4,661 research outputs found
Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy
The control of optically driven high-frequency strain waves in nanostructured
systems is an essential ingredient for the further development of
nanophononics. However, broadly applicable experimental means to quantitatively
map such structural distortion on their intrinsic ultrafast time and nanometer
length scales are still lacking. Here, we introduce ultrafast convergent beam
electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative
retrieval of the time-dependent local distortion tensor. We demonstrate its
capabilities by investigating the ultrafast acoustic deformations close to the
edge of a single-crystalline graphite membrane. Tracking the structural
distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an
acoustic membrane breathing mode with spatially modulated amplitude, governed
by the optical near field structure at the membrane edge. Furthermore, an
in-plane polarized acoustic shock wave is launched at the membrane edge, which
triggers secondary acoustic shear waves with a pronounced spatio-temporal
dependency. The experimental findings are compared to numerical acoustic wave
simulations in the continuous medium limit, highlighting the importance of
microscopic dissipation mechanisms and ballistic transport channels
Real-time observation of a coherent lattice transformation into a high-symmetry phase
Excursions far from their equilibrium structures can bring crystalline solids
through collective transformations including transitions into new phases that
may be transient or long-lived. Direct spectroscopic observation of
far-from-equilibrium rearrangements provides fundamental mechanistic insight
into chemical and structural transformations, and a potential route to
practical applications, including ultrafast optical control over material
structure and properties. However, in many cases photoinduced transitions are
irreversible or only slowly reversible, or the light fluence required exceeds
material damage thresholds. This precludes conventional ultrafast spectroscopy
in which optical excitation and probe pulses irradiate the sample many times,
each measurement providing information about the sample response at just one
probe delay time following excitation, with each measurement at a high
repetition rate and with the sample fully recovering its initial state in
between measurements. Using a single-shot, real-time measurement method, we
were able to observe the photoinduced phase transition from the semimetallic,
low-symmetry phase of crystalline bismuth into a high-symmetry phase whose
existence at high electronic excitation densities was predicted based on
earlier measurements at moderate excitation densities below the damage
threshold. Our observations indicate that coherent lattice vibrational motion
launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk
bismuth brings the lattice structure directly into the high-symmetry
configuration for tens of picoseconds, after which carrier relaxation and
diffusion restore the equilibrium lattice configuration.Comment: 22 pages, 4 figure
Exciton Control in a Room-Temperature Bulk Semiconductor with Coherent Strain Pulses
The coherent manipulation of excitons in bulk semiconductors via the lattice
degrees of freedom is key to the development of acousto-optic and
acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit
strongly bound excitons that are interesting for applications in the
deep-ultraviolet, but their properties have remained elusive due to the lack of
efficient generation and detection schemes in this spectral range. Here, we
perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO
single crystals at room temperature, and reveal a dramatic modulation of the
exciton peak amplitude due to coherent acoustic phonons. This modulation is
comparable to those of nanostructures where exciton-phonon coupling is enhanced
by quantum confinement, and is accompanied by a giant exciton shift of 30-50
meV. We model these results by many-body perturbation theory and show that the
deformation potential coupling within the nonlinear regime is the main
mechanism for the generation and detection of the coherent acoustic phonons.
Our findings pave the way to the design of exciton control schemes in the
deep-ultraviolet with propagating strain pulses
Picosecond strain dynamics in GeSbTe monitored by time-resolved x-ray diffraction
Coherent phonons (CP) generated by laser pulses on the femtosecond scale have
been proposed as a means to achieve ultrafast, non-thermal switching in
phase-change materials such as GeSbTe(GST). Here we use
ultrafast optical pump pulses to induce coherent acoustic phonons and
stroboscopically measure the corresponding lattice distortions in GST using 100
ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a
good description of the observed changes in the diffraction signal, however,
the magnitudes of the measured shifts are too large to be explained by thermal
effects alone implying the presence of transient non-equilibrium electron
heating in addition to temperature driven expansion. The information on the
movement of atoms during the excitation process can lead to greater insight
into the possibilities of using CP-induced phase-transitions in GST.Comment: 7 pages, 4 figures, Phys. Rev. B, in pres
Control of carrier transport in GaAs by longitudinal-optical phonon-carrier scattering using a pair of laser pump pulses
We demonstrate optical control of the LO phonon-plasmon coupled (LOPC) modes
in GaAs by using a femtosecond pump-pulse pair. The relaxation time of the
plasmon-like LOPC mode significantly depends on the separation time (\Delta t)
of the pump-pulse pair. Especially it is maximized when \Delta t becomes
simultaneously comparable to the half period of the longitudinal optical (LO)
phonon oscillation and resonant to the 3/4 period of the plasmon-like LOPC
oscillation. We attribute these observations to the modification of carrier-LO
phonon scattering and ballistic motion of the plasmon-like LOPC mode.Comment: 5 pages, 4 figures, submitted to Journal of Applied Physic
Ultrafast relaxation of hot phonons in Graphene-hBN Heterostructures
Fast carrier cooling is important for high power graphene based devices.
Strongly Coupled Optical Phonons (SCOPs) play a major role in the relaxation of
photoexcited carriers in graphene. Heterostructures of graphene and hexagonal
boron nitride (hBN) have shown exceptional mobility and high saturation
current, which makes them ideal for applications, but the effect of the hBN
substrate on carrier cooling mechanisms is not understood. We track the cooling
of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast
pump-probe spectroscopy. We find that the carriers cool down four times faster
in the case of graphene on hBN than on a silicon oxide substrate thus
overcoming the hot phonon (HP) bottleneck that plagues cooling in graphene
devices.Comment: Pages 1-12: Main manuscript. Pages 13-18: Supplementary materia
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