343 research outputs found
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Optimal seismic upgrade timing of seaports in high-capital cost and high-throughput growth environments using a real options approach
Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities
A massive redistribution of the polariton occupancy to two specific wave vectors is observed under conditions of continuous wave excitation of a semiconductor microcavity.
The “condensation” of the polaritons to the two specific states arises from stimulated scattering at final
state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of
the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch.
High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in
the vertical cavity structure
Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire
We show that a single exciton state in an individual ordered conjugated
polymer chain exhibits macroscopic quantum spatial coherence reaching tens of
microns, limited by the chain length. The spatial coherence of the k=0 exciton
state is demonstrated by selecting two spatially separated emitting regions of
the chain and observing their interference.Comment: 12 pages with 2 figure
Theory of neutral and charged exciton scattering with electrons in semiconductor quantum wells
Electron scattering on both neutral () and charged () excitons in
quantum wells is studied theoretically. A microscopic model is presented,
taking into account both elastic and dissociating scattering. The model is
based on calculating the exciton-electron direct and exchange interaction
matrix elements, from which we derive the exciton scattering rates. We find
that for an electron density of in a GaAs QW at ,
the linewidth due to electron scattering is roughly twice as large as
that of the neutral exciton. This reflects both the larger interaction
matrix elements compared with those of , and their different dependence on
the transferred momentum. Calculated reflection spectra can then be obtained by
considering the three electronic excitations of the system, namely, the
heavy-hole and light-hole 1S neutral excitons, and the heavy-hole 1S charged
exciton, with the appropriate oscillator strengths.Comment: 18 pages, 12 figure
Sculpting oscillators with light within a nonlinear quantum fluid
Seeing macroscopic quantum states directly remains an elusive goal. Particles
with boson symmetry can condense into such quantum fluids producing rich
physical phenomena as well as proven potential for interferometric devices
[1-10]. However direct imaging of such quantum states is only fleetingly
possible in high-vacuum ultracold atomic condensates, and not in
superconductors. Recent condensation of solid state polariton quasiparticles,
built from mixing semiconductor excitons with microcavity photons, offers
monolithic devices capable of supporting room temperature quantum states
[11-14] that exhibit superfluid behaviour [15,16]. Here we use microcavities on
a semiconductor chip supporting two-dimensional polariton condensates to
directly visualise the formation of a spontaneously oscillating quantum fluid.
This system is created on the fly by injecting polaritons at two or more
spatially-separated pump spots. Although oscillating at tuneable THz-scale
frequencies, a simple optical microscope can be used to directly image their
stable archetypal quantum oscillator wavefunctions in real space. The
self-repulsion of polaritons provides a solid state quasiparticle that is so
nonlinear as to modify its own potential. Interference in time and space
reveals the condensate wavepackets arise from non-equilibrium solitons. Control
of such polariton condensate wavepackets demonstrates great potential for
integrated semiconductor-based condensate devices.Comment: accepted in Nature Physic
Relaxation bottleneck and its suppression in semiconductor microcavities
A polariton relaxation bottleneck is observed in angle-resolved measurements of photoluminescence emission from a semiconductor microcavity. For low power laser excitation, low k polariton states are found to have a very small population relative to those at high k. The bottleneck is found to be strongly suppressed at higher powers in the regime of superlinear emission of the lower polariton states. Evidence for the important role of carrier-carrier scattering in suppression of the bottleneck is presented
Angle-resonant stimulated polariton amplifier
We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification
induced by stimulated energy relaxation
Monochromatic Propagation-Based Phase-Contrast Microscale Computed-Tomography System with a Rotating-Anode Source
We present an experimental setup for monochromatic propagation-based x-ray phase-contrast imaging based on a conventional rotating-copper-anode source, capable of an integrated flux up to 108 photons/s at 8 keV. In our study, the system is characterized in terms of spatial coherence, resolution, contrast sensitivity, and stability. Its quantitativeness is demonstrated by comparing theoretical predictions with experimental data on simple wire phantoms both in planar and computerized-tomography-scan geometries. Application to two biological samples of medical interest shows the potential for bioimaging on the millimeter scale with spatial resolution of the order of 10 \u3bcm and contrast resolution below 1%. All the scans are performed within laboratory-compatible exposure times, from 10 min to a few hours, and trade-offs between scan time and image quality are discussed
Uncoupled excitons in semiconductor microcavities detected in resonant Raman scattering
We present an outgoing resonant Raman-scattering study of a GaAs/AlGaAs based microcavity embedded in a p-i-n junction. The p-i-n junction allows the vertical electric field to be varied, permitting control of exciton-photon detuning and quenching of photoluminescence which otherwise obscures the inelastic light scattering signals. Peaks corresponding to the upper and lower polariton branches are observed in the resonant Raman cross sections, along with a third peak at the energy of uncoupled excitons. This third peak, attributed to disorder activated Raman scattering, provides clear evidence for the existence of uncoupled exciton reservoir states in microcavities in the strong-coupling regime
Vortices in polariton OPO superfluids
This chapter reviews the occurrence of quantised vortices in polariton
fluids, primarily when polaritons are driven in the optical parametric
oscillator (OPO) regime. We first review the OPO physics, together with both
its analytical and numerical modelling, the latter being necessary for the
description of finite size systems. Pattern formation is typical in systems
driven away from equilibrium. Similarly, we find that uniform OPO solutions can
be unstable to the spontaneous formation of quantised vortices. However,
metastable vortices can only be injected externally into an otherwise stable
symmetric state, and their persistence is due to the OPO superfluid properties.
We discuss how the currents charactering an OPO play a crucial role in the
occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure
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