Ultrafast Dynamical Processes Research Articles

Electron transfer tuned by pressure-dependent aggregation-induced emission in InP/ZnS quantum dot–anthraquinone complexes

Electron transfer (ET) process is considered a substantial factor in influencing the photoelectric conversion efficiency of optoelectronic devices. While pressure has demonstrated effective tune ET, a comprehensive investigation into the mechanisms for both restraining and promoting ET remains elusive. Herein, we have performed measurements using in situ high-pressure steady-state photoluminescence (PL), Raman scattering spectra, and femtosecond transient absorption (fs-TA) spectroscopy on InP/ZnS quantum dot–anthraquinone (InP/ZnS QD-AQ) complexes. The experimental results have demonstrated that the pressure-suppressed ET process in the InP/ZnS QD-AQ complexes arises from both the aggregation-induced emission (AIE) effect of AQ in toluene and the quantum confinement effect of the InP/ZnS QDs. The reduction in the distance between InP/ZnS QD and AQ under pressure emerges as a key factor that promotes the ET process in the InP/ZnS QD-AQ complexes. Furthermore, we observed that the pressure not only enhances the ET process but also suppresses the auger recombination process in liquid phase I of toluene, consequently leading to an enhancement in the photoelectric conversion efficiency. This study contributes to understanding the mechanism of the ultrafast dynamic processes in the pressure-induced QD-receptor complexes, and it has great potential for preparing efficient and stable optoelectronic devices.

Ultrafast damage dynamics and ablation mechanism of GaSe induced by femtosecond laser irradiation

With the development of high-power infrared lasers and optical devices based on GaSe, more attention has been paid to its optical damage under laser irradiation. Understanding the interactions at the surface of GaSe during femtosecond laser irradiation is essential for its applications as infrared nonlinear optical and optoelectronic devices. Here, ultrafast damage dynamic process and ablation mechanism of GaSe under femtosecond laser irradiation are studied. Two types of damage/ablation mechanisms are proposed according to two distinct types of damaged morphologies and damaged products, and further confirmed by ultrafast pump–probe spectroscopy. At lower fluences, the laser-induced damage of GaSe can be attributed to the formation of new products by chemical bonds broken and phase transitions, changing the stoichiometry and spectra while barely destroy surface morphologies. At higher fluences, surface plasma is formed with massive hot free electrons in the conduction band from the multi-photon ionization and avalanche ionization. The plasma heats the surface and forms shock wave, which removes the surface layer by vaporization, causing a damage pit on the surface. Our results gain insight into damage dynamics of GaSe under laser irradiation, which can help to have a deeper comprehension of the laser-materials interaction.

Full Temporal Characterization of Ultrabroadband Few-Cycle Laser Pulses Using Atomically Thin WS2

Nowadays, the accurate and full temporal characterization of ultrabroadband few-cycle laser pulses with pulse durations below 7 fs is of great importance in fields of science that investigate ultrafast dynamic processes. There are several indirect methods that use nonlinear optical signals to retrieve the complex electric field of femtosecond lasers. However, the precise characterization of few-cycle femtosecond laser sources with an ultrabroadband spectrum presents additional difficulties, such as reabsorption of nonlinear signals, partial phase matching, and spatiotemporal mismatches. In this work, we combine the dispersion scan (d-scan) method with atomically thin WS2 flakes to overcome these difficulties and fully characterize ultrabroadband laser pulses with a pulse duration of 6.9 fs and a spectrum that ranges from 650 to 1050 nm. Two-dimensional WS2 acts as a remarkably efficient nonlinear medium that offers a broad transparency range and allows for achieving relaxed phase-matching conditions due to its atomic thickness. Using mono- and trilayers of WS2, we acquire d-scan traces by measuring the second-harmonic generation (SHG) signal, originated via laser–WS2 interaction, as a function of optical dispersion (i.e., glass thickness) and wavelength. Our retrieval algorithm extracts a pulse duration at full-width half-maximum of 6.9 fs and the same spectral phase function irrespective of the number of layers. We benchmark and validate our results obtained using WS2 by comparing them with those obtained using a 10-μm-thick BBO crystal. Our findings show that atomically thin media can be an interesting alternative to micrometer-thick bulk crystals to characterize ultrabroadband femtosecond laser pulses using SHG-d-scan with an error below 100 as (attoseconds).

Ultrafast dynamic processes and nonlinear optical properties of Platinum(II) fluoren-acetylides

Two N-(4-ethylnylphenyl)-9H-fluoren-9-imine (EFT)arylacetylides (1, 2) and three EFT based Platinum(II) arylacetylides (3, 4, 5) were synthesized. The ultrafast dynamic processes after excitation and the nonlinear optical properties for picosecond lasers were investigatedusing femtosecond transient absorption and Z-scan techniques. The influence of conjugated length and Pt(II) content percentage on the properties ofthese moleculeswas discussed in detail. For compounds 1 and 2, two dynamic decay processes are observed after excitationby the pump laser beams. The first process is an ultrafast singlet (first singlet excited state, S1S*) → singlet state (ground state, GS), followed by a geometric twisting charge transfer state (CTS*) of the fluoren units relative to the central arylacetylide plane. For 3, 4, and 5, three dynamic decay processes are observed. The first process is the same as those of 1 and 2. However,the geometric twisting of 3, 4, and 5 is easier than 1 and 2 due to the presence of Pt atoms in the molecular skeleton of 3, 4, and 5, which makes a fast S1S* → geometric twisting state (TS*) occurred before the charge transfer occurring, and then a subsequent geometric twisting state (TS*) to geometric twisting charge transfer state (CTS*) occurs. The nonlinear optical properties of these compounds, studied using the open aperture Z-scan technique (λ = 532 nm, pulse width 23 ps), show that the values of the title compounds are around tens of cm/GW, which are comparable to newly reported porphyrin-based MOF materials and much better than Ru(II) terpyridine complexes..The mechanism of the nonlinear optical properties is attributed to the excited state absorption based on S1S*, TS*, and CTS* states, according to the femtosecond transient absorption and Z-scan results. The increase of Pt content percentage does not benefit the nonlinear absorption of Pt-containing compounds for the picosecond pulse laser. This is because the increase of the intersystem crossing rate weakly contributes to the nonlinear optical properties.

Ultra-Fast Heating Process of Cu-Pd Bimetallic Nanoparticles Unraveled by Molecular Dynamics Simulation

This paper investigates the ultra-fast heating process of Cu-Pd bimetallic nanoparticles from an atomic-scale perspective, which is essential for laser manufacturing processes, such as laser cladding and selective laser melting. The behavior of high surface ratio nanoparticles during these processes is strongly influenced by their properties and the heating process, which is governed by atomic dynamics. Previous studies have mainly focused on the combination process in pure metallic nanoparticles under slow or isothermal heating, but this work demonstrates that the ultra-fast atomic dynamic process between bimetallic nanoparticles differs significantly. Specifically, in Cu-Pd nanoparticles, the combination process is primarily dependent on the surface atomic motion of the lower melting point particles rather than plastic deformation in the grain boundary between particles. Moreover, the ultra-fast heating process is size-dependent. For small nanoparticles, the atomic kinetics exhibit two different mechanisms depending on temperature: Low-temperature jointing is controlled by localized atomic rearrangement, while high-temperature coalition is governed by the atomic flow of surface atomic melting in the low-temperature melting particle. The combination mechanism is the same for large particles as it is for small particles at high temperatures. The findings of this study provide important insights into the behavior of bimetallic nanoparticles during ultra-fast heating and can inform the development of coat and lubricant.

Ultrafast optical nonlinearity in natural van der Waals heterostructure nanosheets of franckeite

Franckeite, a natural van der Waals heterostructure with excellent physicochemical characteristics, has shown great potential for high-performance optoelectronic device. Here, the few-layer franckeite nanosheets have been prepared by liquid phase-exfoliation method successfully, and the nonlinear optical absorption and carrier dynamics behaviors of the nanosheets have been studied experimentally. The franckeite nanosheets show wavelength- and intensity-dependent nonlinear absorption characteristics, and exhibit ultrafast carrier dynamic process. The ultrafast optical nonlinearity in franckeite nanosheets has been further validated via an Er3+-doped fiber laser to deliver stable femtosecond pulse. The experimental results reveal that franckeite heterostructure nanosheets can be an ideal candidate in the design and manufacture of ultrafast nonlinear optoelectronic devices.

Nutation Excitations in the Gyrotropic Vortex Dynamics in a Circular Magnetic Nanodot.

A significant activity is devoted to the investigation of the ultrafast spin dynamic processes, holding a great potential for science and applications. However, a challenge of the understanding of the mechanisms of underlying spin dynamics in nanomaterials at pico- and femtosecond timescales remains under discussion. In this article, we explore the gyrotropic vortex dynamics in a circular soft magnetic nanodot, highlighting the impacts given by nutations in the high-frequency part of the dot spin excitation spectrum. Using a modified Thiele equation of the vortex core motion with a nutation term, we analyze the dynamic response of the vortex to an oscillating magnetic field applied in the dot plane. It is found that nutations affect the trajectory of the vortex core. Namely, we show that the directions of the vortex core motion in the low-frequency gyrotropic mode and the high-frequency nutation mode are opposite. The resonant frequencies of gyrotropic and nutational vortex core motions reveal themselves on different scales: gigahertz for the gyrotropic motion and terahertz for the nutations. We argue that the nutations induce a dynamic vortex mass, present estimates of the nutational mass, and conduct comparison with the mass appearing due to moving vortex interactions with spin waves and Doering domain wall mass.

High-resolution cavity bunch arrival-time monitor for charges less than 1 pC

Pump-probe experiments using x-ray pulses or MeV ultrafast electrons as probes are important for studying ultrafast dynamic processes at the atomic level. Increasing demand is being focused on ultrashort x-ray pulses and electron pulses to research processes down to a few femtoseconds. Due to the limitation of the space charge force, only low charge electron bunches with critical synchronization can achieve such short pulses. A high-precision beam arrival-time monitor for the low charge is especially important. In this paper, a low ${Q}_{l}$ cavity with high sensitivity is designed to measure the arrival time of charges below 1 pC. The pickup is particularly optimized with high $R/Q$ at 4.76 GHz to provide the maximum signal, and the cold test results are in good agreement with the simulations. An IQ demodulation scheme is developed for the readout electronics. In the beam tests, the measured resolution of the beam arrival-time monitor is 33.46 fs at 0.5 pC.

Wavelength encoded single-shot high-spatiotemporal resolution all-optical probe

The laser probe is one of the main techniques for capturing ultrafast dynamic processes and has extensive applications in fields such as plasma physics, photochemistry, and biomedical science. In this work, a time-wavelength encoded optical probe generation scheme is proposed, which uses cascaded frequency doubling crystals with different phase-matching angles and independent delay lines to achieve time-wavelength encoding. This method offers single-shot high-spatiotemporal resolution, high frame rate, and a wide range of adjustable time windows. The temporal resolution of the optical probe depends on the pulse width of the second harmonic, which can be adjusted by changing the phase-matching angle of the frequency-doubling crystal. The time window of the optical probe is only related to the change in the delay line, which can be adjusted by changing the length of the delay line. Therefore, the time resolution and time window of the optical probe are independent of each other. An optical probe generation system is constructed with 247 fs temporal resolution, 4 μm spatial resolution, 4.05 THz maximal frame rate, and an adjustable time window from sub-picosecond to 3 ns. The three-dimensional spatiotemporal evolution process of plasma filaments is captured within a single shot by using the optical probe. The experimental results show that the ionization front of the plasma propagates forward at a velocity of <inline-formula><tex-math id="M2">\begin{document}$ {\left(2.963\pm 0.024\right)\times 10}^{8}\;{\rm{m}}/{\rm{s}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20230727_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20230727_M2.png"/></alternatives></inline-formula>, which is consistent with the theoretical prediction. This demonstrates the feasibility of using the probe for capturing ultrafast events. In the part of discussion, we analyze that the key parameters of the optical probe can reach a maximum frame rate of 35.7 THz, a maximum time resolution of 28 fs, and a time window range that can be adjusted from hundreds of femtoseconds to tens of nanoseconds. Finally, the optimal design parameters of the optical probe are given for different application scenarios. The optical probe generation scheme has good scalability and versatility, and can be combined with any wavelength decoding device, diffraction imaging, holographic imaging, tomography scanning, and other technologies. The high spatiotemporal resolution of the optical probe and the independent adjustability of its parameters provide a feasible solution for single-shot high spatiotemporal resolution captures of ultrafast dynamic processes on a multiple time scale.

Rapid hydrogen detection with low temperature realized by regulating chemisorbed oxygen species of mesoporous indium tin oxide microsphere

With the quick development of hydrogen energy, it is necessary to fabricate hydrogen sensor with high sensitivity, fast response/recovery and low detection limit to monitor the leakage of hydrogen. In this work, a series of indium tin oxide (ITOs) microspheres were successfully synthesized by alkoxide precursors method. Among them, 10.7% In-doped SnO2 (named ITO-II) based sensor exhibits high sensitivity, excellent selectivity, great stability and ultra-fast dynamic process (response time and recovery time<3 s), at a low working temperature of 200 °C. The excellent hydrogen sensing performance of ITO-II should be attributed to the increased chemisorbed oxygen and the high intrinsic reactivity of SnO2 toward hydrogen. This work provides inspiration for the design of novel hydrogen sensing material.

Efficient and Stable Self-Assembly Blue-Emitting CsPbBr3 Nanoplatelets with Self-Repaired Surface Defects

The development of efficient and stable blue-emitting CsPbX3 is critically important for the balance of the three primary colors and advanced lighting and displays. Here, a single ligand passivation strategy is proposed to synthesize highly efficient self-assembly CsPbBr3 nanoplatelets (NPLs). With such a single ligand passivation strategy, the Br vacancy of the Pb–Br octahedron is eliminated effectively. The ultrafast exciton dynamic processes confirmed by transient absorption indicates the lower defect density for the self-assembly CsPbBr3 NPLs. Obvious emission improvement of the pristine CsPbBr3 NPLs was observed as the resting time increased to 24 h, which is associated with self-repaired surface defects and suppressed nonradiative recombination. Photoluminescence (PL) intensity of the CsPbBr3 NPLs in deionized water for 20 h was decreased by 40%, while PL still kept about 73% of the initial intensity even after the continuous irradiation of 395 nm UV for 30 h. The efficient and stable blue-emitting CsPbBr3 NPLs provide great opportunities to develop efficient lighting and displays.

First commissioning results of the coherent scattering and imaging endstation at the Shanghai soft X-ray free-electron laser facility

The Shanghai soft X-ray free-electron laser (SXFEL) user facility project started in 2016 and is expected to be open to users by 2022. It aims to deliver ultra-intense coherent femtosecond X-ray pulses to five endstations covering a range of 100–620 eV for ultrafast X-ray science. Two undulator lines are designed and constructed, based on different lasing modes: self-amplified spontaneous emission and echo-enabled harmonic generation. The coherent scattering and imaging (CSI) endstation is the first of five endstations to be commissioned online. It focuses on high-resolution single-shot imaging and the study of ultrafast dynamic processes using coherent forward scattering techniques. Both the single-shot holograms and coherent diffraction patterns were recorded and reconstructed for nanoscale imaging, indicating the excellent coherence and high peak power of the SXFEL and the possibility of “diffraction before destruction” experiments at the CSI endstation. In this study, we report the first commissioning results of the CSI endstation.

All-optical reconstruction of three-band transition dipole moments by the crystal harmonic spectrum from a two-color laser pulse.

When a bulk solid is irradiated by an intense laser pulse, transition dipole moments (TDMs) between different energy bands have an important influence on the ultra-fast dynamic process. In this paper, we propose a new all-optical method to reconstruct the k-dependent TDMs between multi-bands using a crystal high-order harmonic generation (HHG). Taking advantage of an obvious separation of bandgaps between three energy bands of an MgO crystal along the <001 > direction, a continuous harmonic spectrum with two plateaus can be generated by a two-color laser pulse. Furthermore, the first harmonic platform is mainly dominated by the polarization between the first conduction band and the valence band, and the second one is largely attributed to the interband HHG from the second conduction band and the valence band. Therefore, the harmonic spectrum from a single quantum trajectory can be adopted to map TDMs between the first, second conduction bands, and the valence one. Our work is of great significance for understanding the instantaneous properties of solid materials in the strong laser field, and will strongly promote the development of the HHG detection technology.

Ultrafast imaging for uncovering laser–material interaction dynamics

AbstractThe physical mechanism of the dynamics in laser–material interaction has been an important research area. In addition to theoretical analysis, direct imaging‐based observation of ultrafast dynamic processes is an important approach to understand many fundamental issues in laser–material interaction such as inertial confinement fusion (ICF), laser accelerator construction, and advanced laser production. In this review, the principles and applications of three types of commonly used ultrafast imaging methods are introduced, including the pump–probe, X‐ray diagnosis, and single‐shot optical burst imaging. We focus on the technical features such as the spatial and temporal resolution for each technique, and present several conventional applications.

Optical Properties of V2O5 Thin Films on Different Substrates and Femtosecond Laser-Induced Phase Transition Studied by Pump-Probe Method.

Vanadium pentoxide can undergo a reversible phase transition by heating above 260 °C; its non-thermal phase transition, as well as ultrafast dynamical processes, is still not known. Here, femtosecond laser-induced phase transition properties in V2O5 thin films were first explored using femtosecond time-resolved pump–probe spectroscopy. The results show that the phase transient processes occur on a 10−15–10−13 temporal scale. The phase transition and recovery properties are dependent on both the substrates and pump laser energy densities. We propose the oxygen vacancies theory to explain the results, and we provide valuable insights into V2O5 films for potential applications.

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.