2,160 research outputs found
Temperature dependent polariton emission from strongly coupled organic semiconductor microcavities
We investigated the absorption and photoluminescence (PL) of J-aggregates of a cyanine dye both in a thin film format and when used as the active layer in a strongly-coupled microcavity. We show that as temperature is reduced, the absorption linewidth of the J-aggregates narrows and shifts to higher energy. When the J-aggregate is placed in a microcavity we find that the energy of the polariton modes also shifts to higher energies as temperature is reduced. We compare the intensity of PL emission from the upper and lower branches at resonance as a function of temperature, and find that it can be described by an activation energy of 25 meV. PL emission spectra at resonance also suggest that uncoupled excitons inside the microcavity populate the upper polariton branch states
Melt viscosities of lattice polymers using a Kramers potential treatment
Kramers relaxation times and relaxation times and
for the end-to-end distances and for center of mass diffusion are
calculated for dense systems of athermal lattice chains. is defined
from the response of the radius of gyration to a Kramers potential which
approximately describes the effect of a stationary shear flow. It is shown that
within an intermediate range of chain lengths N the relaxation times
and exhibit the same scaling with N, suggesting that N-dependent
melt-viscosities for non-entangled chains can be obtained from the Kramers
equilibrium concept.Comment: submitted to: Journal of Chemical Physic
Optimizing Replica Exchange Moves For Molecular Dynamics
In this short note we sketch the statistical physics framework of the replica
exchange technique when applied to molecular dynamics simulations. In
particular, we draw attention to generalized move sets that allow a variety of
optimizations as well as new applications of the method.Comment: 4 pages, 3 figures; revised version (1 figure added), PRE in pres
Voltage controlled nuclear polarization switching in a single InGaAs quantum dot
Sharp threshold-like transitions between two stable nuclear spin
polarizations are observed in optically pumped individual InGaAs self-assembled
quantum dots embedded in a Schottky diode when the bias applied to the diode is
tuned. The abrupt transitions lead to the switching of the Overhauser field in
the dot by up to 3 Tesla. The bias-dependent photoluminescence measurements
reveal the importance of the electron-tunneling-assisted nuclear spin pumping.
We also find evidence for the resonant LO-phonon-mediated electron
co-tunneling, the effect controlled by the applied bias and leading to the
reduction of the nuclear spin pumping rate.Comment: 5 pages, 2 figures, submitted to Phys Rev
Fast preparation of single hole spin in InAs/GaAs quantum dot in Voigt geometry magnetic field
The preparation of a coherent heavy-hole spin via ionization of a
spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt
geometry magnetic field is investigated. For a dot with a 17 ueV bright-exciton
fine-structure splitting, the fidelity of the spin preparation is limited to
0.75, with optimum preparation occurring when the effective fine-structure of
the bright-exciton matches the in-plane hole Zeeman energy. In principle,
higher fidelities can be achieved by minimizing the bright-exciton
fine-structure splitting.Comment: 8 pages, 10 figs, published PRB 85 155310 (2012
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
Phonon-Induced Rabi-Frequency Renormalization of Optically Driven Single InGaAs/GaAs Quantum Dots
The authors thank the EPSRC (U.K.) EP/G001642, and the QIPIRC U.K. for financial support. A. N. is supported by the EPSRC and B.W. L. by the Royal Society.We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14 pi. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.Peer reviewe
On-chip electrically controlled routing of photons from a single quantum dot
Electrical control of on-chip routing of photons emitted by a single InAs/GaAs self-assembled quantum dot (SAQD) is demonstrated in a photonic crystal cavity-waveguide system. The SAQD is located inside an H1 cavity, which is coupled to two photonic crystal waveguides. The SAQD emission wavelength is electrically tunable by the quantum-confined Stark effect. When the SAQD emission is brought into resonance with one of two H1 cavity modes, it is preferentially routed to the waveguide to which that mode is selectively coupled. This proof of concept provides the basis for scalable, low-power, high-speed operation of single-photon routers for use in integrated quantum photonic circuits
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