Showing papers by "Alexey Kavokin published in 2012"

Spontaneous coherence in a cold exciton gas

Abstract: In theory, excitons can form a coherent state like a Bose–Einstein condensate, but this is difficult to produce; it is now shown that spontaneous coherence, characteristic of a condensate, can occur in a cold exciton gas.

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

Abstract: 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.

Propagation and amplification dynamics of 1D polariton condensates.

Abstract: The dynamics of propagating polariton condensates in one-dimensional microcavities is investigated through time resolved experiments. We find a strong increase in the condensate intensity when it travels through the nonresonantly excited area. This amplification is shown to come from bosonic stimulated relaxation of reservoir excitons into the polariton condensate, allowing for the repopulation of the condensate through nonresonant pumping. Thus, we experimentally demonstrate a polariton amplifier with a large band width, opening the way towards the transport of polaritons with high densities over macroscopic distances.

Exciton mediated self-organization in glass driven by ultrashort light pulses

Abstract: We propose an exciton-polariton-mediated self-organization effect in transparent SiO2 glass under intense femtosecond light irradiation. Interference and dipole-dipole interaction of polaritons causes formation of gratings of dielectric polarization. Due to an ultrafast exciton self-localization into a quasicrystal structure, the polariton gratings remain frozen in glass and a permanent three-dimensional image of exciton-polariton gas is created. We show that coherent effects in propagation of exciton-polaritons can serve as a tool for nanostructuring and fabrication of 5-dimensional optical memories in glass, opening new horizons for polaritronics.

Spontaneous symmetry breaking in a polariton and photon laser.

Abstract: We report on the simultaneous observation of spontaneous symmetry breaking and long-range spatial coherence both in the strong- and the weak-coupling regime in a semiconductor microcavity. Under pulsed excitation, the formation of a stochastic order parameter is observed in polariton and photon lasing regimes. Single-shot measurements of the Stokes vector of the emission exhibit the buildup of stochastic polarization. Below threshold, the polarization noise does not exceed 10%, while above threshold we observe a total polarization of up to 50% after each excitation pulse, while the polarization averaged over the ensemble of pulses remains nearly zero. In both polariton and photon lasing regimes, the stochastic polarization buildup is accompanied by the buildup of spatial coherence. We find that the Landau criterion of spontaneous symmetry breaking and Penrose-Onsager criterion of long-range order for Bose-Einstein condensation are met in both polariton and photon lasing regimes.

Vertical cavity surface emitting terahertz laser

Abstract: Vertical cavity surface emitting terahertz lasers can be realized in conventional semiconductor microcavities with embedded quantum wells in the strong coupling regime. The cavity is to be pumped optically at half the frequency of the 2p exciton state. Once a threshold population of 2p excitons is achieved, a stimulated terahertz transition populates the lower exciton-polariton branch, and the cavity starts emitting laser light both in the optical and terahertz ranges. The lasing threshold is sensitive to the statistics of photons of the pumping light.

Crossover from photon to exciton-polariton lasing

Abstract: We report on a real-time observation of the crossover between photon and exciton-polariton lasing in a semiconductor microcavity. Both lasing phases are observed at different times after a high-power excitation pulse. Energy-, time- and angle-resolved measurements allow for the transient characterization of carrier distribution and effective temperature. We find signatures of Bose–Einstein condensation, namely macroscoping occupation of the ground state and narrowing of the linewidth in both lasing regimes. The Bernard–Douraffourgh condition for inversion was tested and the polariton laser as well as the photon laser under continuous wave excitation were found to operate at estimated densities below the theoretically predicted inversion threshold.

Parabolic polarization splitting of Tamm states in a metal-organic microcavity

Abstract: We observe hybrid states of cavity photons and Tamm plasmons in an organic microcavity with an incorporated thin silver layer of increasing thickness up to 40 nm. Via μ-photoluminescence spectroscopy, we investigate their angular dependence. At oblique angles, we observe a TE-TM polarization splitting of more than 40 meV for each mode. An analytical model is developed to describe the coupling of Tamm plasmons and cavity photons and to account for the splitting of the orthogonally polarized resonances.

The behaviour of exciton-polaritons

Abstract: To the Editor — In a recent correspondence, Butov and Kavokin discussed exciton– polaritons and their transition into a coherent state1. In particular, they wished to point out that describing polaritons and their coherent state in terms of a Bose–Einstein condensate (BEC) and superfluidity may not be appropriate. However, I cannot agree with most of their comments. Work on polariton quantum fluids and condensates flourished following our observation of a polariton BEC2. Our claim was based mainly on the spontaneous appearance of long-range order from a thermalized polariton cloud, in addition to all other properties expected for a BEC, such as a Bose distribution of polaritons. Condensation effects have subsequently been reported in many different configurations, and even up to room temperature3. Butov and Kavokin state that the “longrange coherence of polaritons is governed by their photonic component”. The ordering of polariton condensates has been observed by different groups across a range of samples. This ordering is not governed by the photonic component of the polariton. The condensate self-orders in the regime of positive detuning; that is, when the exciton fraction in the polariton is larger than 0.5, which provides a higher polariton collision rate and therefore faster thermalization. Ordering also takes place at negative detuning, where the system does not have time to reach full thermal equilibrium. All experimental observations are accurately reproduced by dynamical Gross–Pitaevskii equations for the lower polariton branch, where the important ingredient is the interaction between polaritons; that is, the excitonic fraction. However, contrary to what Butov and Kavokin imply, we are still awaiting for a clear demonstration of BEC for the case of excitons, whether standard or indirect. Discussion within the polariton community about the best way to qualify this observed phenomenon is linked with the fact that polaritons are part-light, partmatter quasiparticles, and that polariton condensates form two populations in a system: polaritons around the lower energy minimum of the dispersion, and polaritons with larger momenta at the ‘reservoir’. These two populations are weakly coupled, which allows us to define a quasi-thermal equilibrium for each. Although they have a very short lifetime, polaritons in the lower branch are able to come to thermal equilibrium, and the condensate builds up from this thermal cloud, as expected for a BEC. The condensation of quasiparticles in solids, including exciton, polaritons, Cooper pairs and magnons, are usually described by disregarding various factors, starting with the crystal lattice. The continuously adjustable proportion of exciton to photon in a polariton allows us to explore a wide range of its properties, ranging from very excitonic, where a condensed phase does not occur, to polaritons with a mass that allows for condensation, and thermalization below the threshold. We still observe the build-up of a coherent phase in lighter, more photonic polaritons. Such a condensed phase, usually called a ‘polariton laser’, differs strongly from conventional vertical-cavity surface-emitting lasers. This makes it possible with polaritons to change from a regime of condensation at quasi-equilibrium to a regime of condensation out of equilibrium. Butov and Kavokin also contest the observation of polariton superfluidity. The original paper by Amo et al.4 has been supported by a numerous collection of other studies. For low enough fluid speeds, there is no turbulence. For higher speeds, turbulence occurs in the form of dark solitons5, quantized vortex pairs6 and vortex streets7. Not only does the fluid exhibit long-range coherence far beyond the initial laser drive, but also dark solitons show the expected π phase shift and vortices exhibit the expected 2π phase shifts — all of which would be highly improbable in a conventional fluid. Polaritons emit light by composition because their light component has a very short lifetime. This allows us to study their phase and coherence in a direct way that is not accessible to other bosonic quasiparticles. Nevertheless, polaritons do not behave as pure photons and undergo efficient interactions with their environment via their excitonic component. This allows manipulation via optical means8 or observation of Josephson oscillations. Polaritons also carry a spin, which brings additional degrees of freedom. As outstanding examples, I could quote the observation of half vortices9 or half solitons (G. Malpuech and A. Amo, private communication). The limited space available in this letter has allowed me to give only a few experimental examples. The theoretical side of the field is progressing perhaps even more rapidly than the experimental side. To end on a positive note, I can easily agree with the last sentiment expressed by Butov and Kavokin: “The physics of polariton condensates, although different from that of atom BECs, is nevertheless rich and interesting.” ☐

Superconductivity with excitons and polaritons: review and extension

Abstract: A system where a Bose-Einstein condensate of exciton-polaritons coexists with a Fermi gas of electrons has been recently proposed as promising for realization of room- temperature superconductivity. In order to find the optimum conditions for exciton and exciton- polariton mediated superconductivity, we studied the attractive mechanism between electrons of a Cooper pair mediated by the exciton and exciton-polariton condensate. We also analyzed the gap equation that follows. We specifically examined microcavities with embedded n-doped quantum wells as well as coupled quantum wells hosting a condensate of spatially indirect exci- tons, put in contact with a two-dimensional electron gas. An effective potential of interaction between electrons was derived as a function of their exchanged energy ℏω, taking into account the retardation effect that allows two negatively charged carriers to feel an attraction. In the polariton case, the interaction is weakly attractive at long times, followed by a succession of strongly attractive and strongly repulsive windows. Strikingly, this allows high critical tempera- ture solutions of the gap equation. An approximate three-steps potential is used to explain this result that is also obtained numerically. The case of polaritons can be compared with that of excitons, which realize the conventional scenario of high-Tc superconductivity where a large coupling strength accounts straightforwardly for the high critical temperatures. Excitons are less advantageous than polaritons but may be simpler systems to realize experimentally. It is concluded that engineering of the interaction in these peculiar Bose-Fermi mixtures is complex and sometimes counter-intuitive, but leaves much freedom for optimization, thereby promising the realization of high-temperature superconductivity in multilayered semiconductor structures. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10.1117/1.JNP.6.064502)

Exciton supersolidity in hybrid Bose-Fermi systems.

Abstract: We investigate the ground states of a Bose-Einstein condensate of indirect excitons coupled to an electron gas. We show that in a properly designed system the crossing of a roton minimum into the negative energy domain can result in the appearance of the supersolid phase, characterized by periodicity in both real and reciprocal space. Accounting for the spin-dependent exchange interaction of excitons we obtain ferromagnetic supersolid domains. The Fourier spectra of excitations of weakly perturbed supersolids show pronounced diffraction maxima which may be detected experimentally.

Generic picture of the emission properties of III-nitride polariton laser diodes: Steady state and current modulation response

Abstract: The main emission characteristics of electrically driven polariton lasers based on planar GaN microcavities with embedded InGaN quantum wells are studied theoretically. The polariton emission dependence on pump current density is first modeled using a set of semiclassical Boltzmann equations for the exciton polaritons that are coupled to the rate equation describing the electron-hole plasma population. Two experimentally relevant pumping geometries are considered, namely the direct injection of electrons and holes into the strongly coupled microcavity region and intracavity optical pumping via an embedded light-emitting diode. The theoretical framework allows the determination of the minimum threshold current density J(thr),(min) as a function of lattice temperature and exciton-cavity photon detuning for the two pumping schemes. A J(thr, min) value of 5 and 6 A cm(-2) is derived for the direct injection scheme and for the intracavity optical pumping one, respectively, at room temperature at the optimum detuning. Then an approximate quasianalytical model is introduced to derive solutions for both the steady-state and high-speed current modulation. This analysis makes it possible to show that the exciton population, which acts as a reservoir for the stimulated relaxation process, gets clamped once the condensation threshold is crossed, a behavior analogous to what happens in conventional laser diodes with the carrier density above threshold. Finally, the modulation transfer function is calculated for both pumping geometries and the corresponding cutoff frequency is determined.

Spontaneous coherence in a cold exciton gas

Abstract: We report on the measurement of spontaneous coherence in a gas of indirect excitons. Extended spontaneous coherence, spin texture, and phase singularities emerge when the exciton gas is cooled below a few Kelvin.

Spin-orbit coupling and the topology of gases of spin-degenerate cold excitons in photoexcited GaAs-AlGaAs quantum wells

Abstract: We calculate the spatial structure of four-component spinor systems of mixed bright and dark exciton condensates in coupled quantum wells. The spin-dependent bright-dark exciton conversion and Dresselhaus spin-orbit coupling is found to generate a rich variety of topological elements. By propagating the Gross-Pitaevskii equation in imaginary time, we observe the following: single and multiple polarized vortices; the phase separation of bright and dark excitons; and exotic spatial structures in density and spin polarization.

Spin Effects in Exciton–Polariton Condensates

Abstract: Exciton–polaritons in microcavities form an unusual gas of weakly interacting bosons. It has no direct analogy in cold atomic gases, superfluids or superconductors due to its two-component spin structure: in typical planar microcavities the polaritons have two allowed spin projections to the structure axis. This is why the order parameter of a polariton condensate is a complex spinor. The magnitude and, possibly, sign of polariton–polariton interaction constant depends on the spin state of polaritons. The energy of an exciton–polariton condensate is also spin-dependent. These specific features make polariton condensates a unique laboratory for studies of spin effects in interacting Bose gases. Several new spin-dependent effects in polariton condensates have been theoretically predicted and experimentally observed during the recent decade. This review chapter addresses some of these effects: polarisation multistability, spin switching, spin rings and spin Meissner effect. In the last section we address the perspective of observation of spin superfluidity in microcavities.

Bosonic condensates: Polariton pendulum

Abstract: A macroscopic quantum pendulum has now been created by confining a polariton condensate in a parabolic optical trap. Spectacular images of multiparticle wavefunctions are obtained by purely optical means.

Bosonic Cascade Laser

Abstract: We propose a concept of a quantum cascade laser based on transitions of bosonic quasiparticles (excitons and exciton-polaritons) in a parabolic potential trap in a semiconductor microcavity. This laser would emit terahertz radiation due to bosonic stimulation of excitonic transitions. Dynamics of a bosonic cascade is strongly different from the dynamics of a conventional fermionic cascade laser. We show that populations of excitonic ladders are parity-dependent and quantized if the laser operates without an external terahertz cavity.

Photon and polariton condensates in microcavities

Abstract: In this work we study thermalisation-, coherence- and spin dynamics of photon and polariton Bose-Einstein condensates (BECs). We witness carrier distributions following the Bose Einstein distribution, buildup of long-range order and spontaneous symmetry breaking.