Subpicosecond Dynamics Research Articles

Orientation independent vibrational dynamics of lipid-bound interfacial water.

Zwitterionic phospholipids are one of the main constituents of biological membranes. The electric field associated with the two opposite headgroup charges aligns water molecules in the headgroup region. Here, we study the role of water alignment on the sub-picosecond vibrational dynamics of lipid-bound water. To this end, we compare the dynamics of oppositely oriented water associated with, respectively, a phosphocholine (PC) headgroup and an inverse-phosphocholine with non-ethylated phosphate groups (CP). We find that the dynamics are independent of the water orientation, implying that the vibrational dynamics report on the local properties of the water molecules.

Tunable electron beam pulser for picoseconds stroboscopic microscopy in transmission electron microscopes

For two decades, time-resolved transmission electron microscopes (TEM) have relied on pulsed-laser photoemission to generate electron bunches to explore sub-microsecond to sub-picosecond dynamics. Despite the vast successes of photoemission time-resolved TEMs, laser-based systems are inherently complex, thus tend not to be turn-key. In this paper, we report on the successful retrofit of a commercial 200 keV TEM, without an external laser, capable of producing continuously tunable pulsed electron beams with repetition rates from 0.1 GHz up to 12 GHz and a tunable bunch length from tens of nanoseconds down to 10 ps. This innovation enables temporal access into previously inaccessible regimes: i.e., high repetition rate stroboscopic experiments. Combination of a pair of RF-driven traveling wave stripline elements, quadrupole magnets, and a variable beam aperture enables operation of the instrument in (1) continuous waveform (CW) mode as though the instrument was never modified (i.e. convention TEM operation mode, where the electrons from the emission cathode randomly arrive at the sample without resolvable time information), (2) stroboscopic (pump-probe) mode, and (3) pulsed beam mode for dose rate sensitive materials. To assess the effect of a pulsed beam on image quality, we examined Au nanoparticles using bright field, high-resolution TEM imaging and selected area diffraction in both continuous and pulsed-beam mode. In comparison of conventional TEMs, the add-on beam pulser enables the observation of ultrafast dynamic behavior in materials that are reversible under synchronized excitation.

Interfacial H-Bond Dynamics in Reverse Micelles: The Role of Surfactant Heterogeneity.

Characterizing the hydrogen bond structure and dynamics at surfactant interfaces is essential for understanding how microscopic interactions translate to bulk microemulsion properties. Heterogeneous blends containing tens or hundreds of surfactants are common in the industry, but the most fundamental studies have been carried out on micelles composed of a single surfactant species. Therefore, the effect of surfactant heterogeneity on the interfacial structure and dynamics remains poorly understood. Here, we use ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations to characterize sub-picosecond solvation dynamics as a function of the surfactant composition in ∼120 nm water-in-oil reverse micelles. We probe the ester carbonyl vibrations of nonionic sorbitan surfactants, which are located precisely at the interface between the polar and nonpolar regions, and as such, report on the interfacial water dynamics. We show a 7% increase in hydrogen bond populations together with a 37% slowdown of interfacial hydrogen bond dynamics in heterogeneous mixtures containing hundreds of species, compared to more uniform compositions. Simulations, which are in semiquantitative agreement with experiments, indicate that structural diversity leads to decreased packing efficiency, which in turn drives water further into the otherwise hydrophobic region. Interestingly, this increase in hydration is accompanied by a slowdown of dynamics, indicating that water molecules solvating surfactants are conformationally constrained. These studies demonstrate that the composition and heterogeneity are key factors in determining interfacial properties.

Inertial water response dominates protein solvation dynamics

Abstract Establishing the mechanism of protein folding and other physiological processes requires a detailed comprehension of protein-solvent interactions. The fastest protein solvation dynamics have not yet been thoroughly investigated due to challenges associated with controlling few-cycle laser pulses and identifying ideal model systems. Here we use 6 fs laser pulses to quantify the sub-picosecond solvation dynamics of an engineered pigment-protein complex that serves as an ideal probe of solvation dynamics. The data reveal that protein solvation dynamics are described well by a single lifetime of 33 fs, indicating that the mechanism of protein solvation is dominated by the inertial water component.

Ultrafast spectroscopy of shift-current in ferroelectric semiconductor Sn2P2S6

We report sub-picosecond photocarrier dynamics observed via emitted terahertz waves in a ferroelectric semiconductor Sn2P2S6. Excitation photon energy, intensity and polarization dependences of the photocarrier dynamics testify that the ultrafast photocurrent originates from the shift-current under interband excitation. The photocurrent excitation spectrum shows a quantitative agreement with that derived from first-principles calculations with the Berry connection integrated, showing the shift-current to be a sensitive feature in ferroelectrics. The terahertz emission spectroscopy is a powerful tool to identify the intricate carrier dynamics under pulsed photoexcitation.

PH Dependence of Ultrafast Charge Dynamics in Graphene Oxide Dispersions

The pH dependence of emission from graphene oxide is believed to be due to the protonation of surface functional groups. In this study, we use transient absorption spectroscopy to study the sub-picosecond charge dynamics in graphene oxide over a range of pH values, observing dynamics consistent with an excited-state protonation step for pH < 9.3. The timescale of this process is ∼1.5 ps, and a corresponding change in recombination dynamics follows. A broad photo-induced absorption peak centered at 530 nm associated with excited-state protonation is also observed.

Genuine Dynamics vs Cross Phase Modulation Artifacts in Femtosecond Stimulated Raman Spectroscopy

Femtosecond stimulated Raman scattering is a time-resolved vibrational spectroscopic technique able to access sub-picosecond dynamics and providing accurate structural information. Thanks to an appropriate combination of three laser pulses, triggering vibrational coherences delayed with respect to the photoinduced event of interest, it is capable of uncompromised temporal precision (down to 50 fs) and spectral resolution (a few wavenumbers), better than spontaneous Raman. Reaching such extreme time scales requires significant temporal overlap of pulses, which gives rise to undesired nonlinear artifacts, often hampering an immediate interpretation of the Raman spectra. Building on a perturbative expansion of the density matrix, we identify the origin of such artifacts in cross phase modulation effects and show how they can be theoretically evaluated and factored out from the signals. We experimentally benchmark the theoretical predictions in a nonreactive model system, namely cyclohexane.

Room temperature quantum coherence vs. electron transfer in a rhodanine derivative chromophore.

Understanding electron transfer in organic molecules is of great interest in quantum materials for light harvesting, energy conversion and integration of molecules into solar cells. This, however, poses the challenge of designing specific optimal molecular structure for which the processes of ultrafast quantum coherence and electron transport are not so well understood. In this work, we investigate subpicosecond time scale quantum dynamics and electron transfer in an efficient electron acceptor rhodanine chromophoric complex. We consider an open quantum system approach to model the complex-solvent interaction, and compute the crossover from weak to strong dissipation on the reduced system dynamics for both a polar (methanol) and a non polar solvent (toluene). We show that the electron transfer rates are enhanced in the strong chromophore-solvent coupling regime, being the highest transfer rates those found at room temperature. Even though the computed dynamics are highly non-Markovian, and they may exhibit a quantum character up to hundreds of femtoseconds, we show that quantum coherence does not necessarily optimise the electron transfer in the chromophore.

A first peek into sub-picosecond dynamics of spin energy levels in magnetic biomolecules.

We estimate the time- and temperature-evolution of spin energy levels in a metallopeptide by combining molecular dynamics with crystal field analysis. Fluctuations of tens of cm-1 for spin energy levels at fs times gradually average out at longer times. We confirm that local vibrations are key in spin dynamics.

Impact of ultrafast transport on the high-energy states of a photoexcited topological insulator

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.

Retinal isomerization in bacteriorhodopsin captured by a femtosecond x-ray laser.

Ultrafast isomerization of retinal is the primary step in photoresponsive biological functions including vision in humans and ion transport across bacterial membranes. We used an x-ray laser to study the subpicosecond structural dynamics of retinal isomerization in the light-driven proton pump bacteriorhodopsin. A series of structural snapshots with near-atomic spatial resolution and temporal resolution in the femtosecond regime show how the excited all-trans retinal samples conformational states within the protein binding pocket before passing through a twisted geometry and emerging in the 13-cis conformation. Our findings suggest ultrafast collective motions of aspartic acid residues and functional water molecules in the proximity of the retinal Schiff base as a key facet of this stereoselective and efficient photochemical reaction.

Picosecond Electronic and Structural Dynamics in Photo-excited Monolayer MoSe2

Monolayers of semiconducting transitional metal dichalcogenides (TMDC) are emerging as strong candidate materials for next generation electronic and optoelectronic devices, with applications in field-effect transistors, valleytronics, and photovoltaics. Prior studies have demonstrated strong light-matter interactions in these materials, suggesting optical control of material properties as a promising route for their functionalization. However, the electronic and structural dynamics in response to electronic excitation have not yet been fully elucidated. In this work, we use non-adiabatic quantum molecular dynamics simulations based on time-dependent density functional theory to study lattice dynamics of a model TMDC monolayer of MoSe2 after electronic excitation. The simulation results show rapid, sub-picosecond lattice response, as well as finite-size effects. Understanding the sub-picosecond atomic dynamics is important for the realization of optical control of the material properties of monolayer TMDCs, which is a hopeful, straightforward tactic for functionalizing these materials.

Femtosecond Dynamics of Spin-Polarized Electrons in Topological Insulators

A fast control of spins is a major quest in spintronic systems. Ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators have received attention due to the existence, within the insulating band gap of bulk states, of spin-polarized surface states that are protected from backscattering by time-reversal symmetry. We studied sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of Bi2Te3, employing circular-polarized light in time- and angle-resolved photoemission spectroscopy. Noncollinear optical parametric amplification and several nonlinear optical processes result in tunable, ultrashort visible pump pulses with a duration of 30 fs and 1.8 eV energy and ultraviolet probe pulses with about 60 fs duraction and about 6 eV energy. The stable optical setup and the high repetition rate of an Yb-laser source gives a high signal-to-noise ratio in our photoemission process. The 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight energy analyzer, provides the opportunity to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism allows access to the spin state of the photoemitted electrons. A signature of femtosecond unpolarized bulk bands dynamics appears in the presence of spin-polarized electrons of the surface states. This helps distinguish the bulk and surface contributions in the spin-electronic current.

Ultrafast hot-electron induced quenching of Tb 4f magnetic order

We have investigated ultrafast quenching of the Tb4f magnetic order in Co74Tb26 alloys, induced by femtosecond hot-electrons pulses. The hot-electron pulses were produced in specific non-magnetic capping layers by infrared fs laser pulses. Our experimental results show that sub-picosecond dynamics of Tb4f magnetic moments can be induced by non-thermal and thermal hot-electrons. We further demonstrate that the demagnetization efficiencies of non-thermal and thermal hot electrons are similar. However, the characteristic demagnetization times show values of 0.35 ps for non-thermal hot-electrons excitations and 1.2 ps for thermal hot-electrons excitations. We explain this temporal elongation by the propagation time of thermal hot-electrons through the 15 nm thick CoTb film.

Subpicosecond dynamics of the molecular polyalanine dipole moment

Instantaneous dipole moments of polyalanine peptides in vacuum and in the aqueous medium have been calculated on the picosecond time scale in order to evaluate the external influence of temperature, of the electrostatic field's amplitude and direction. Computer simulation was performed using the molecular dynamics method. The dynamic scenarios induced by the external electrostatic field above 100 MV/m were obtained for polyalanine molecules of different lengths (from 2 to 24 groups) placed in vacuum and in aqueous medium, the time step of 1 fs and the simulation time up to 100 ns being taken. The simulated scenarios can be used for a further analysis and a generalized description of structural properties and conformational dynamics of molecules. The mastered software packages are appropriate for computing the representational scenarios of biomolecular behavior under various conditions.

Delocalization and stretch-bend mixing of the HOH bend in liquid water.

Liquid water's rich sub-picosecond vibrational dynamics arise from the interplay of different high- and low-frequency modes evolving in a strong yet fluctuating hydrogen bond network. Recent studies of the OH stretching excitations of H2O indicate that they are delocalized over several molecules, raising questions about whether the bending vibrations are similarly delocalized. In this paper, we take advantage of an improved 50 fs time-resolution and broadband infrared (IR) spectroscopy to interrogate the 2D IR lineshape and spectral dynamics of the HOH bending vibration of liquid H2O. Indications of strong bend-stretch coupling are observed in early time 2D IR spectra through a broad excited state absorption that extends from 1500 cm-1 to beyond 1900 cm-1, which corresponds to transitions from the bend to the bend overtone and OH stretching band between 3150 and 3550 cm-1. Pump-probe measurements reveal a fast 180 fs vibrational relaxation time, which results in a hot-ground state spectrum that is the same as observed for water IR excitation at any other frequency. The fastest dynamical time scale is 80 fs for the polarization anisotropy decay, providing evidence for the delocalized or excitonic character of the bend. Normal mode analysis conducted on water clusters extracted from molecular dynamics simulations corroborate significant stretch-bend mixing and indicate delocalization of δHOH on 2-7 water molecules.

Guest Hydrogen Bond Dynamics and Interactions in the Metal–Organic Framework MIL-53(Al) Measured with Ultrafast Infrared Spectroscopy

MIL-53(Al) is among the most “flexible” metal–organic frameworks and significantly changes its structure when guest molecules are introduced into the framework’s pores. The guest molecules interact with μ2-OH, which is a hydroxyl group bridging two aluminums. This interaction is thought to play an important role in the adsorption of molecules into the pores. Here, we report infrared absorption, infrared pump–probe, and two-dimensional infrared spectroscopy on a deuterated bridging hydroxyl, μ2-OD, in MIL-53(Al) loaded with various guest molecules. Depending on guest, large vibrational frequency shifts, broadening of absorption band, enhancement of absorption coefficient, and acceleration of vibrational relaxation were observed, which are all signatures of hydrogen bonding interactions between the bridging hydroxyls and the guest. 2D IR spectroscopy reveals that the time evolution of the vibrational frequency occurs on multiple time scales: subpicosecond dynamics caused by localized hydrogen bond fluctuati...

Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging.

Among the variety of magnetic textures available in nature, antiferromagnetism is one of the most 'discrete' because of the exact cancellation of its staggered internal magnetization. It is therefore very challenging to probe. However, its insensitivity to external magnetic perturbations, together with the intrinsic sub-picosecond dynamics, make it very appealing for tomorrow's information technologies. Thus, it is essential to understand the microscopic mechanisms governing antiferromagnetic domains to achieve accurate manipulation and control. Using optical second-harmonic generation, a unique and laboratory-available tool, we succeeded in imaging with sub-micrometre resolution both electric and antiferromagnetic orders in the model multiferroic BiFeO3. We show here that antiferromagnetic domains can be manipulated with low power consumption, using sub-coercive electric fields and sub-picosecond light pulses. Interestingly, we also show that antiferromagnetic and ferroelectric domains can behave independently, thus revealing that magneto-electric coupling can lead to various arrangements of the two orders.

Selective Excitation of Terahertz Magnetic and Electric Dipoles in Er^{3+} Ions by Femtosecond Laser Pulses in ErFeO_{3}.

We show that femtosecond laser pulse excitation of the orthoferrite ErFeO_{3} triggers pico- and subpicosecond dynamics of magnetic and electric dipoles associated with the low energy electronic states of the Er^{3+} ions. These dynamics are readily revealed by using polarization sensitive terahertz emission spectroscopy. It is shown that by changing the polarization of the femtosecond laser pulse one can excite either electric dipole-active or magnetic dipole-active transitions between the Kramers doublets of the ^{4}I_{15/2} ground state of the Er^{3+} (4f^{11}) ions. These observations serve as a proof of principle of polarization-selective control of both electric and magnetic degrees of freedom at terahertz frequencies, opening up new vistas for optical manipulation of magnetoelectric materials.

Jahn-Teller-induced femtosecond electronic depolarization dynamics of the nitrogen-vacancy defect in diamond.

Single-photon emission from the nitrogen-vacancy defect in diamond constitutes one of its many proposed applications. Owing to its doubly degenerate 3E electronic excited state, photons from this defect can be emitted by two optical transitions with perpendicular polarization. Previous measurements have indicated that orbital-selective photoexcitation does not, however, yield photoluminescence with well-defined polarizations, thus hinting at orbital-averaging dynamics even at cryogenic temperatures. Here we employ femtosecond polarization anisotropy spectroscopy to investigate the ultrafast electronic dynamics of the 3E state. We observe subpicosecond electronic dephasing dynamics even at cryogenic temperatures, up to five orders of magnitude faster than dephasing rates suggested by previous frequency- and time-domain measurements. Ab initio molecular dynamics simulations assign the ultrafast depolarization dynamics to nonadiabatic transitions and phonon-induced electronic dephasing between the two components of the 3E state. Our results provide an explanation for the ultrafast orbital averaging that exists even at cryogenic temperatures.