38 research outputs found
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
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
Quasi-particles dynamics in underdoped Bi2212 under strong optical perturbation.
In this work an optical pump-probe set-up is used to study the photo-induced non-equilibrium dynamics of a superconducting underdoped Bi2212 single crystal in a strong excitation regime (10<<600 \ub5J/cm2). The use of a tunable repetition rate 120 fs pulsed laser source allows us to avoid significant average heating of the sample and to optimize the signal-to-noise ratio in the detection of the transient reflectivity variation. A discontinuity of the transient reflectivity is observed at high excitation intensities (~70 \ub5J/cm2). Numerical simulations of the heat diffusion problem indicate that, in this regime, the local temperature of the sample is lower than TC, confirming the impulsive nature of this phenomenon. The quasi-particles (QP) dynamics in the strongly perturbed superconducting state (10<<70 \ub5J/cm2) is analysed within the framework of the Rotwarf-Taylor model. The picture emerging from the data is consistent with a dynamics governed by high-frequency phonon (HFP) population, which causes a bottleneck effect in the QP recombinatio
Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy
Charge-transfer excitations are of paramount importance for understanding the
electronic structure of copper-oxide based high-temperature superconductors. In
this study, we investigate the response of a
BiSrCaCuO crystal to the charge
redistribution induced by an infrared ultrashort pulse. Element-selective
time-resolved core-level photoelectron spectroscopy with a high energy
resolution allows disentangling the dynamics of oxygen ions with different
coordination and bonds thanks to their different chemical shifts. Our
experiment shows that the O\, component arising from the Cu-O planes is
significantly perturbed by the infrared light pulse. Conversely, the apical
oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected.
This result highlights the peculiar behavior of the electronic structure of the
Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic
structure of copper-oxide-based high-temperature superconductors by identifying
the O\, core-level emission originating from the oxygen ions in the Cu-O
planes. This ability could be critical to gain information about the
strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane
in the normal and superconducting phases
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES)
experiments the presence of the pump pulse adds a new level of complexity to
the photoemission process in comparison to conventional ARPES. This is
evidenced by pump-induced vacuum space-charge effects and surface
photovoltages, as well as multiple pump excitations due to internal reflections
in the sample-substrate system. These processes can severely affect a correct
interpretation of the data by masking the out-of-equilibrium electron dynamics
intrinsic to the sample. In this study, we show that such effects indeed
influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In
particular, we find a time- and laser fluence-dependent spectral shift and
broadening of the acquired spectra, and unambiguously show the presence of a
double pump excitation. The dynamics of these effects is slower than the
electron dynamics in the graphene sample, thereby permitting us to deconvolve
the signals in the time domain. Our results demonstrate that complex
pump-related processes should always be considered in the experimental setup
and data analysis.Comment: 9 pages, 4 figure
Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene
Bilayer graphene is a highly promising material for electronic and
optoelectronic applications since it is supporting massive Dirac fermions with
a tuneable band gap. However, no consistent picture of the gap's effect on the
optical and transport behavior has emerged so far, and it has been proposed
that the insulating nature of the gap could be compromised by unavoidable
structural defects, by topological in-gap states, or that the electronic
structure could be altogether changed by many-body effects. Here we directly
follow the excited carriers in bilayer graphene on a femtosecond time scale,
using ultrafast time- and angle-resolved photoemission. We find a behavior
consistent with a single-particle band gap. Compared to monolayer graphene, the
existence of this band gap leads to an increased carrier lifetime in the
minimum of the lowest conduction band. This is in sharp contrast to the second
sub-state of the conduction band, in which the excited electrons decay through
fast, phonon-assisted inter-band transitions.Comment: 5 pages, 4 figure
Ultrafast insulator-to-metal phase transition as a switch to measure the spectrogram of a supercontinuum light pulse
In this letter we demonstrate the possibility to determine the temporal and spectral structure (spectrogram) of a complex light pulse exploiting the ultrafast switching character of a nonthermal photoinduced phase transition. As a proof, we use a VO2 multifilm, undergoing an ultrafast insulator-to-metal phase transition when excited by femtosecond near-infrared laser pulses. The abrupt variation in the multifilm optical properties, over a broad infrared/visible frequency range, is exploited to determine, in situ and in a simple way, the spectrogram of a supercontinuum pulse produced by a photonic crystal fiber. The determination of the structure of the pulse is mandatory to develop pump-probe experiments with frequency resolution over a broad spectral range (700-1100 nm)
Direct view on the ultrafast carrier dynamics in graphene
The ultrafast dynamics of excited carriers in graphene is closely linked to
the Dirac spectrum and plays a central role for many electronic and
optoelectronic applications. Harvesting energy from excited electron-hole
pairs, for instance, is only possible if these pairs can be separated before
they lose energy to vibrations, merely heating the lattice. While the hot
carrier dynamics in graphene could so far only be accessed indirectly, we here
present a direct time-resolved view on the Dirac cone by angle-resolved
photoemission (ARPES). This allows us to show the quasi-instant thermalisation
of the electron gas to a temperature of more than 2000 K; to determine the
time-resolved carrier density; to disentangle the subsequent decay into
excitations of optical phonons and acoustic phonons (directly and via
supercollisions); and to show how the presence of the hot carrier distribution
affects the lifetime of the states far below the Fermi energy.Comment: 15 pages, 4 figure
Ultrafast orbital manipulation and Mott physics in multi-band correlated materials
Multiorbital correlated materials are often on the verge of multiple electronic phases (metallic, insulating, superconducting, charge and orbitally ordered), which can be explored and controlled by small changes of the external parameters. The use of ultrashort light pulses as a mean to transiently modify the band population is leading to fundamentally new results. In this paper we will review recent advances in the field and we will discuss the possibility of manipulating the orbital polarization in correlated multi-band solid state systems. This technique can provide new understanding of the ground state properties of many interesting classes of quantum materials and offers a new tool to induce transient emergent properties with no counterpart at equilibrium. We will address: the discovery of high-energy Mottness in superconducting copper oxides and its impact on our understanding of the cuprate phase diagram; the instability of the Mott insulating phase in photoexcited vanadium oxides; the manipulation of orbital-selective correlations in iron-based superconductors; the pumping of local electronic excitons and the consequent transient effective quasiparticle cooling in alkali-doped fullerides. Finally, we will discuss a novel route to manipulate the orbital polarization in a a k-resolved fashion