32 research outputs found
Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit
We present the design and implementation of a highly compact femtosecond
electron diffractometer working at electron energies up to 100 keV. We use a
multi-body particle tracing code to simulate electron bunch propagation through
the setup and to calculate pulse durations at the sample position. Our
simulations show that electron bunches containing few thousands of electrons
per bunch are only weakly broadened by space-charge effects and their pulse
duration is thus close to the one of a single-electron wavepacket. With our
compact setup we can create electron bunches containing up to 5000 electrons
with a pulse duration below 100 femtoseconds on the sample. We use the
diffractometer to track the energy transfer from photoexcited electrons to the
lattice in a thin film of titanium. This process takes place on the timescale
of few-hundred femtoseconds and a fully equilibrated state is reached within
one picosecond.Comment: 5 pages, 3 figure
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials
The extreme electro-optical contrast between crystalline and amorphous states
in phase change materials is routinely exploited in optical data storage and
future applications include universal memories, flexible displays,
reconfigurable optical circuits, and logic devices. Optical contrast is
believed to arise due to a change in crystallinity. Here we show that the
connection between optical properties and structure can be broken. Using a
unique combination of single-shot femtosecond electron diffraction and optical
spectroscopy, we simultaneously follow the lattice dynamics and dielectric
function in the phase change material Ge2Sb2Te5 during an irreversible state
transformation. The dielectric function changes by 30% within 100 femtoseconds
due to a rapid depletion of electrons from resonantly-bonded states. This
occurs without perturbing the crystallinity of the lattice, which heats with a
2 ps time constant. The optical changes are an order-of-magnitude larger than
those achievable with silicon and present new routes to manipulate light on an
ultrafast timescale without structural changes
Time-domain separation of optical properties from structural transitions in resonantly bonded materials
The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage1 and future applications include universal memories2, flexible displays3, reconfigurable optical circuits4, 5, and logic devices6. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.Peer ReviewedPostprint (author's final draft
Ultrafast Heat Flow in Heterostructures of Au Nanoclusters on Thin Films: Atomic Disorder Induced by Hot Electrons
We study the ultrafast structural dynamics, in response to electronic excitations, in heterostructures composed of size-selected Au nanoclusters on thin-film substrates with the use of femtosecond electron diffraction. Various forms of atomic motion, such as thermal vibrations, thermal expansion, and lattice disordering, manifest as distinct and quantifiable reciprocal-space observables. In photoexcited supported nanoclusters, thermal equilibration proceeds through intrinsic heat flow between their electrons and their lattice and extrinsic heat flow between the nanoclusters and their substrate. For an in-depth understanding of this process, we have extended the two-temperature model to the case of 0D/2D heterostructures and used it to describe energy flow among the various subsystems, to quantify interfacial coupling constants and to elucidate the role of the optical and thermal substrate properties. When lattice heating of Au nanoclusters is dominated by intrinsic heat flow, a reversible disordering of atomic positions occurs, which is absent when heat is injected as hot substrate phonons. The present analysis indicates that hot electrons can distort the lattice of nanoclusters, even if the lattice temperature is below the equilibrium threshold for surface premelting. Based on simple considerations, the effect is interpreted as activation of surface diffusion due to modifications of the potential energy surface at high electronic temperatures. We discuss the implications of such a process in structural changes during surface chemical reactions
Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe/MoSe heterobilayers
We identify an optical excitation mechanism that transfers a valley
polarization from photo-excited electron-hole pairs to free charge carriers in
twisted WSe/MoSe heterobilayers. For small twist angles, the valley
lifetimes of the charge carriers are surprisingly short, despite the occurrence
of interlayer excitons with their presumably long recombination and
polarization lifetimes. For large twist angles, we measure an increase in both
the valley polarization and its respective lifetime by more than two orders of
magnitude. Interestingly, in such heterobilayers we observe an interlayer
transfer of valley polarization from the WSe layer into the MoSe layer.
This mechanism enables the creation of a photo-induced valley polarization of
free charge carriers in MoSe, which amplitude scales with the gate-induced
charge carrier density. This is in contrast to monolayer MoSe, where such a
gate-tunable valley polarization cannot be achieved. By combining time-resolved
Kerr rotation, photoluminesence and angle-resolved photoemission spectroscopy
measurements with first principles calculations, we show that these findings
can be explained by twist angle dependent interlayer scattering mechanisms
involving the Q- and -valleys.Comment: 21 pages, 10 figure
Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2
Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe2) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼g2s pol—pol ∼46:4±13:9 μeVμm2 which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies