116 research outputs found
Mixed regime of light-matter interaction revealed by phase sensitive measurements of the dynamical Franz-Keldysh effect
The speed of ultra-fast optical switches is generally limited by the
intrinsic electronic response time of the material. Here we show that the phase
content of selected electromagnetic pulses can be used to measure the
timescales characteristic for the different regimes of matter-light
interactions. By means of combined single cycle THz pumps and broadband optical
probes, we explore the field-induced opacity in GaAs (the Franz-Keldysh
effect). Our phase-resolved measurements allow to identify a novel quasi-static
regime of saturation where memory effects are of relevance
Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure
Nanometer-thick quantum-well structures are quantum model systems offering a
few discrete unoccupied energy states that can be impulsively filled and that
relax back to equilibrium predominantly via spontaneous emission of light. Here
we report on the response of an indirectly excited quantum-well
heterostructure, probed by means of time and frequency resolved
photoluminescence spectroscopy. This experiment provides access to the
sub-nanosecond evolution of the free electron density, indirectly injected in
the quantum-wells. In particular, the modelling of the time-dependent
photoluminescence spectra unveils the time evolution of the temperature and of
the chemical potentials for electrons and holes, from which the sub-nanosecond
time-dependent electron density is determined. This information allows to prove
that the recombination of excited carriers is mainly radiative and bimolecular
at early delays after excitation, while, as the carrier density decreases, a
monomolecular and non-radiative recombination channel becomes relevant. Access
to the sub-nanosecond chronology of the mechanisms responsible for the
relaxation of charge carriers provides a wealth of information for designing
novel luminescent devices with engineered spectral and temporal behavior
Non-linearity observed in the direct sub-ps photoemission regime in Mo
The total charge emitted from a polycrystalline Mo surface by 500 fs-264 nm laser pulses has been measured. Though a one-photon photoelectric effect is expected, a non-linear increase of the photoelectric yield was observed as a function of laser peak intensity, confirming earlier observations on Au, W and Zr. The threshold intensity for this non-linearity is 2 between 0.1 and 0.2 GW/ cm . The linear and non-linear regimes were clearly discerned in the experimental data. The non-equilibrium heating of the conduction electrons is considered as the cause of the observed non-linear behaviour. © 1999 Elsevier Science B.V. All rights reserved
Linear and nonlinear total-yield photoemission observed in the subpicosecond regime in Mo
The total charge emitted from a polycrystalline Mo sample by 500 fs laser pulses at normal incidence is measured as a function of the laser peak intensity. Total yield data are taken at wavelengths of 527 and 264 nm. In both cases, a nonlinearity higher than expected is measured. A thermally enhanced regime is clearly observed when using 264 nm pulses for laser peak intensity larger than 0.1--0.2 . This effect is interpreted on the basis of the nonequilibrium heating of the conduction electrons. Pump and probe photoemission data at 527 nm show a significant enhancement of the photoelectric sensitivity when the probe pulse is delayed by 1 ps from the pump. This enhancement is related to the growth of the available electron density induced by the nonequilibrium heating. Single pulse photoemission at this wavelength is not properly explained by a thermally assisted photoemission regime. This may indicate that other processes have a role in determining the photoemission yield
Photon number statistics uncover the fluctuations in non-equilibrium lattice dynamics
Fluctuations of the atomic positions are at the core of a large class of
unusual material properties ranging from quantum para-electricity to high
temperature superconductivity. Their measurement in solids is the subject of an
intense scientific debate focused on seeking a methodology capable of
establishing a direct link between the variance of the atomic displacements and
experimentally measurable observables. Here we address this issue by means of
non-equilibrium optical experiments performed in shot-noise limited regime. The
variance of the time dependent atomic positions and momenta is directly mapped
into the quantum fluctuations of the photon number of the scattered probing
light. A fully quantum description of the non-linear interaction between
photonic and phononic fields is benchmarked by unveiling the squeezing of
thermal phonons in -quartz.Comment: 7 pages (main text), 5 figures, 11 pages (supplementary information
Pulsed homodyne Gaussian quantum tomography with low detection efficiency
Pulsed homodyne quantum tomography usually requires a high detection
efficiency limiting its applicability in quantum optics. Here, it is shown that
the presence of low detection efficiency () does not prevent the
tomographic reconstruction of quantum states of light, specifically, of
Gaussian type. This result is obtained by applying the so-called "minimax"
adaptive reconstruction of the Wigner function to pulsed homodyne detection. In
particular, we prove, by both numerical and real experiments, that an effective
discrimination of different Gaussian quantum states can be achieved. Our
finding paves the way to a more extensive use of quantum tomographic methods,
even in physical situations in which high detection efficiency is unattainable
Interband characterization and electronic transport control of nanoscaled GeTe/SbTe superlattices
The extraordinary electronic and optical properties of the
crystal-to-amorphous transition in phase-change materials led to important
developments in memory applications. A promising outlook is offered by
nanoscaling such phase-change structures. Following this research line, we
study the interband optical transmission spectra of nanoscaled
GeTe/SbTe chalcogenide superlattice films. We determine, for films with
varying stacking sequence and growth methods, the density and scattering time
of the free electrons, and the characteristics of the valence-to-conduction
transition. It is found that the free electron density decreases with
increasing GeTe content, for sub-layer thickness below 3 nm. A simple
band model analysis suggests that GeTe and SbTe layers mix, forming a
standard GeSbTe alloy buffer layer. We show that it is possible to control the
electronic transport properties of the films by properly choosing the
deposition layer thickness and we derive a model for arbitrary film stacks
Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon
We study the hole binding energy and pairing correlations in the three-band
Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and
constrained-path Monte Carlo simulations. In the physically relevant
charge-transfer regime, we find that the hole binding energy is strongly
enhanced by the electron-phonon interaction, which is due to a novel
potential-energy-driven pairing mechanism involving reduction of both
electronic potential energy and phonon related energy. The enhancement of hole
binding energy, in combination with a phonon-induced increase of quasiparticle
weight, leads to a dramatic enhancement of the long-range part of d-wave
pairing correlations. Our results indicate that the apical oxygen phonon plays
a significant role in the superconductivity of high- cuprates.Comment: 5 pages, 5 figure
Hubbard exciton revealed by time-domain optical spectroscopy
We use broadband ultra-fast pump-probe spectroscopy in the visible range to
study the lowest excitations across the Mott-Hubbard gap in the orbitally
ordered insulator YVO3. Separating thermal and non-thermal contributions to the
optical transients, we show that the total spectral weight of the two lowest
peaks is conserved, demonstrating that both excitations correspond to the same
multiplet. The pump-induced transfer of spectral weight between the two peaks
reveals that the low-energy one is a Hubbard exciton, i.e. a resonance or bound
state between a doublon and a holon. Finally, we speculate that the pump-driven
spin-disorder can be used to quantify the kinetic energy gain of the excitons
in the ferromagnetic phase.Comment: 5 pages and 6 figures, 9 pages and 12 figures with additional
material
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