432 research outputs found
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
Electrically injected cavity polaritons
We have realised a semiconductor quantum structure that produces
electroluminescence while operating in the light-matter strong coupling regime.
The mid-infrared light emitting device is composed of a quantum cascade
structure embedded in a planar microcavity, based on the GaAs/AlGaAs material
system. At zero bias, the structure is characterised using reflectivity
measurements which show, up to room temperature, a wide polariton anticrossing
between an intersubband transition and the resonant cavity photon mode. Under
electrical injection the spectral features of the emitted light change
drastically, as electrons are resonantly injected in a reduced part of the
polariton branches. Our experiment demonstrates that electrons can be
selectively injected into polariton states up to room temperature.Comment: 10 pages, 4 figure
Quantum density matrix theory for a laser without adiabatic elimination of the population inversion: transition to lasing in the class-B limit
Despite the enormous technological interest in micro and nanolasers,
surprisingly, no class-B quantum density-matrix model is available to date,
capable of accurately describing coherence and photon correlations within a
unified theory. In class-B lasers applicable for most solid-state lasers at
room temperature, the macroscopic polarization decay rate is larger than the
cavity damping rate which, in turn, exceeds the upper level population decay
rate. Here we carry out a density-matrix theoretical approach for generic
class-B lasers, and provide closed equations for the photonic and atomic
reduced density matrix in the Fock basis of photons. Such a relatively simple
model can be numerically integrated in a straightforward way, and exhibits all
the expected phenomena, from one-atom photon antibunching, to the well-known
S-shaped input-output laser emission and super-Poissonian autocorrelation for
many atoms (), and from few photons (large spontaneous
emission factors, ) to the thermodynamic limit ( and
). Based on the analysis of , we conclude that
super-Poissonian fluctuations are clearly related to relaxation oscillations in
the photon number. We predict a strong damping of relaxation oscillations with
an atom number as small as . This model enables the study of
few-photon bifurcations and non-classical photon correlations in class-B laser
devices, also leveraging quantum descriptions of coherently coupled nanolaser
arrays.Comment: 23 pages, 6 figure
Insights into the phototautomerism of free-base 5, 10, 15, 20-tetrakis(4-sulfonatophenyl) porphyrin
: Phototautomerism in the excited states of free-base 5, 10, 15, 20-tetrakis(4-sulfonatophenyl) porphyrin (H2TPPS4-) has been investigated combining, for the first time, advanced Electron Paramagnetic Resonance (EPR) with fluorescence and Raman spectroscopy. Triplet EPR spectroscopy, performed in protic and deuterated solvents and in the presence of photoselection, confirms the occurrence of phototautomerization and additionally suggests the formation of the cis tautomer as a minor component. The zero-field splitting parameters and triplet sublevel populations indicate that the process is slow in the triplet state. The results obtained by EPR combined with photoselection and fluorescence anisotropy have been interpreted within a model which accounts for a fast trans-trans tautomerization promoted by a spin-vibronic coupling mechanism for intersystem crossing, with an even distribution of the two trans tautomers at liquid nitrogen temperatures for H2TPPS4-
Automatic Cyclic Alternating Pattern (CAP) analysis: Local and multi-trace approaches
The Cyclic Alternating Pattern (CAP) is composed of cycles of two different electroencephalographic features: an activation A-phase followed by a B-phase representing the background activity. CAP is considered a physiological marker of sleep instability. Despite its informative nature, the clinical applications remain limited as CAP analysis is a time-consuming activity. In order to overcome this limit, several automatic detection methods were recently developed. In this paper, two new dimensions were investigated in the attempt to optimize novel, efficient and automatic detection algorithms: 1) many electroencephalographic leads were compared to identify the best local performance, and 2) the global contribution of the concurrent detection across several derivations to CAP identification. The developed algorithms were tested on 41 polysomnographic recordings from normal (n = 8) and pathological (n = 33) subjects. In comparison with the visual CAP analysis as the gold standard, the performance of each algorithm was evaluated. Locally, the detection on the F4-C4 derivation showed the best performance in comparison with all other leads, providing practical suggestions of electrode montage when a lean and minimally invasive approach is preferable. A further improvement in the detection was achieved by a multi-trace method, the Global Analysis—Common Events, to be applied when several recording derivations are available. Moreover, CAP time and CAP rate obtained with these algorithms positively correlated with the ones identified by the scorer. These preliminary findings support efficient automated ways for the evaluation of the sleep instability, generalizable to both normal and pathological subjects affected by different sleep disorders
Many-body physics of a quantum fluid of exciton-polaritons in a semiconductor microcavity
Some recent results concerning nonlinear optics in semiconductor
microcavities are reviewed from the point of view of the many-body physics of
an interacting photon gas. Analogies with systems of cold atoms at thermal
equilibrium are drawn, and the peculiar behaviours due to the non-equilibrium
regime pointed out. The richness of the predicted behaviours shows the
potentialities of optical systems for the study of the physics of quantum
fluids.Comment: Proceedings of QFS2006 conference to appear on JLT
Steady states of a chi-three parametric oscillator with coupled polarisations
Polarisation effects in the microcavity parametric oscillator are studied
using a simple model in which two chi-three optical parametric oscillators are
coupled together. It is found that there are, in general, a number of steady
states of the model under continuous pumping. There are both continuous and
discontinuous thresholds, at which new steady-states appear as the driving
intensity is increased: at the continuous thresholds, the new state has zero
output intensity, whereas at the discontinuous threshold it has a finite output
intensity. The discontinuous thresholds have no analog in the uncoupled device.
The coupling also generates rotations of the linear polarisation of the output
compared with the pump, and shifts in the output frequencies as the driving
polarisation or intensity is varied. For large ratios of the interaction
between polarisations to the interaction within polarisations, of the order of
5, one of the thresholds has its lowest value when the pump is elliptically
polarised. This is consistent with recent experiments in which the maximum
output was achieved with an elliptically polarised pump.Comment: 7 pages, 4 figure
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