Picosecond Time Scale Research Articles

Near-Unity All-Optical Modulation of Third-Harmonic Generation with a Fano-Resonant Dielectric Metasurface.

We demonstrate all-optical modulation with a near-unity contrast of nonlinear light generation in a dielectric metasurface. We study third-harmonic generation from silicon Fano-resonant metasurfaces excited by femtosecond pulses at 1480 nm wavelength. We modulate the metasurface resonance by free carrier excitation induced by absorption of an 800 nm pump pulse, leading to up to 93% suppression of third-harmonic generation. Modulation and recovery occur on (sub)picosecond time scales. According to the Drude model, the pump-induced refractive index change blue-shifts the metasurface resonance away from the generation pulse, causing a strong modulation of third-harmonic conversion efficiency. The principle holds great promise for spatiotemporal programmability of nonlinear light generation.

Time-resolved Auger-Meitner spectroscopy of the photodissociation dynamics of CS2

Abstract The photodissociation dynamics of UV excited CS2 are investigated using time-resolved Auger-Meitner spectroscopy. Auger-Meitner decay is initiated by inner-shell ionisation with a femtosecond duration X-ray (179.9 eV) probe generated by the FERMI free electron laser. The time-delayed X-ray probe removes an electron from the S(2p) orbital leading to secondary emission of a high energy electron through Auger-Meitner decay. We monitor the electron kinetic energy of the Auger-Meitner emission as a function of pump-probe delay and observe time-dependent changes in the spectrum that correlate with the formation of bound, excited-state CS2 molecules at early times, and CS + S fragments on the picosecond timescale. The results are analysed based on a simplified kinetic scheme that provides a time constant for dissociation of approximately 1.2 ps, in agreement with previous time-resolved X-ray photoelectron spectroscopy measurements (Gabalski, et al. J. Phys. Chem. Lett. 2023, 14, 7126–7133).

Investigation into the carrier recombination in Sb2Se3: Photo thermal effect, trapped carrier absorption and hot carrier cooling

Understanding the carrier recombination processes in Sb2Se3 is essential for its optoelectronic applications. In this work, carrier recombination dynamics in Sb2Se3 were studied by broad band transient absorption spectroscopy. Firstly, the contribution of photothermal effect to the transient absorption spectrum was thoroughly discussed. It is confirmed that the excited state absorption (ESA) band with lifetime of several nanoseconds results from co-contribution of photo thermal effect and deep trapped carrier absorption. Secondly, the features of transient absorption spectrum on picosecond time scale were interpreted. The short-lived ESA band around 1000 nm was assigned to shallow trapped carrier absorption, while not band gap renormalization (BGR) or free carrier absorption. By globally fitting the transient absorption spectrum, the hot carrier cooling time and time constant for free carrier relax into deep trap state were determined to be 0.25∼0.45 ps and 3.1∼8.7 ps, respectively. Finally, we built up the carrier recombination model of Sb2Se3. The experimental results in this work will improve the understanding on the carrier recombination in Sb2Se3.

Formation mechanism and luminescence properties of silicon carbide nanowires with core-shell structure

For the preparation of SiC nanowires (SiC NWs) by carbothermal reduction, it is of great significance to study the formation mechanism of sic nanowires for process optimization and the controllable preparation. Although first-principles molecular dynamics (FPMD) can simulate systems of hundreds of atoms on a picosecond time scale, the results of FPMD are still insufficient to elucidate the formation mechanism of core-shell SiC nanowires. To make up this deficiency, this article has conducted Deep Potential Molecular Dynamics (DPMD) simulation on nanoseconds timescales with near FPMD accuracy for systems containing thousands of atoms, the results more concretively show the formation process of core-shell SiC nanowires. DPMD simulation results show when CO molecules react with SiO molecules, CO molecules are wrapped by SiO molecules to form agglomerates, SiO molecules in the agglomerates that can come into contact with the wrapped CO molecules will react with wrapped CO molecules to form the SiC core, and SiO molecules that in the outer layer of the agglomerates will disproportionate into the amorphous SiO2 shell. Furthermore, the formation mechanism of SiC nanowires was validated in combination with thermodynamics and experimental methods. Thermodynamic results show that vacuum conditions are favorable for the formation of SiO and CO and lower the temperature of SiC preparation. Finally, the core-shell structure of SiC NWs with good luminescence properties were successfully prepared by carbothermal reduction method using carbon black and silicon dioxide as raw materials under the pressure of less than 5 kPa.

CO2 and NO2 formation on amorphous solid water

Context. The dynamics of molecule formation, relaxation, diffusion, and desorption on amorphous solid water (ASW) is studied in a quantitative fashion. Aims. The formation probability, stabilization, energy relaxation, and diffusion dynamics of CO2 and NO2 on cold ASW following atom+diatom recombination reactions are characterized quantitatively. Methods. Accurate machine-learned energy functions combined with fluctuating charge models were used to investigate the diffusion, interactions, and recombination dynamics of atomic oxygen with CO and NO on ASW. Energy relaxation to the ASW and into water internal degrees of freedom were determined from the analysis of the vibrational density of states. The surface diffusion and desorption energetics were investigated with extended and nonequilibrium MD simulations. Results. The reaction probability is determined quantitatively and it is demonstrated that surface diffusion of the reactants on the nanosecond time scale leads to recombination for initial separations of up to 20 Å. After recombination, both CO2 and NO2 stabilize by energy transfer to water internal and surface phonon modes on the picosecond timescale. The average diffusion barriers and desorption energies agree with those reported from experiments, which validates the energy functions. After recombination, the triatomic products diffuse easily, which contrasts with the equilibrium situation, in which both CO2 and NO2 are stationary on the multinanosecond timescale.

Vibrational Energy Transfer in Energetic Ionic Liquid 4-Amino-1H-1,2,4-triazolium Nitrate: Ab Initio Molecular Dynamics Simulations.

Energetic ionic liquids (EILs) represent a distinctive class of energetic materials with substantial research significance and promising energetic applications. In this work, we delved into the vibrational energy transfer mechanism within the EILs, specifically focusing on 4-amino-1H-1,2,4-triazolium nitrate (ATN), utilizing ab initio molecular dynamics simulations. Our work illustrates distinct energy transfer patterns for different vibrational modes. Upon exciting the stretching vibration of the NH group in the cationic group, vibrational energy preferentially migrates to the neighboring CH bond within the aromatic ring on the femtosecond to picosecond time scales and notably in an in-phase coherent energy transfer fashion. In contrast, exciting the stretching vibration of the N9H11 bond triggers the transfer of vibrational energy to its neighboring N9H10 bond in an out-of-phase coherent fashion. Conversely, exciting the stretching vibration of the N9H10 bond leads to energy transfer predominantly through intermolecular pathways due to the hydrogen-bonding interaction between this bond and the anion. The vibrational energy of the excited N9H10 stretch is shown to dissipate very rapidly, displaying a fast component (with a time constant as short as ca. 7 fs) and a slow component (ca. 230 fs).

Exploring the Dynamics of Charge Transfer in Photocatalysis: Applications of Femtosecond Transient Absorption Spectroscopy.

Artificial photocatalytic energy conversion is a very interesting strategy to solve energy crises and environmental problems by directly collecting solar energy, but low photocatalytic conversion efficiency is a bottleneck that restricts the practical application of photocatalytic reactions. The key issue is that the photo-generated charge separation process spans a huge spatio-temporal scale from femtoseconds to seconds, and involves complex physical processes from microscopic atoms to macroscopic materials. Femtosecond transient absorption (fs-TA) spectroscopy is a powerful tool for studying electron transfer paths in photogenerated carrier dynamics of photocatalysts. By extracting the attenuation characteristics of the spectra, the quenching path and lifetimes of carriers can be simulated on femtosecond and picosecond time scales. This paper introduces the principle of transient absorption, typical dynamic processes and the application of femtosecond transient absorption spectroscopy in photocatalysis, and summarizes the bottlenecks faced by ultrafast spectroscopy in photocatalytic applications, as well as future research directions and solutions. This will provide inspiration for understanding the charge transfer mechanism of photocatalytic processes.

(Invited) Ultrafast Spectroscopy of Nanostructured Plasmonic and Energy Conversion Materials

Ultrafast processes that occur upon the absorption of photons by a material frequently play important roles in the efficiency of a desired energy conversion or photocatalytic outcome. For example, energetic carriers produced upon photoexcitation of plasmonic systems can quickly dissipate energy through electron scattering processes on femtosecond timescales, followed by electron-phonon coupling on picosecond timescales. Once the carriers are thermalized, less energy is available to drive a charge separation or photocatalytic process. At the same time, there are number of handles with which to improve the efficiency of photoinduced processes, and ultrafast spectroscopy can be used to characterize that improvement. Nanostructuring, for example, can produce higher efficiency of photocatalytic processes as free charges are more likely to reach a surface following photoexcitation, and local anisotropies at the surface can drive electromagnetic field enhancements and the greater production of “hot” electrons. Hybridization of differing materials is also a powerful handle with which to manipulate or improve charge separation and photocatalytic outcomes. New materials can lead to greater robustness under illumination, as well as lead to changes in dissipation processes. In this talk, I describe recent efforts to characterize novel nanostructures of interest for plasmonics and energy conversion via ultrafast spectroscopy, as well as explore the ultrafast dissipation processes in refractory plasmonic nanomaterials. Particular attention is paid to dissipation of energy to the local environment of the nanoparticles as well as to ultrafast electron scattering processes. Hybrid nanostructures are also explored. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

(Invited) Experimentally Measuring the Role of Ion-Phonon and Ion-Electron Correlations in Solid-State Li Ion Conductors

In superionic conductors, ion hopping involves a complex interplay of ion-phonon, ion-electron, and ion-ion correlations that is challenging to measure experimentally. In this presentation, we present a new experimental technique that can directly measure ultrafast ion hopping on its inherent picosecond and longer timescale. The technique works by measuring the time-resolved perturbation to a GHz impedance signal when potential ion-coupling interactions are driven with UV to THz irradiation. High-bandwidth, real-time electronics allow synchronization of the impedance measurement to ultrafast laser pulses for varying carrier frequencies. The result of the measurement is the relative strength of different correlations in the many-body ion hopping Hamiltonian. We demonstrate the technique on Li0.5La0.5TiO3 (LLTO) and single crystalline Li7La3Zr2O12 (LLZO), both solid-state Li+ conductors. The ultrafast laser-driven impedance measurements reveal that the dominant ion hopping mechanism in LLTO is through a coupled phonon-ion THz rocking mode. Although this higher frequency mode is less than one-quarter of the overall phonon density of states, it leads to the majority of ion hops as compared to other optical and acoustic phonon modes. Metastable states lasting tens of minutes were also measured for ion-electron perturbations. Further work on extending the technique to measure ion-ion correlations is currently underway. In general, the new technique applies to any complex ion conducting system such as polymers, fuel cells, supercapacitors, and membranes.

Mitigation of laser plasma filamentation by rotating beam smoothing scheme

The propagation of intense laser beams in plasma inevitably gives rise to laser plasma instabilities, which have a significant impact on the illumination uniformity of the focused spot on the target in inertial confinement fusion (ICF) facilities. Here we propose an ultrafast smoothing scheme using a rotating beam (RB) to mitigate the laser plasma filamentation. Using the propagation model of the rotating beam in plasma for the laser-plasma self-focusing (SF) and filamentation, the filamentation characteristics of laser spots were analyzed. The results indicate that the rotating beam smoothing scheme, operating at picosecond timescale, exhibits superior mitigation effect of laser plasma filamentation.

Effect of monolayer ratio on single-shot all-optical switching in Gd/Fe multilayers

Ultrafast thermally induced magnetization switching (TIMS) with femtosecond lasers has attracted much attention due to its ability to trigger a single switching on the picosecond time scale. Currently, most of the studies on TIMS have focused on various ferrimagnetic alloys. In this paper, TIMS of Gd/Fe multilayers in different monolayer ratios is investigated by atomic spin dynamics. The results show that an increase in the monolayer Gd ratio narrows the energy density window of the switching. Further studies found that a lower damping ratio decreases the laser energy density threshold for magnetization reversal. Moreover, reducing the ratio of Gd in the monolayer at the appropriate energy density can shorten the duration of the transient ferromagnetic-like state, which can lead to faster realization of TIMS. Our simulation results provide new insights to explore the physical mechanism of TIMS in Gd/Fe multilayers.

Photonic quantum walk with ultrafast time-bin encoding

The quantum walk (QW) has proven to be a valuable testbed for fundamental inquiries in quantum technology applications such as quantum simulation and quantum search algorithms. Many benefits have been found by exploring implementations of QWs in various physical systems, including photonic platforms. Here, we propose a platform to perform quantum walks based on ultrafast time-bin encoding (UTBE) and all-optical Kerr gating. This platform supports the scalability of quantum walks to a large number of steps and walkers while retaining a significant degree of programmability. More importantly, ultrafast time bins are encoded at the picosecond time scale, far away from mechanical fluctuations. This enables the scalability of our platform to many modes while preserving excellent interferometric phase stability over extremely long periods of time without requiring active phase stabilization. Our 18-step QW is shown to preserve interferometric phase stability over a period of 50 h, with an overall walk fidelity maintained above 95%.

Spin vacuum switching.

Ultrafast control over the magnetic orientation of matter represents a vital element of potential future spin-based electronics ("spintronics"). While physical mechanisms underpinning spin switching are established for picosecond time scales, we here present a physical route to magnetization toggle control, i.e., multiple switching events, at <100 femtoseconds. A minority spin current injected into a ferromagnet is shown to generate rapid depopulation of the minority channel below the ground-state Fermi level, creating a minority "spin vacuum" that then drives rapid charge redistribution from the majority channel and spin switching. We demonstrate that this mechanism reproduces many of the features of recent subpicosecond switching of ferromagnetic Co/Pt multilayers and provide simple practical rules for the design of materials via tailoring the electronic density of states to optimize spin vacuum control over magnetic order.

Adaptation processes in Halomicronema hongdechloris, an example of the light-induced optimization of the photosynthetic apparatus on hierarchical time scales.

Oxygenic photosynthesis in Halomicronema hongdechloris, one of a series of cyanobacteria producing red-shifted Chl f, is adapted to varying light conditions by a range of diverse processes acting over largely different time scales. Acclimation to far-red light (FRL) above 700 nm over several days is mirrored by reversible changes in the Chl f content. In several cyanobacteria that undergo FRL photoacclimation, Chl d and Chl f are directly involved in excitation energy transfer in the antenna system, form the primary donor in photosystem I (PSI), and are also involved in electron transfer within photosystem II (PSII), most probably at the ChlD1 position, with efficient charge transfer happening with comparable kinetics to reaction centers containing Chl a. In H. hongdechloris, the formation of Chl f under FRL comes along with slow adaptive proteomic shifts like the rebuilding of the D1 complex on the time scale of days. On shorter time scales, much faster adaptation mechanisms exist involving the phycobilisomes (PBSs), which mainly contain allophycocyanin upon adaptation to FRL. Short illumination with white, blue, or red light leads to reactive oxygen species-driven mobilization of the PBSs on the time scale of seconds, in effect recoupling the PBSs with Chl f-containing PSII to re-establish efficient excitation energy transfer within minutes. In summary, H. hongdechloris reorganizes PSII to act as a molecular heat pump lifting excited states from Chl f to Chl a on the picosecond time scale in combination with a light-driven PBS reorganization acting on the time scale of seconds to minutes depending on the actual light conditions. Thus, structure-function relationships in photosynthetic energy and electron transport in H. hongdechloris including long-term adaptation processes cover 10-12 to 106 seconds, i.e., 18 orders of magnitude in time.

Temporal Separation between Lattice Dynamics and Electronic Spin‐State Switching in Spin‐Crossover Thin Films Evidenced by Time‐Resolved X‐Ray Diffraction

AbstractSpin‐crossover (SCO) complexes have drawn significant attention for the possibility to photoswitch their electronic spin state on a sub‐picosecond timescale at the molecular level. However, the multi‐step mechanism of laser‐pulse‐induced switching in solid state is not yet fully understood. Here, time‐resolved synchrotron X‐ray diffraction is used to follow the dynamics of the crystal lattice in response to a picosecond laser excitation in nanometric thin films of the SCO complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2]. The observed structural dynamics unambiguously reveal a lattice expansion on the 100 picosecond timescale, which is temporally decoupled both from the ultrafast molecular photoswitching process (occurring within 100 fs) and from the delayed, thermo‐elastic (Arrhenius‐driven) conversion (taking place ≈10 ns). These time‐separated dynamics are also manifested by the observation of damped acoustic oscillations in the time evolution of the lattice volume, whereas no such oscillations are observed in the electronic spin‐state dynamics. Overall, these results suggest the existence of a universal behavior whereby the intramolecular energy barrier between low‐spin and high‐spin states acts as an intrinsic dynamical bottleneck in the out‐of‐equilibrium spin‐state switching dynamics of SCO materials.

Laser-driven ultrafast impedance spectroscopy for measuring complex ion hopping processes.

Superionic conductors, or solid-state ion-conductors surpassing 0.01 S/cm in conductivity, can enable more energy dense batteries, robust artificial ion pumps, and optimized fuel cells. However, tailoring superionic conductors requires precise knowledge of ion migration mechanisms that are still not well understood due to limitations set by available spectroscopic tools. Most spectroscopic techniques do not probe ion hopping at its inherent picosecond timescale nor the many-body correlations between the migrating ions, lattice vibrational modes, and charge screening clouds-all of which are posited to greatly enhance ionic conduction. Here, we develop an ultrafast technique that measures the time-resolved change in impedance upon light excitation, which triggers selective ion-coupled correlations. We also develop a cost-effective, non-time-resolved laser-driven impedance method that is more accessible for lab-scale adoption. We use both techniques to compare the relative changes in impedance of a solid-state Li+ conductor Li0.5La0.5TiO3 (LLTO) before and after UV to THz frequency excitations to elucidate the corresponding ion-many-body-interaction correlations. From our techniques, we determine that electronic screening and phonon-mode interactions dominate the ion migration pathway of LLTO. Although we only present one case study, our technique can extend to O2-, H+, or other charge carrier transport phenomena where ultrafast correlations control transport. Furthermore, the temporal relaxation of the measured impedance can distinguish ion transport effects caused by many-body correlations, optical heating, correlation, and memory behavior.

Dynamics of Polyalkylfluorene Conjugated Polymers: Insights from Neutron Spectroscopy and Molecular Dynamics Simulations.

The dynamics of the conjugated polymers poly(9,9-dioctylfluorene) (PF8) and poly(9,9-didodecylfluorene) (PF12), differing by the length of their side chains, is investigated in the amorphous phase using the temperature-dependent quasielastic neutron scattering (QENS) technique. The neutron spectroscopy measurements are synergistically underpinned by molecular dynamics (MD) simulations. The probe is focused on the picosecond time scale, where the structural dynamics of both PF8 and PF12 would mainly be dominated by the motions of their side chains. The measurements highlighted temperature-induced dynamics, reflected in the broadening of the QENS spectra upon heating. The MD simulations reproduced well the observations; hence, the neutron measurements validate the MD force fields, the adopted amorphous model structures, and the numerical procedure. As the QENS spectra are dominated by the signal from the hydrogens on the backbones and side chains of PF8 and PF12, extensive analysis of the MD simulations allowed the following: (i) tagging these hydrogens, (ii) estimating their contributions to the self-part of the van Hove functions and hence to the QENS spectra, and (iii) determining the activation energies of the different motions involving the tagged hydrogens. PF12 is found to exhibit QENS spectra broader than those of PF8, indicating a more pronounced motion of the didodecyl chains of PF12 as compared to dioctyl chains of PF8. This is in agreement with the outcome of our MD analysis: (i) confirming a lower glass transition temperature of PF12 compared to PF8, (ii) showing PF12 having a lower density than PF8, and (iii) highlighting lower activation energies of the motions of PF12 in comparison with PF8. This study helped to gain insights into the temperature-induced side-chain dynamics of the PF8 and PF12 conjugated polymers, influencing their stability, which could potentially impact, on the practical side, the performance of the associated optoelectronic active layer.

Ultra-high spin emission from antiferromagnetic FeRh

An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.

Photoexcited Polaron Relaxation as a Structurally Sensitive Reporter of Charge Trapping in a Conducting Polymer

AbstractConjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity.

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump – X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of the crystal-liquid transition. According to the experimental data, the melting process gradually accelerates with the increasing density of deposited energy. The molecular dynamics simulations reveal that the transition mechanism progressively varies from heterogeneous, initiated inside the material at structurally disordered grain boundaries, to homogenous, proceeding catastrophically in the crystal volume on a picosecond timescale comparable to that of electron-phonon coupling. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state.