Stimulated parametric scattering of excitonic polaritons in planar GaAs microcavities: Distinctive feature of QW electric field

Instabilities in the electric field at the QW in the active region of a GaAs based microcavity have been investigated under the resonance excitation near the inflection point of the low polariton (LP) dispersion curve with the use of four wave mixing technique. The electric field jump due to an LP bistability has been found to precede the development of a stimulated parametric scattering of LPs. The latter has been found to develop with a delay of a few hundreds ps from the beginning of the excitation pulse. These results are in qualitative agreement with hard regime of excitation of the stimulated parametric LP scattering predicted recently.     

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.     

Bose glass and superfluid phase transitions of exciton-polaritons in GaN microcavities

Microcavity exciton-polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross-Pitaevskii theory provide further insight into the properties of exciton-polaritons in GaN-based microcavities.     

Bipolariton laser emission from a GaAs microcavity

Biexciton emission properties were studied in a single GaAs quantum well semiconductor planar microcavity by photoluminescence measurements at low temperatures. At high pump intensity a bipolariton emission appears close to the lower polariton mode. This new mode appears when we detune the cavity resonance out of the lower polariton branch, showing a laser-like behavior. Very small linewidths were measured, lying below 110 μeV and 150 μeV for polariton and bipolariton emission respectively. The input/output power (I/O) measurements show that the bipolariton emission has a weaker coupling efficiency compared to previous results for polariton emission. Varying the pump laser polarization, we were able to show the selection rules for the biexciton particle creation in the quantum well. Simultaneous photoluminescence and near-field measurements show that the polariton and bipolariton emission are spectrally and spatially separated.     

Modulation of Splitting Beam Angle with Metal--Nonlinear Optical Material--Metal (M-NL-M) Array Structure

We demonstrate active manipulating plasmonic signals with metal--nonlinear optical material--metal (M-NL-M) arrays consisting of slits with variant widths. The parameters of the M-NL-M array structure are derived by theoretical analysis of dispersion relationship. The splitting angle can be modulated by the incident light intensity, and verified by a nonlinear two-dimensional finite difference time domain method. The physical principle of this phenomenon is analysed from the phase of surface plasmon polaritons and Fabry--Pérot (F-P) resonance in slits     

Localized interface phonon polaritons in superlattice with a structural defect consisting of ternary mixed crystal

Using a transfer matrix method, we investigate the existence and characteristics of the localized interface phonon-polariton modes (IPPMs) in superlattice (SL) with a structural defect consisting of ternary mixed crystal (Al_xGa_1−xAs). The results show that the introduction of two-mode behavior of the ternary mixed crystal leads to the rich and varied localized IPPMs spectra with new features. The characteristics and positions of the localized IPPMs in the Reststrahlen region are strongly dependent on the concentration x of the ternary mixed crystal. Moreover, it is found that the localized IPPMs are sensitive to the thicknesses of the defect and constituent layers as well as to the transverse wavenumber q_∥.     

Characterization of aluminium concentration in shallow quantum wells AlxGa1−xAs/GaAs types

We report a reflectance study on series of shallow quantum wells GaAs/Al_xGa_1−xAs types with different aluminium concentration. The observed barrier exciton reflectance line shape depends strongly on the shift in aluminium concentration in the two barriers, with the appropriate choice of the cap layer thickness. This observation was based on the reflectivity line shape analysis of anti-Bragg structures.     

Polaritons in anisotropic semiconductors

We show how to compute the optical properties (reflection and absorption) of anisotropic semiconductors in the exciton energy region, taking into account polariton and electron-hole coherence effects. The method is applied to a GaAs/Ga_1–x Al _x As superlattice, and the modifications in the optical properties with respect to GaAs are related to the anisotropy.     

Strong correlation effects in 2D Bose-Einstein condensed dipolar excitons

By doing quantum Monte Carlo ab initio simulations we show that dipolar excitons, which are now under experimental study, actually are strongly correlated systems. Strong correlations manifest in significant deviations of excitation spectra from the Bogoliubov one, large Bose condensate depletion, short-range order in the pair correlation function, and peak(s) in the structure factor.     

Propagation and localization of polaritons in disordered organic microcavities

The effect of disorder on the polaritonic states in organic microcavities utilizing J aggregates of cyanine dyes is examined. The comparison between the elastic mean free path, the phase breaking length and the wavelength of polaritons shows that, by varying two control parameters, one can achieve different regimes of cavity polariton propagation and localization (including weak and strong localization) in one sample at room temperature. We analyze the role of different parameters of the sample in the possibility of realization of each regime.     

Secondary peak in the optical absorption spectra: A possible criterion for Bose condensation of excitons

By solving the BCS and Bethe-Salpeter equations, we confirm the result by Chu and Chang that a secondary peak appears in the optical absorption spectrum immediately after the 1S-exciton peak in the presence of a condensed phase. The observation of the secondary peak indicates the presence of exciton condensate.     

Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density , an exponential polarization decay is obtained, which is characterized by a dephasing rate . This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p⁰ with that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions . The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function F_k exhibits a clear threshold behaviour at the critical density. Received 13 December 1999     

Polariton-polariton kinematic interaction modulation and kinematic bipolaritons in organic microcavities

We study polariton-polariton kinematic interactions in organic microcavities. Using the Agranovich-Toshich transformation, to transform the Frenkel excitons from Paulions into Bosons, the exciton-exciton kinematic interaction is derived. In the strong coupling regime, the polariton excitonic part results in the polariton-polariton kinematic interaction. The scattering amplitude is calculated and the effective potential is obtained for a scattering between two free polaritons. The effective potential can be modulated by changing the exciton-cavity photon detuning, and we show the crossover of the effective potential from attractive into repulsive one. A pole in the two-particle Green's function is the signature of the formation of polariton bound state, i.e. bipolariton. Due to the smallness of the polariton effective mass, the obtained bound state is very shallow and appears below the minimum of the lower polariton branch, and falls inside the natural bandwidth of the polariton branch.     

Analysis of nonclassical behavior for a two-mode coupled-photon system

We consider a model two-mode coupled-photon system and verify that the photon distribution for this system is exactly super-Poissonian. We calculate the Glauber–Sudarshan diagonal P representation for both the individual photon subsystem and the complete photon–photon complex. We present the detailed analysis on the threshold temperature of the nonclassical behavior for the both cases. We discuss the effect of the interaction between two photons on the threshold temperature.     

Dynamics of microcavity polaritons in the Markovian limit

A master equation for the reduced density matrix of the microcavity polaritons coupled with the reservoir of high energy excitons is derived. It is allowed both the polaritons and the excitons to be self-interacting systems. Long time asymptotic properties of the polariton population is studied in the whole range of the reservoir temperatures and the corresponding decoherence effects are reported.     

Bose-Einstein Condensates in a One-Dimensional Optical Lattice: from Superfluidity to Number-Squeezed States

We study the phase coherence property of Bose-Einstein condensates confined in a one-dimensional optical lattice formed by a standing-wave laser field. The lattice depth is determined using a method of Kapitza-Dirac scattering between a condensate and a short pulse lattice potential. Condensates are then adiabatically loaded into the optical lattice. The phase coherence property of the confined condensates is reflected by the interference patterns of the expanded atomic cloud released from the optical lattice. For weak lattice, nearly all of the atoms stay in a superfluid state. However, as the lattice depth is increased, the phase coherence of the whole condensate sample is gradually lost, which confirms that the sub-condensates in each lattice well have evolved into number-squeezed states.     

Ratchet-induced matter-wave transport and soliton collisions in Bose-Einstein condensates

We study the dynamics of bright matter-wave solitons in a Bose-Einstein condensate with negative scattering length under the influence of a time-periodic ratchet potential. The potential is formed by a one-dimensional bichromatic optical lattice which flashes on and off so that the time average of its amplitude vanishes. Due to the broken space and time-reversal symmetries of the potential, the soliton is transported with a nonzero average velocity. By employing the non-dissipative mean-field model for the matter waves, we study the dependence of the transport velocity on the initial state of the soliton and show how the properties of the individual localized states affect the outcome of their collisions. A useful insight into the transport properties is provided by Hamiltonian theory for the mean field, which treats the extended matter-wave excitation as an effective classical particle.