Excitonic lasing of strain-free InP(As) quantum dots in AlInAs microdisk
Formation, emission, and lasing properties of strain-free InP(As)/AlInAs quantum dots (QDs) embedded in AlInAs microdisk (MD) cavity were investigated using transmission electron microscopy and photoluminescence (PL) techniques. In MD structures, the QDs have the nano-pan-cake shape with the height of ˜2 nm, the lateral size of 20–50 nm, and the density of ˜5x109 cm-2. Their emission observed at ˜940 nm revealed strong temperature quenching, which points to exciton decomposition. It also showed unexpected type-I character, indicating In-As intermixing as confirmed by band structure calculations. We observed lasing of InP(As) QD excitons into whispering gallery modes in MD having the diameter of ˜3.2 lm and providing a free spectral range of ˜27 nm and quality factors up to Q˜13 000. Threshold of ˜50 W/cm2 and spontaneous emission coupling coefficient of ˜0.2 were measured for this MD-QD system.
D. V. Lebedev, M. M. Kulagina, S. I. Troshkov, A. S. Vlasov, V. Y. Davydov, A. N. Smirnov, A. A. Bogdanov, J. L. Merz, J. Kapaldo, A. Gocalinska, G. Juska, S. T. Moroni, E. Pelucchi, D. Barettin, S. Rouvimov, and A. M. Mintairov
Fast 3C 279 γ flares by a merging medium size black hole jet aligned to the AGN one by tidal torque?
The shorter-than-Schwarzschild 3C 279 variability flare on June 2015 is very puzzling. Its nature cannot be due to any NS merging nor to a medium sized (˜ 102 M) BH collapse. Our preliminary model is based on the long-life (˜ 107 s) merging of a medium size BH (˜ 102 M) spiralling towards the more massive AGN (BH) that is tidally dragging the medium size BH jet in alignment with the jet of the more massive BH of AGN 3C 279. The tidal torque is aligning both jets towards Earth. The twin overlapping blazars may offer at once a long and a short scale variability consistent with the surprising Fermi discoveries.
D. Fargion, P. Oliva
Nano-carbon pixels array for ionizing particles monitoring
The paper deals on the response of a polycrystalline diamond sensor, 500 μm thick, to particles from a Sr β-source. 21×21 nano-carbon pads, with 0.18 mm×0.18 mm area each, were realized by ArF excimer laser irradiation on one diamond face, whereas a 7×7 mm2 backside contact was fabricated and used for sensor biasing during characterization of sensor under β-source irradiation. The carbon pads embrace a number of grains, which show different degrees of surface graphitization dependent on the grain orientations. Each carbon pad exhibits a linear I(V) response up to 200 V. The average number of charge carriers collected by a single pixel, as well as the distribution of pixels involved by the impinging particle tracking, is analyzed as a function of the applied voltage recording the signals acquired by 16 pixels at a time. The pulse height distribution is not affected by reversing the bias polarity. For a single pixel, the most probable collected charge value is 1.40±0.02 fC whereas the main value gives Qcoll=1.67±0.02 fC (10430 ±120 electrons). The charge collection distance was measured tacking into account the effect induced by highenergy electrons and found to be 285±3 μm, demonstrating the absence of bulk defects induced by the laser graphitization processing. Cross-talk effects between nearest-neighbor pixels has been excluded analyzing the results obtained in a batch of more than 1000 events even if the same cannot be excluded under higher energy particles.
S. Salvatori, P. Oliva, M. Pacilli, P. Allegrini, G. Conte, M. Komlenok, A.A. Khomich, A. Bolshakov, V. Ralchenko, V. Konov
Diamond device architectures for UV laser monitoring
The paper reviews the status of diamond detectors for UV laser monitoring and imaging. Single pixel detectors, position sensitive architectures, optically activated switches and sensor arrays for beam positioning and imaging are analyzed. The performances of natural diamond and synthetic diamond produced by chemical vapor deposition are compared to evaluate the suitability of such an outstanding material for the described applications.
S Salvatori, M Girolami, P Oliva, G Conte, A Bolshakov, V Ralchenko and V Konov
Enhanced thermoelectric coupling near electronic phase transition: The role of fluctuation Cooper pairs
Thermoelectric energy conversion is a direct but low-efficiency process, which precludes the development of long-awaited wide-scale applications. As a breakthrough permitting a drastic performance increase is seemingly out of reach, we fully reconsider the problem of thermoelectric coupling enhancement. The cornerstone of our approach is the observation that heat engines are particularly efficient when their operation involves a phase transition of their working fluid. We derive and compute the thermoelastic coefficients of various systems, including Bose and Fermi gases, and fluctuation Cooper pairs. Combination of these coefficients yields the definition of the thermodynamic figure of merit, the divergence of which at finite temperature indicates that conditions are fulfilled for the best possible use of the thermoelectric working fluid. Here, this situation occurs in the fluctuation regime only, as a consequence of the increased compressibility of the working fluid near its phase transition. Our results and analysis clearly show that efforts in the field of thermoelectricity can now be productively directed towards systems where electronic phase transitions are possible.
Henni Ouerdane, Andrey A. Varlamov, Alexey V. Kavokin, Christophe Goupil, and Cronin B. Vining
Dispersion characteristics of electromagnetic excitations in a disordered one-dimensional lattice of coupled microresonators
Localized photonic modes are studied in a non-ideal chain of coupled microcavities with the use of the virtual crystal approximation. The approach proves sufficient to elucidate the effects of varying composition and nearest-neighbor distances on the spectrum. It permits to obtain the density of states of the studied quasiparticles as well as the dispersion dependence of collective excitation frequencies on defect concentration. Based upon the developed description of ideal photonic structures we proceed to study a non-ideal polaritonic crystal constituted by an array of spatially ordered cavities containing atomic clusters. Frequency, effective mass and group velocity of polaritons are analytically derived as functions of vacancy concentration.
Exciton-like electromagnetic excitations in non-ideal microcavity supercrystals
We study localized photonic excitations in a quasi-two-dimensional non-ideal binary microcavity lattice with use of the virtual crystal approximation. The effect of point defects (vacancies) on the excitation spectrum is investigated by numerical modelling. We obtain the dispersion and the energy gap of the electromagnetic excitations which may be considered as Frenkel exciton-like quasiparticles and analyze the dependence of their density of states on the defect concentrations in a microcavity supercrystal.
Vladimir Rumyantsev, Stanislav Fedorov, Kostyantyn Gumennyk, Marina Sychanova, Alexey Kavokin
Exciton-like electromagnetic excitations in non-ideal microcavity supercrystals
We report the observation of spin currents in a coherent gas of indirect excitons. The realized longrange spin currents originate from the formation of a coherent gas of bosonic pairs—a new mechanism to suppress the spin relaxation. The spin currents result in the appearance of a variety of polarization patterns, including helical patterns, four-leaf patterns, spiral patterns, bell patterns, and periodic patterns. We demonstrate control of the spin currents by a magnetic field. We also present a theory of coherent exciton spin transport that describes the observed exciton polarization patterns and indicates the trajectories of the spin currents.
A. A. High, A. T. Hammack, J. R. Leonard, Sen Yang, L.V. Butov, T. Ostatnicky, M. Vladimirova, A.V. Kavokin, T. C. H. Liew, K. L. Campman, and A. C. Gossard
Nonlinear effects in multi-photon polaritonics
We consider theoretically nonlinear effects in a semiconductor quantum well embedded inside a photonic microcavity. Two-photon absorption by a 2p exciton state is considered and investigated; the matrix element of two-photon absorption is calculated. We compute the emission spectrum of the sample and demonstrate that under coherent pumping the nonlinearity of the two photon absorption process gives rise to bistability.
A. A. Pervishko, T. C. H. Liew, V. M. Kovalev, I. G. Savenko, and I. A. Shelykh
Stochastic Gross-Pitaevskii Equation for the Dynamical Thermalization of Bose-Einstein Condensates
We present a theory for the description of energy relaxation in a nonequilibrium condensate of bosonic particles. The approach is based on coupling to a thermal bath of other particles (e.g., phonons in a crystal, or noncondensed atoms in a cold atom system), which are treated with a Monte Carlo type approach. Together with a full account of particle-particle interactions, dynamic driving, and particle loss, this offers a complete description of recent experiments in which Bose-Einstein condensates are seen to relax their energy as they propagate in real space and time. As an example, we apply the theory to the solid-state system of microcavity exciton polaritons, in which nonequilibrium effects are particularly prominent.
I. G. Savenko, T. C. H. Liew, and I. A. Shelykh
Holographic arrays based on semiconductor microstructures
A concept of complex reflectivity modulation is proposed based on the electrical control of quantum well exciton resonances that influence the propagation of light in a layered semiconductor structure. By variation in exciton energies, both the intensity and the phase of reflected light can be fully controlled. Unlike previous devices, for full complex light modulation, the design is based on a single device in a single structure. The device allows complete 100% intensity contrast and allows for the construction of small pixel sizes with fast response times.
T. C. H. Liew
Dissociation dynamics of singly charged vortices into half-quantum vortex pairs
The quest for identification and understanding of fractional vorticity is a major subject of research in the quantum fluids domain, ranging from superconductors, superfluid Helium-3 to cold atoms. In a two-dimensional Bose degenerate gas with a spin degree of freedom, the fundamental topological excitations are fractional vortical entities, called half-quantum vortices. Convincing evidence for the existence of half-quantum vortices was recently provided in spinor polariton condensates. The half-quantum vortices can be regarded as the fundamental structural components of singly charged vortices but, so far, no experimental evidence of this relation has been provided. Here we report on the direct and time-resolved observation of the dynamical process of the dissociation of a singly charged vortex into its primary components, a pair of half-quantum vortices. The physical origin of the observed phenomenology is found in a spatially inhomogeneous static potential that couples the two spin components of the condensate.
F. Manni, K. G. Lagoudakis, T. C. H. Liew, R. André, V. Savona & B. Deveaud
Propagation and Amplification Dynamics of 1D Polariton Condensates
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.
E. Wertz,1 A. Amo, D. D. Solnyshkov, L. Ferrier, T. C. H. Liew, D. Sanvitto, P. Senellart, I. Sagnes, A. Lemaitre, A.V. Kavokin, G. Malpuech, and J. Bloch
Spin-orbit coupling and the topology of gases of spin-degenerate cold excitons in photoexcited GaAs-AlGaAs quantum wells
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.
H. Ohadi, E. Kammann, T. C. H. Liew, K. G. Lagoudakis, A.V. Kavokin, and P. G. Lagoudakis