597 research outputs found
Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy
We present the first study of nonlinear optical third harmonic generation in
the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental
excitation in the near-infrared, the THG spectrum reveals a strongly resonant
response for photon energies near 0.7 eV. Polarization analysis reveals this
novel resonance to be only partially accounted for by three-photon excitation
to the optical charge-transfer exciton, and indicates that an even-parity
excitation at 2 eV, with a_1g symmetry, participates in the third harmonic
susceptibility.Comment: Requires RevTeX v4.0beta
Collective oscillations driven by correlation in the nonlinear optical regime
We present an analytical and numerical study of the coherent exciton
polarization including exciton-exciton correlation. The time evolution after
excitation with ultrashort optical pulses can be divided into a slowly varying
polarization component and novel ultrafast collective modes. The frequency and
damping of the collective modes are determined by the high-frequency properties
of the retarded two-exciton correlation function, which includes Coulomb
effects beyond the mean-field approximation. The overall time evolution depends
on the low-frequency spectral behavior. The collective mode, well separated
from the slower coherent density evolution, manifests itself in the coherent
emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
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Photocarrier relaxation in the quantum kinetics regime
We present a study of carrier relaxation in GaAs in the quantum- kinetics regime. The results cannot be explained by Bolzmann kinetics. They are in satisfactory agreement with quantum-kinetic theory calculation of the occupation number in k-space
Radiative corrections to the excitonic molecule state in GaAs microcavities
The optical properties of excitonic molecules (XXs) in GaAs-based quantum
well microcavities (MCs) are studied, both theoretically and experimentally. We
show that the radiative corrections to the XX state, the Lamb shift
and radiative width , are
large, about of the molecule binding energy , and
definitely cannot be neglected. The optics of excitonic molecules is dominated
by the in-plane resonant dissociation of the molecules into outgoing
1-mode and 0-mode cavity polaritons. The later decay channel,
``excitonic molecule 0-mode polariton + 0-mode
polariton'', deals with the short-wavelength MC polaritons invisible in
standard optical experiments, i.e., refers to ``hidden'' optics of
microcavities. By using transient four-wave mixing and pump-probe
spectroscopies, we infer that the radiative width, associated with excitonic
molecules of the binding energy meV, is
meV in the microcavities and
meV in a reference GaAs single quantum
well (QW). We show that for our high-quality quasi-two-dimensional
nanostructures the limit, relevant to the XX states, holds at
temperatures below 10 K, and that the bipolariton model of excitonic molecules
explains quantitatively and self-consistently the measured XX radiative widths.
We also find and characterize two critical points in the dependence of the
radiative corrections against the microcavity detuning, and propose to use the
critical points for high-precision measurements of the molecule bindingenergy
and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Proportional Relations Between Systolic, Diastolic and Mean Pulmonary Artery Pressure are Explained by Vascular Properties
Recently, it was shown that proportional relationships exist between systolic, diastolic and mean pulmonary artery pressure (Psys, Pdia and Pmean) and that they are maintained under various conditions in both health and disease. An arterial-ventricular interaction model was used to study the contribution of model parameters to the ratios Psys/Pmean, and Pdia/Pmean. The heart was modeled by a time-varying elastance function, and the arterial system by a three-element windkessel model consisting of peripheral resistance, Rp, arterial compliance Ca, and pulmonary artery characteristic impedance Z0. Baseline model parameters were estimated in control subjects and compared to values estimated in patients with pulmonary hypertension. Results indicate that experimentally derived ratios Psys/Pmean and Pdia/Pmean could be accurately reproduced using our model (1.59 and 0.61 vs. 1.55 and 0.64, respectively). Sensitivity analysis showed that the (empirical) constancy of Psys/Pmean and Pdia/Pmean was primarily based on the inverse hyperbolic relation between total vascular resistance (RT; calculated as RpĀ +Ā Z0) and Ca, (i.e. constant RTCa product). Of the cardiac parameters, only heart rate affected the pressure ratios, but the contribution was small. Therefore, we conclude that proportional relations between systolic, diastolic and mean pulmonary artery pressure result from the constancy of RTCa thus from pulmonary arterial properties, with only little influence of heart rate
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