Ultrafast Dynamical Processes Research Articles

Laser-induced two-step demagnetization process study in Ni–Mn–Sn Heusler alloy film

The ultrafast magnetization dynamic processes are investigated in a wide time scale range for different laser pump fluences in Ni- and Mn-rich Heusler alloy film of Ni54.3Mn31.9Sn13.8 composition using time-resolved magneto-optical Kerr effect (TR-MOKE) in perpendicular magnetic field geometry. For all fluences used, two distinct types of magnetization dynamics of different time scales: ultrafast up to 2 picoseconds and slower from 2 ps to hundreds of ps, were observed. The description of the two-step de- and remagnetization processes was performed in the frame of modified model developed, based on the microscopic three-temperature model (M3TM). The model is based on the assumption that two magnetic sublattices of the Heusler alloy which are weakly exchange coupled due to Curie temperature proximity are responsible for the fast and slow magnetization dynamics. It is demonstrated excellent agreement of the model prediction with experimental data for all the fluences used. It is shown that the values of model parameters determined — demagnetization rates R and spin-flip probabilities asf for one sublattice are of the order reported for the Ni film. It has been found that R parameter value for the second sublattice related to Mn site is over two orders, and asf is up to one order of magnitude lower than in Ni site. It is shown that strong reduction of the spin-flip probability and electron–phonon coupling, and large magnetic moment in the Mn sublattice as compared to the Ni one is the cause of the observed demagnetization slowing down effect in the Heusler alloy studied.

Electrically driven transport of photoinduced hot carriers in carbon nanotube fibers.

Hot carriers play a significant role in applications of photovoltaics, photodetection, and photocatalysis. However, effective methods for observing the ultrafast dynamic processes of hot carriers are concentrated on the time domain, on which it is difficult and complex to operate. We propose a novel, to the best of our knowledge, and creative strategy to convert the time-domain dynamic process into a spatially thermal redistribution in suspended carbon nanotube fibers. The large average free path of photoinduced hot holes ensures a prominent offset of temperature distribution. The experimental results confirm the theory about electrically driven transport of hot holes, which has rarely been reported.

Crystallographic features and microstructure evolution of sandwich warm laser polishing: The case of aluminum foil

To obtain high-performance metal foil (the case of aluminum foil) with an ultra-smooth surface and deeply study its polishing mechanism, sandwich warm laser polishing (SWLP) is proposed in this study as a generic polishing approach for metal foils. Investigations of the surface roughness and micro-morphology of aluminum foils polished using SWLP technology revealed that the surface roughness was reduced by 97.5%. However, the inadequate understanding of the polishing and strengthening mechanism may limit the practical application of such technology. Therefore, the crystallographic features and microstructure evolution of the aluminum foil before and after SLP/SWLP were analyzed using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and molecular dynamics (MD) simulations, and the mechanism for the reduction in roughness and improvement in tensile properties of the aluminum foil caused by SWLP and the fundamental difference between SLP and SWLP were determined, even the ultrafast dynamical process as small as nanoscale was observed by MD simulation. The nanoscale ultrafast dynamic process, crystallographic features, microstructure evolution, and the fundamental difference between SLP and SWLP revealed in this study are of great significance for understanding the polishing/materials modification mechanism of the aluminum foil, and for the development of future laser polishing technology.

Ultrafast dynamics control on ablation of Cu using shaped femtosecond pulse trains

The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.

Revealing the Dynamic Mechanism by Which Transferrin Promotes the Cellular Uptake of HAIYPRH Peptide-Conjugated Nanostructures by Force Tracing.

The HAIYPRH (T7) peptide has been widely used as a ligand for constructing tumor-targeted nanodrug delivery systems since it can target the transferrin receptor (TfR) and then enter cells easily with the help of transferrin (Tf). However, the dynamic mechanism by which transferrin promotes the entry of T7-conjugated nanostructures into cells remains unclear. Herein, a force tracing technique based on atomic force microscopy (AFM) was used to track the ultrafast dynamic process of a T7-conjugated gold nanoparticle (AuNP-T7) entering a cell at the single-particle level in real time. Tf helped decrease the endocytosis force and increase the endocytosis speed of AuNP-T7 in A549 cells. However, Tf only increased the endocytosis speed of AuNP-T7 in HeLa cells. In contrast, in Vero cells without TfR overexpression, Tf decreased the endocytosis speed. This report provides important insights for redesigning and developing T7-conjugated nanodrug carriers in targeted nanodrug delivery systems.

Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide

Femtosecond transient absorption spectroscopy has been employed to unravel separate initial nonequilibrium dynamic processes of photo-injected electrons and holes during the formation process of the lowest excitons at the K-valley in few-layer tungsten disulfide. Charge carrier thermalization and cooling, as well as concomitant many-body effects on the exciton resonances, are distinguished. The thermalization of holes is observed to be faster than that of electrons. Both of them proceed predominantly via carrier-carrier scattering, as evidenced by the observed dependence of the thermalization time on pump fluences. The fluence dependent time constants also suggest that the subsequent cooling for electrons is probably dominated by acoustic phonons, whereas for holes it is mostly controlled by LO phonons. An extremely fast red- and blue-shift crossover followed by a slow blue-shift of exciton resonance was observed in the temporal evolution of exciton resonances by resonant exciton A excitation. The rapid red-shift could be due to the strong screening of the Coulomb interaction between quasi-free charge carriers in electron-hole plasma. The subsequent slow blue-shift is the net result of the competition among many-body effects in the hot-exciton cooling process. Our findings elucidate the carrier-selective ultrafast dynamics and their many-body effects, underpinning new possibilities for developing optoelectronic devices based on transport properties of a single type of carrier.

Coincident angle-resolved state-selective photoelectron spectroscopy of acetylene molecules: a candidate system for time-resolved dynamics.

The acetylene-vinylidene system serves as a benchmark for investigations of ultrafast dynamical processes where the coupling of the electronic and nuclear degrees of freedom provides a fertile playground to explore the femto- and sub-femto-second physics with coherent extreme-ultraviolet (EUV) photon sources both on the table-top as well as free-electron lasers. We focus on detailed investigations of this molecular system in the photon energy range 19-40 eV where EUV pulses can probe the dynamics effectively. We employ photoelectron-photoion coincidence (PEPICO) spectroscopy to uncover hitherto unrevealed aspects of this system. In this work, the role of excited states of the C2H2+ cation, the primary photoion, is specifically addressed. From photoelectron energy spectra and angular distributions, the nature of the dissociation and isomerization channels is discerned. Exploiting the 4π-collection geometry of the velocity map imaging spectrometer, we not only probe pathways where the efficiency of photoionization is inherently high but also perform PEPICO spectroscopy on relatively weak channels.

Experimental measurement of the wake field in a plasma filament created by a single-color ultrafast laser pulse

A laser plasma wake field in a single-color femtosecond laser filament determines the acceleration of ionized electrons, which affects the intensity and bandwidth of the emitted terahertz wave and is important for understanding the fundamental nonlinear process of THz generation. Since the THz wave generated by a laser wake field is extremely small and easily hidden by other THz generation mechanisms, no method exists to measure this wake field directly. In this paper, a simple and stable method for determining the amplitude of the laser plasma wake field is presented. Based on the cancellation of a positive laser plasma wake field and an external negative electric field, the "zero point" of the intensity of the generated THz wave at some frequency can be used to determine the exact amplitude of the corresponding laser plasma wake field. This finding opens an avenue toward the clarification of ultrafast electronic dynamic processes in laser-induced plasmas.

Integrating terahertz metamaterial and water nanodroplets for ultrasensitive detection of amyloid β aggregates in liquids

The association of amyloid β (Aβ) peptide and its ordered aggregates with the onset of Alzheimer’s disease raises the rational use as biomarker for therapeutic and diagnostic. However, due to the heterogeneity among aggregates and relatively low concentration at the physiological conditions, the fast and sensitive detection of the Aβ aggregates remains technical challenges. In this work, we develop a label-free method for probing the hydration dynamics of collective Aβ aggregates in liquids by using terahertz spectroscopy. Aβ aggregates-containing water nanodroplets provide an aqueous confinement environment for improving signal-to-noise ratio of terahertz response. The integration of a metamaterial sensor and nanoconfined droplets further extremely enhances the sensitivity and enables to detect 1 nM Aβ aggregates in a buffer solution. This novel development offers a valuable toolkit towards bioanalytical applications in neurobiology but also paves the way for tracing the ultrafast dynamic biological process in fundamental science.

Nonlinear absorption and the ultrafast dynamic process of Au-Ag nanoshuttles

Nonlinear optical absorption of Au-Ag nanoshuttles (NSs) was studied using an open-aperture Z-scan experiment with a 532 nm nanosecond laser at different energies. It was found that, when the laser energy is relatively low, the Au-Ag NSs exhibit saturated absorption (SA). When the laser energy is high, a conversion from SA to reverse saturated absorption (RSA) occurs. The ultrafast dynamic process of Au-Ag NSs was also investigated by using a femtosecond pump-probe technique. It is found that the process contains a fast and slow decay component that depends strongly on the laser intensity. Furthermore, when the probe wavelength is far away from the plasma resonance peak, the decay shows modulation due to the vibration mode of the coherent excitation.

Strong-field control and attosecond probing of multielectron dynamics

The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.

Direct Probe of Room-Temperature Quantum-Tunneling Processes in Type-II Heterostructures Using Terahertz Emission Spectroscopy

Charge-carrier-transport phenomena on nanoscopic length and ultrashort timescales are of great interest for a multitude of ultrafast dynamic processes occurring in chemical reactions as well as modern devices such as solar cells or transistors. However, the investigation of such ultrashort current pulses is very challenging and mostly based on indirect methods such as spectroscopic changes induced by the charge transport. Here we monitor the short current pulse generated by the dissipative tunneling of charge carriers through a barrier between two adjacent quantum wells via its radiated legacy in the terahertz frequency range. By examining quantum structures with intermediate barriers of different thickness, we demonstrate that the spectrum of the emitted radiation is a direct measurement of the charge-transfer dynamics associated with the tunneling process. Our findings indicate that these incoherent tunnel currents do not start instantaneously but build up on a timescale of approximately 170 fs.

Modeling the Ultrafast Response of Two‐Magnon Raman Excitations in Antiferromagnets on the Femtosecond Timescale

AbstractIlluminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid‐state physics. Here, this problem is tackled, addressing the purely optical, nonmagnetic artifacts on the time resolved two‐magnon stimulated Raman spectrum of an antiferromagnet, rationalizing the recent observation on the exchange energy modification upon photo‐excitation. It is demonstrated how the genuine dynamics of the magnetic eigenmode can be disentangled from the nonlinear optical effects, generated by cross phase modulation, on the femtosecond timescale. The introduced approach can be extended for the investigation of <100 fs dynamic processes by means of coherent Raman scattering.

Improvement of 6D brightness by a 1.4-cell photocathode RF gun for MeV Ultrafast electron diffraction

Recent research indicates that ultrafast electron diffraction and microscopy (UED/M) have unprecedented potential in probing ultrafast dynamic processes, especially in organic and biological materials. However, reaching the required brightness while maintaining high spatiotemporal resolution requires new design of electron source. In order to produce ultrashort electron beam with extreme high brightness, a 1.4-cell RF gun is being developed to reach higher acceleration gradient near the photocathode and thus suppress the space charge effect in the low energy region. Simulation of the 1.4-cell RF photocathode gun shows considerable improvement in bunch length, emittance and energy spread, which all lead to better temporal and spatial resolution comparing to traditional 1.6-cell RF photocathode gun. The results demonstrate the feasibility of sub-ps temporal resolution with normalized emittance less than 0.1 πmm·mrad while maintaining 1 pC electron pulse.

Wavelength-dependent nonlinear absorption and ultrafast dynamics process of WS2

The nonlinear absorption and ultrafast dynamic process of WS2 nanosheets were investigated by using a broadband (ranging from 450 to 700 nm) nanosecond Z-scan technique. Z-scan measurements showed that WS2 nanosheets exhibit saturable absorption (SA), and the magnitude of SA is wavelength-dependent. Besides, the ultrafast dynamics process of WS2 was also investigated with femtosecond transient absorption spectrum. It was found that there is a double-exponential energy relaxation in the process. The investigation shows that WS2 nanosheets can be used for ultrashort pulse generation and a wide spectral range optical absorber.

Bayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities.

This paper employs Bayesian probability theory for analyzing data generated in femtosecond pump-probe photoelectron-photoion coincidence (PEPICO) experiments. These experiments allow investigating ultrafast dynamical processes in photoexcited molecules. Bayesian probability theory is consistently applied to data analysis problems occurring in these types of experiments such as background subtraction and false coincidences. We previously demonstrated that the Bayesian formalism has many advantages, amongst which are compensation of false coincidences, no overestimation of pump-only contributions, significantly increased signal-to-noise ratio, and applicability to any experimental situation and noise statistics. Most importantly, by accounting for false coincidences, our approach allows running experiments at higher ionization rates, resulting in an appreciable reduction of data acquisition times. In addition to our previous paper, we include fluctuating laser intensities, of which the straightforward implementation highlights yet another advantage of the Bayesian formalism. Our method is thoroughly scrutinized by challenging mock data, where we find a minor impact of laser fluctuations on false coincidences, yet a noteworthy influence on background subtraction. We apply our algorithm to data obtained in experiments and discuss the impact of laser fluctuations on the data analysis.

Time-dependent characteristics of secondary electron emission

The recent development of the time-resolving capability for scanning electron microscopy (SEM) enables it to be a real 4D space-time imaging technique, which is extremely suitable for investigating the ultrafast dynamic processes concerned with secondary electron emission (SEE). This paper attempts to investigate the dynamic SEE process with the aid of a Monte Carlo method; the understanding of the mechanism will surely benefit the construction and application of various kinds of time-resolved SEMs. Our simulation modeling is based on the use of the Mott cross section and a dielectric function approach for the respective description of the electron elastic and inelastic scattering. One secondary electron is assumed to be produced in an inelastic scattering event, and the owned kinetic energy enables it to transport and produce other secondary electrons, forming the cascade production process. From the simulation, not only the time delay of SEE from the incidence instant of primary electrons but also the time dependences of the involved physical quantities, including the energy-, depth-, direction-, emission site-, and production site-distributions can be theoretically derived. The calculations provide useful knowledge on the time dependence of SEE from the theoretical perspective for the applications to the available time-resolved SEMs.

RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY

Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.

Transient Aspects and Ultrafast Dynamical Processes of Amplified Spontaneous Emission in Conjugated Polymers.

Experimental research has revealed that the stimulated emission in organic semiconductors is associated with lattice vibrations. To reveal the dynamical aspects of the excited state of the conjugated polymer, the electron-transition process has been incorporated into the molecular dynamics, making it possible to simulate in detail the whole ultrafast process of amplified spontaneous emission (ASE). A typical conjugated polymer, poly( p-phenylene vinylene), is chosen as a model for the research. When an external laser beam of 60 μJ/cm2 is applied to photoexcite the polymer, the energy levels begin to oscillate within 100 fs. Thanks to the prominent self-trapping effect of the conjugated polymer, the laser beam also drives the lattice of the polymer chain to strongly vibrate. Until about 200 fs, four energy levels are pulled to the center of the energy gap, resulting in a transient discrete four-energy-level electronic structure for ASE. Simultaneously, in this ultrafast dynamical process, within the first 1 ps, inversion of the electron population occurs along with the appearance of a localized electronic cloud and locally distorted alternating bonds of the polymer chain. This detailed description of the whole ultrafast process of ASE helps in the understanding of the microscopic dynamical evolution of excitation in conjugated polymers.

Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H_{2} molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of abinitio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.