361 research outputs found
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.Comment: 5 pages, 3 eps figures (published version
The new physics of non-equilibrium condensates: insights from classical dynamics
We discuss the dynamics of classical Dicke-type models, aiming to clarify the
mechanisms by which coherent states could develop in potentially
non-equilibrium systems such as semiconductor microcavities. We present
simulations of an undamped model which show spontaneous coherent states with
persistent oscillations in the magnitude of the order parameter. These states
are generalisations of superradiant ringing to the case of inhomogeneous
broadening. They correspond to the persistent gap oscillations proposed in
fermionic atomic condensates, and arise from a variety of initial conditions.
We show that introducing randomness into the couplings can suppress the
oscillations, leading to a limiting dynamics with a time-independent order
parameter. This demonstrates that non-equilibrium generalisations of polariton
condensates can be created even without dissipation. We explain the dynamical
origins of the coherence in terms of instabilities of the normal state, and
consider how it can additionally develop through scattering and dissipation.Comment: 10 pages, 4 figures, submitted for a special issue of J. Phys.:
Condensed Matter on "Optical coherence and collective phenomena in
nanostructures". v2: added discussion of links to exact solution
Direct evidence of reduced dynamic scattering in the lower polariton of a semiconductor microcavity
The temperature dependent linewidths of homogeneously broadened GaAs/AlxGa1-xAs microcavity polaritons are investigated. The linewidths of the lower, middle, and upper polariton resonances are measured directly from reflection spectra at normal incidence (k‖=0). The linewidth of the lower polariton is found to be smaller than the linewidths of the middle and upper polaritons at all investigated temperatures ranging from 11 to 100 K. The results clearly show the reduction of dynamic scattering processes in the lower polariton compared to the middle and upper polaritons, in agreement with theoretical predictions in literature. A nontrivial temperature dependence of the linewidth is found and its physical origin is discussed
Banking failure prediction: a boosting classification tree approach
The recent financial crisis shows that failure of some financial institutions can cause other banks to fail and ultimately cause damage to the financial system worldwide. Eurozone banks that experienced either liquidity or solvency problems during the finan- cial markets turmoil were bailed out by their national governments with the financial support and supervision of the European Union. This paper applies the boosted classification tree methodology to predict failure in the banking sector and identifies four key scor- ecard variables that are worth tracking closely in order to anticipate and prevent bank financial distress. The data used in this study comprises 2006-2012 annual series of 25 financial ratios of 155 banks in the Eurozone. The findings indicate that the greater the size and the higher the income from non-operating items and net loans to deposits, the more likely is bank failure; conversely, the higher the Interbank ratio the lower the chances of bank financial distress. For the sake of their own financial soundness, banks should fund lending activities through clients' deposits and should avoid relying excessively on non-recurring sources of income
Engineering the spatial confinement of exciton-polaritons in semiconductors
We demonstrate the spatial confinement of electronic excitations in a solid
state system, within novel artificial structures that can be designed having
arbitrary dimensionality and shape. The excitations under study are
exciton-polaritons in a planar semiconductor microcavity. They are confined
within a micron-sized region through lateral trapping of their photon
component. Striking signatures of confined states of lower and upper polaritons
are found in angle-resolved light emission spectra, where a discrete energy
spectrum and broad angular patterns are present. A theoretical model supports
unambiguously our observations
Resonant Rayleigh scattering of exciton-polaritons in multiple quantum wells
A theoretical concept of resonant Rayleigh scattering (RRS) of exciton-polaritons in multiple quantum wells (QWs) is presented. The optical coupling between excitons in different QWs can strongly affect the RRS dynamics, giving rise to characteristic temporal oscillations on a picosecond scale. Bragg and anti-Bragg arranged QW structures with the same excitonic parameters are predicted to have drastically different RRS spectra. Experimental data on the RRS from multiple QWs show the predicted strong temporal oscillations at small scattering angles, which are well explained by the presented theory
Electron-Phonon Interacation in Quantum Dots: A Solvable Model
The relaxation of electrons in quantum dots via phonon emission is hindered
by the discrete nature of the dot levels (phonon bottleneck). In order to
clarify the issue theoretically we consider a system of discrete fermionic
states (dot levels) coupled to an unlimited number of bosonic modes with the
same energy (dispersionless phonons). In analogy to the Gram-Schmidt
orthogonalization procedure, we perform a unitary transformation into new
bosonic modes. Since only of them couple to the fermions, a
numerically exact treatment is possible. The formalism is applied to a GaAs
quantum dot with only two electronic levels. If close to resonance with the
phonon energy, the electronic transition shows a splitting due to quantum
mechanical level repulsion. This is driven mainly by one bosonic mode, whereas
the other two provide further polaronic renormalizations. The numerically exact
results for the electron spectral function compare favourably with an analytic
solution based on degenerate perturbation theory in the basis of shifted
oscillator states. In contrast, the widely used selfconsistent first-order Born
approximation proves insufficient in describing the rich spectral features.Comment: 8 pages, 4 figure
Noise-free scattering of the quantized electromagnetic field from a dispersive linear dielectric
We study the scattering of the quantized electromagnetic field from a linear,
dispersive dielectric using the scattering formalism for quantum fields. The
medium is modeled as a collection of harmonic oscillators with a number of
distinct resonance frequencies. This model corresponds to the Sellmeir
expansion, which is widely used to describe experimental data for real
dispersive media. The integral equation for the interpolating field in terms of
the in field is solved and the solution used to find the out field. The
relation between the in and out creation and annihilation operators is found
which allows one to calculate the S-matrix for this system. In this model, we
find that there are absorption bands, but the input-output relations are
completely unitary. No additional quantum noise terms are required.Comment: Revtex, submitted to Physical Review
Polariton Condensation in a One-Dimensional Disordered Potential
We study the coherence and density modulation of a non-equilibrium
exciton-polariton condensate in a one-dimensional valley with disorder. By
means of interferometric measurements we evidence a modulation of the
first-order coherence function and we relate it to a disorder-induced
modulation of the condensate density, that increases as the pump power is
increased. The non-monotonous spatial coherence function is found to be the
result of the strong non-equilibrium character of the one-dimensional system,
in the presence of disorder
Probing the Dynamics of Spontaneous Quantum Vortices in Polariton Superfluids
The experimental investigation of spontaneously created vortices is of utmost
importance for the understanding of quantum phase transitions towards a
superfluid phase, especially for two dimensional systems that are expected to
be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time
resolved near-field interferometry we track the path of such vortices, created
at random locations in an exciton-polariton condensate under pulsed
non-resonant excitation, to their final pinning positions imposed by the
stationary disorder. We formulate a theoretical model that successfully
reproduces the experimental observations
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