56 research outputs found
Bypassing the energy-time uncertainty in time-resolved photoemission
The energy-time uncertainty is an intrinsic limit for time-resolved
experiments imposing a tradeoff between the duration of the light pulses used
in experiments and their frequency content. In standard time-resolved
photoemission, this limitation maps directly onto a tradeoff between the time
resolution of the experiment and the energy resolution that can be achieved on
the electronic spectral function. Here we propose a protocol to disentangle the
energy and time resolutions in photoemission. We demonstrate that dynamical
information on all time scales can be retrieved from time-resolved
photoemission experiments using suitably shaped light pulses of quantum or
classical nature. As a paradigmatic example, we study the dynamical buildup of
the Kondo peak, a narrow feature in the electronic response function arising
from the screening of a magnetic impurity by the conduction electrons. After a
quench, the electronic screening builds up on timescales shorter than the
inverse width of the Kondo peak and we demonstrate that the proposed
experimental scheme could be used to measure the intrinsic time scales of such
electronic screening. The proposed approach provides an experimental framework
to access the nonequilibrium response of collective electronic properties
beyond the spectral uncertainty limit and will enable the direct measurement of
phenomena such as excited Higgs modes and, possibly, the retarded interactions
in superconducting systems.Comment: Extended introduction, added references to section IIB, improved
wording in section II
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
Probing ultrafast symmetry breaking in photo-stimulated matter
The nature of a phase transition is inherently connected to the changes in
the crystalline symmtry, which is typically probed by elastic or inelastic
scattering with neutrons, electrons or photons. When such a phase transition is
stimulated by light or other sudden perturbations the solid evolves along a
non-equilibrium pathway of which the underlying physics is poorly understood.
Here we use picosecond Raman scattering to study the photo-induced ultrafast
dynamics in Peierls distorted Antimony. We find evidence for an ultrafast
non-thermal reversible structural phase transition. Most surprisingly, we find
evidence that this transition evolves toward a lower symmetry, in contrast to
the commonly accepted rhombohedral-to-simple cubic transition path. Our study
demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of
photo-induced non-thermal transient phases
Polaronic conductivity in the photoinduced phase of 1T-TaS2
The transient optical conductivity of photoexcited 1T-TaS2 is determined over
a three-order-of-magnitude frequency range. Prompt collapse and recovery of the
Mott gap is observed. However, we find important differences between this
transient metallic state and that seen across the thermally-driven
insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon
lineshapes, and a mid-infrared absorption band point to polaronic transport.
This is explained by noting that the photo-induced metallic state of 1T-TaS2 is
one in which the Mott gap is melted but the lattice retains its low-temperature
symmetry, a regime only accessible by photo-doping.Comment: 10 pages, 4 figure
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
Generation and detection of squeezed phonons in lattice dynamics by ultrafast optical excitations
We propose a fully quantum treatment for pump and probe experiments applied to the study of phonon excitations in solids. To describe the interaction between photons and phonons, a single effective hamiltonian is used that is able to model both the excitation induced by pump laser pulses and the subsequent measuring process through probe pulses. As the photoexcited phonons interact with their surroundings, mainly electrons and impurities in the target material, they cannot be considered isolated: their dynamics needs to be described by a master equation that takes into account the dissipative and noisy effects due to the presence of the environment. In this formalism, the quantum dynamics of pump excited phonons can be analyzed through suitable probe photon observables; in particular, a clear signature of squeezed phonons can be obtained by looking simultaneously at the behavior of the scattered probe mean photon number and its variance
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
Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse
In organic bulk heterojunction materials, charge delocalization has been
proposed to play a vital role in the generation of free carriers by reducing
the Coulomb attraction via an interfacial charge transfer exciton (CTX).
Pump-push-probe (PPP) experiments produced evidence that the excess energy
given by a push pulse enhances delocalization, thereby increasing photocurrent.
However, previous studies have employed near-IR push pulses in the range
0.4-0.6 eV which is larger than the binding energy of a typical CTX. This
raises the doubt that the push pulse may directly promote dissociation without
involving delocalized states. Here, we perform PPP experiments with mid-IR push
pulses at energies that are well below the binding energy of a CTX state
(0.12-0.25 eV). We identify three types of CTX: delocalized, localized, and
trapped. The excitation resides over multiple polymer chains in delocalized
CTXs, while is restricted to a single chain (albeit maintaining a degree of
intrachain delocalization) in localized CTXs. Trapped CTXs are instead
completely localized. The pump pulse generates a hot delocalized CTX, which
relaxes to a localized CTX, and eventually to trapped states. We find that
photo-exciting localized CTXs with push pulses resonant to the mid-IR charge
transfer absorption can promote delocalization and contribute to the formation
of long-lived charge separated states. On the other hand, we found that trapped
CTX are non-responsive to the push pulses. We hypothesize that delocalized
states identified in prior studies are only accessible in systems where there
is significant interchain electronic coupling or regioregularity that supports
either interchain or intrachain polaron delocalization. This emphasizes the
importance of engineering the micromorphology and energetics of the
donor-acceptor interface to exploit a full potential of a material for
photovoltaic applications
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