bose-Einstein condensation of stationary dark-state polaritons. Dark-state polaritons (DSPs) are formed in the interaction of light with laser-driven 3-level Lambda-type atoms and are the basis of phenomena such as electromagnetically induced transparency, ultraslow, and stored light. They have long intrinsic lifetimes and in a stationary setup, a 3D quadratic dispersion profile with variable effective mass. Since DSPs are bosons, they can undergo a Bose-Einstein condensation at a critical temperature which can be many orders of magnitude larger than that of atoms. We show that thermalization of polaritons can occur via elastic collisions mediated by a resonantly enhanced optical Kerr nonlinearity on a time scale short compared to the decay time. Finally, condensation can be observed by turning stationary into propagating polaritons and monitoring the emitted light.     

Lasing behaviour from the condensation of polaronic excitons in a ZnO nanowire

Atoms under optical and magnetic trapping in a limited space at a very low temperature can lead to Bose-Einstein condensation (BEC),even in a one-dimensional (1D) optical lattice. However,can the confinment of dense excitons in a 1D semiconductor microstructure easily reach the excitonic BEC A lightly Mn(Ⅱ)-doped ZnO nanowire under a femtosecond laser pulse pump at room temperature produces single-mode lasing from coherent bipolaronic excitons,which is much like a macroscopic quantum state due to the condensation of the bipoaronic excitons if not real BEC. In this process,longitudinal biphonon binding with the exciton plays an important role. We revisit this system and propose possibility of bipolaronic exciton condensation. More studies are needed for this condensation phenomenon in 1D microcavity systems.     

非共振激发条件下微腔中激子自发辐射的时间分辨光谱

低温下,测量了InGaAs量子阱平面微腔的时间分辨光谱。在非共振激发条件下,观察到上下两支腔极化激元光荧光的衰退时间与失谐无关;下支腔极化激元光荧光的上升时间也与失谐无关;而上支腔极化激元光荧光的上升时间却与失谐有强烈的依赖关系,随着从负失谐到正失谐的增加,上升时间逐渐减小。对实验结果的物理根源进行了讨论。     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.