Picosecond Time Scale Research Articles

Enhanced Two-Dimensional Exciton Propagation via Strong Light–Matter Coupling with Surface Lattice Plasmons

Exciton–polaritons─hybrid exciton–photon quasi-particles─have enabled exciting long-range coherent transport by taking advantage of the properties of their photonic component. However, most experimental demonstrations of strong coupling have been based on semiconductor Fabry–Pérot microcavities. Here, we report an open and versatile exciton–polariton platform by integrating two-dimensional lead halide perovskites with plasmonic nanoparticle arrays, which support hybridization between excitons and surface lattice resonances in the strong coupling regime. Benefitting from the open architecture, we directly visualized polariton-enhanced exciton transport via angle-dependent transient absorption microscopy measurements. The observed transport enhancement extends into the picosecond timescale, leading to more than 1 order of magnitude improvement of exciton migration, from ∼10 nm to 100s of nanometers. We proposed a polariton-mediated exciton transport model and obtained an average polariton velocity of ∼1 × 106 m/s for states with an ∼40% photonic fraction. These results demonstrate plasmonic particle arrays as a versatile and adaptable cavity to enhance exciton transport.

Anisotropy of water dynamics confined in model silica material

Silica microfibers with highly oriented cylindrical mesopores embedded in alumina membrane were used as model system to characterize the water dynamics anisotropy at a picosecond timescale. To reach this goal, we first studied the optimal preparation conditions to obtain longitudinal silica mesopores orientation at macroscopic scale. Accordingly, we characterized the impact of ethanol content and aspiration pressure on the orientation and filling of the mesopores using scanning electron microscopy, transmission electron microscopy, small angle X-ray scattering, and dynamic vapor sorption. Based on the obtained results we selected the most appropriate sample as a model system. The model system was filled with water and used to characterize the dynamics of water molecules in parallel and perpendicular directions using quasi-elastic neutron scattering measurements collected in two orientations at two timescales (7 ps and 30 ps). The results revealed the anisotropy of water dynamics in the silica mesopores and highlighted a larger perpendicular diffusion coefficient compared to the parallel one at both timescales. This suggests that it may be more difficult for water molecules to move from one hydroxyl surface site to another compared to forming a hydrogen bond with a water molecule not adsorbed at the surface.

Chemical Implications of Rapid Reactive Absorption of I2O4 at the Air-Water Interface.

Marine aerosol formation involving iodine-bearing species significantly affects the global climate and radiation balance. Although recent studies outline the critical role of iodine oxide in nucleation, much less is known about its contribution to aerosol growth. This paper presents molecular-level evidence that the air-water interfacial reaction of I2O4 mediated by potent atmospheric chemicals, such as sulfuric acid (H2SO4) and amines [e.g., dimethylamine (DMA) and trimethylamine (TMA)], can occur rapidly on a picosecond time scale by Born-Oppenheimer molecular dynamics simulations. The interfacial water bridges the reactants while facilitating the DMA-mediated proton transfer and stabilizing the ionic products of H2SO4-involved reactions. The identified heterogeneous mechanisms exhibit the dual contribution to aerosol growth: (i) the ionic products (e.g., IO3-, DMAH+, TMAH+, and HSO4-) formed by reactive adsorption possess less volatility than the reactants and (ii) these ions, such as alkylammonium salts (e.g., DMAH+), are also highly hydrophilic, further facilitating hygroscopic growth. This investigation enhances not only our understanding of heterogeneous iodine chemistry but also the impact of iodine oxide on aerosol growth. Also, these findings can bridge the gap between the abundance of I2O4 in the laboratory and its absence in field-collected aerosols and provide an explanation for the missing source of IO3-, HSO4-, and DMAH+ in marine aerosols.

Revealing the hidden dynamics of confined water in acrylate polymers: Insights from hydrogen-bond lifetime analysis.

Polymers contain functional groups that participate in hydrogen bond (H-bond) with water molecules, establishing a robust H-bond network that influences bulk properties. This study utilized molecular dynamics (MD) simulations to examine the H-bonding dynamics of water molecules confined within three poly(meth)acrylates: poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(1-methoxymethyl acrylate) (PMC1A). Results showed that H-bonding dynamics significantly slowed as the water content decreased. Additionally, the diffusion of water molecules and its correlation with H-bond breakage were analyzed. Our findings suggest that when the H-bonds between water molecules and the methoxy oxygen of PMEA are disrupted, those water molecules persist in close proximity and do not diffuse on a picosecond time scale. In contrast, the water molecules H-bonded with the hydroxy oxygen of PHEMA and the methoxy oxygen of PMC1A diffuse concomitantly with the breakage of H-bonds. These results provide an in-depth understanding of the impact of polymer functional groups on H-bonding dynamics.

The Crystallization of Disordered Materials under Shock Is Governed by Their Network Topology.

When the shock load is applied, materials experience incredibly high temperature and pressure conditions on picosecond timescales, usually accompanied by remarkable physical or chemical phenomena. Understanding the underlying physics that governs the kinetics of shocked materials is of great importance for both physics and materials science. Here, combining experiment and large-scale molecular dynamics simulation, the ultrafast nanoscale crystal nucleation process in shocked soda-lime silicate glass is investigated. By adopting topological constraints theory, this study finds that the propensity of nucleation is governed by the connectivity of the atomic network. The densification of local networks, which appears once the crystal starts to grow, results in the underconstrained shell around the crystal and prevents further crystallization. These results shed light on the nanoscale crystallization mechanism of shocked materials from the viewpoint of topological constraint theory.

Ultrafast demagnetization dynamics in the epitaxial FeGe(111) film chiral magnet

The exploring of ultrafast demagnetization is a significant subject for understanding the versatility of spin dynamics. However, the laser-induced ultrafast demagnetization process in chiral magnet FeGe has remained elusive. Here, we report the ultrafast demagnetization processes of FeGe film with various magnetic structures using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Our work reveals a transition from one-step ultrafast demagnetization (type I) to two-step demagnetization (type II) process by changing ambient temperature, laser fluence, and magnetic field. We confirm that the transition from one-step to two-step ultrafast demagnetization mainly results from the weak electron-phonon (e-p) coupling. In the skyrmion phase region, TRMOKE curves demonstrate a two-step ultrafast demagnetization process. The two-step ultrafast demagnetization will further enhance the magnetization loss, and consequently facilitate the magnetization switching and pave a pathway to engineer chiral magnetic devices on picosecond timescales.

Ultrafast Control of the Optical Transition in Type-II Colloidal Quantum Wells

Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core–crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift–redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p–n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.

Real Space and Time Imaging of Collective Headgroup Dipole Motions in Zwitterionic Lipid Bilayers

Lipid bilayers are supramolecular structures responsible for a range of processes, such as transmembrane transport of ions and solutes, and sorting and replication of genetic materials, to name just a few. Some of these processes are transient and currently, cannot be visualized in real space and time. Here, we developed an approach using 1D, 2D, and 3D Van Hove correlation functions to image collective headgroup dipole motions in zwitterionic phospholipid bilayers. We show that both 2D and 3D spatiotemporal images of headgroup dipoles are consistent with commonly understood dynamic features of fluids. However, analysis of the 1D Van Hove function reveals lateral transient and re-emergent collective dynamics of the headgroup dipoles-occurring at picosecond time scales-that transmit and dissipate heat at longer times, due to relaxation processes. At the same time, the headgroup dipoles also generate membrane surface undulations due a collective tilting of the headgroup dipoles. A continuous intensity band of headgroup dipole spatiotemporal correlations-at nanometer length and nanosecond time scales-indicates that dipoles undergo stretching and squeezing elastic deformations. Importantly, the above mentioned intrinsic headgroup dipole motions can be externally stimulated at GHz-frequency scale, enhancing their flexoelectric and piezoelectric capabilities (i.e., increased conversion efficiency of mechanical energy into electric energy). In conclusion, we discuss how lipid membranes can provide molecular-level insights about biological learning and memory, and as platforms for the development of the next generation of neuromorphic computers.

Ultrafast nonequilibrium dynamics of rotons in superfluid helium

Short-time dynamics of superfluids far from equilibrium remains largely unknown, despite its importance for key processes in these systems. Here, we describe a method for locally perturbing the density of superfluid helium via the excitation of roton pairs with ultrashort laser pulses. By measuring the time dependence of this perturbation, we track the nonequilibrium dynamics of the two-roton states on femtosecond and picosecond timescales. Our results reveal an ultrafast equilibration of roton pairs as they thermalize with the colder equilibrium quasiparticle gas. Future applications of this technique to different temperature and pressure regimes, in various superfluids, will enable to probe rapid nucleation and decay processes, as well as metastable Bose-Einstein condensates of rotons and roton pairs.

Atomistic Insights on Surface Quality Control via Annealing Process in AlGaN Thin Film Growth.

Aluminum gallium nitride (AlGaN) is a nanohybrid semiconductor material with a wide bandgap, high electron mobility, and high thermal stability for various applications including high-power electronics and deep ultraviolet light-emitting diodes. The quality of thin films greatly affects their performance in applications in electronics and optoelectronics, whereas optimizing the growth conditions for high quality is a great challenge. Herein, we have investigated the process parameters for the growth of AlGaN thin films via molecular dynamics simulations. The effects of annealing temperature, the heating and cooling rate, the number of annealing rounds, and high temperature relaxation on the quality of AlGaN thin films have been examined for two annealing modes: constant temperature annealing and laser thermal annealing. Our results reveal that for the mode of constant temperature annealing, the optimum annealing temperature is much higher than the growth temperature in annealing at the picosecond time scale. The lower heating and cooling rates and multiple-round annealing contribute to the increase in the crystallization of the films. For the mode of laser thermal annealing, similar effects have been observed, except that the bonding process is earlier than the potential energy reduction. The optimum AlGaN thin film is achieved at a thermal annealing temperature of 4600 K and six rounds of annealing. Our atomistic investigation provides atomistic insights and fundamental understanding of the annealing process, which could be beneficial for the growth of AlGaN thin films and their broad applications.

Photogrammetry of Ultrafast Excited-State Intramolecular Proton Transfer Pathways in the Fungal Pigment Draconin Red.

Proton transfer processes of organic molecules are key to charge transport and photoprotection in biological systems. Among them, excited-state intramolecular proton transfer (ESIPT) reactions are characterized by quick and efficient charge transfer within a molecule, resulting in ultrafast proton motions. The ESIPT-facilitated interconversion between two tautomers (PS and PA) comprising the tree fungal pigment Draconin Red in solution was investigated using a combination of targeted femtosecond transient absorption (fs-TA) and excited-state femtosecond stimulated Raman spectroscopy (ES-FSRS) measurements. Transient intensity (population and polarizability) and frequency (structural and cooling) dynamics of -COH rocking and -C=C, -C=O stretching modes following directed stimulation of each tautomer elucidate the excitation-dependent relaxation pathways, particularly the bidirectional ESIPT progression out of the Franck-Condon region to the lower-lying excited state, of the intrinsically heterogeneous chromophore in dichloromethane solvent. A characteristic overall excited-state PS-to-PA transition on the picosecond timescale leads to a unique "W"-shaped excited-state Raman intensity pattern due to dynamic resonance enhancement with the Raman pump-probe pulse pair. The ability to utilize quantum mechanics calculations in conjunction with steady-state electronic absorption and emission spectra to induce disparate excited-state populations in an inhomogeneous mixture of similar tautomers has broad implications for the modeling of potential energy surfaces and delineation of reaction mechanisms in naturally occurring chromophores. Such fundamental insights afforded by in-depth analysis of ultrafast spectroscopic datasets are also beneficial for future development of sustainable materials and optoelectronics.

Efficient Generation of Axial Magnetic Field by Multiple Laser Beams with Twisted Pointing Directions.

Strong laser-driven magnetic fields are crucial for high-energy-density physics and laboratory astrophysics research, but generation of axial multikilotesla fields remains a challenge. The difficulty comes from the inability of a conventional linearly polarized laser beam to induce the required azimuthal current or, equivalently, angular momentum (AM). We show that several laser beams can overcome this difficulty. Our three-dimensional kinetic simulations demonstrate that a twist in their pointing directions enables them to carry orbital AM and transfer it to the plasma, thus generating a hot electron population carrying AM needed to sustain the magnetic field. The resulting multikilotesla field occupies a volume that is tens of thousands of cubic microns and it persists on a picosecond timescale. The mechanism can be realized for a wide range of laser intensities and pulse durations. Our scheme is well suited for implementation using multikilojoule petawatt-class lasers, because, by design, they have multiple beamlets and because the scheme requires only linear polarization.

Single-shot ultrafast terahertz photography

Multidimensional imaging of transient events has proven pivotal in unveiling many fundamental mechanisms in physics, chemistry, and biology. In particular, real-time imaging modalities with ultrahigh temporal resolutions are required for capturing ultrashort events on picosecond timescales. Despite recent approaches witnessing a dramatic boost in high-speed photography, current single-shot ultrafast imaging schemes operate only at conventional optical wavelengths, being suitable solely within an optically-transparent framework. Here, leveraging on the unique penetration capability of terahertz radiation, we demonstrate a single-shot ultrafast terahertz photography system that can capture multiple frames of a complex ultrafast scene in non-transparent media with sub-picosecond temporal resolution. By multiplexing an optical probe beam in both the time and spatial-frequency domains, we encode the terahertz-captured three-dimensional dynamics into distinct spatial-frequency regions of a superimposed optical image, which is then computationally decoded and reconstructed. Our approach opens up the investigation of non-repeatable or destructive events that occur in optically-opaque scenarios.

Photosynthesis re-wired on the pico-second timescale.

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to 're-wire' photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H2 evolution or CO2 fixation1,2. Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems3. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems4,5. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.

Optically induced ultrafast magnetization switching in ferromagnetic spin valves.

The discovery of spin-transfer torque (STT) enabled the control of the magnetization direction in magnetic devices in nanoseconds using an electrical current. Ultrashort optical pulses have also been used to manipulate the magnetization of ferrimagnets at picosecond timescales by bringing the system out of equilibrium. So far, these methods of magnetization manipulation have mostly been developed independently within the fields of spintronics and ultrafast magnetism. Here we show optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free archetypal spin valves of [Pt/Co]/Cu/[Co/Pt] commonly used for current-induced STT switching. We find that the magnetization of the free layer can be switched from a parallel to an antiparallel alignment, as in STT, indicating the presence of an unexpected, intense and ultrafast source of opposite angular momentum in our structures. Our findings provide a route to ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism.

Ionic coherence in resonant above-threshold attosecond ionization spectroscopy

The ionization of atoms with sequences of attosecond pulses gives rise to excited ionic states that are entangled with the emitted photoelectron. Still, the ionic ensemble preserves some coherence that can be controlled through the laser parameters. In helium, control of the $2s/2p\phantom{\rule{4pt}{0ex}}{\mathrm{He}}^{+}$ coherence is mediated by the autoionizing states below the $N=2$ threshold [Phys. Rev. Res. 3, 023233 (2021)]. In the present work we study the role of the resonances both below and above the $N=3$ threshold on the coherence of the $N=3\phantom{\rule{4pt}{0ex}}{\mathrm{He}}^{+}$ ion, in the attosecond pump-probe ionization of the helium atom, which we simulate using the newstock ab initio code. Due to the fine-structure splitting of the $N=3{\mathrm{He}}^{+}$ level, the ionic dipole beats on a picosecond timescale. We show how, from the dipole beating, it is possible to reconstruct the polarization of the ion at its inception.

Efficient Reverse Intersystem Crossing in Carbene‐Copper‐Amide TADF Emitters via an Intermediate Triplet State

AbstractThe mechanism behind reverse intersystem crossing (rISC) in metal‐based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor‐acceptor systems with charge‐transfer excited states. However, their associated spin‐orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene‐copper(I)‐carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher‐lying triplet state of metal‐to‐ligand charge transfer character (3MLCT), which enables large spin‐orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S1 state, where the complex can return to a co‐planar coordination geometry that presents high fluorescence efficiency.

Efficient Reverse Intersystem Crossing in Carbene-Copper-Amide TADF Emitters via an Intermediate Triplet State.

The mechanism behind reverse intersystem crossing (rISC) in metal-based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor-acceptor systems with charge-transfer excited states. However, their associated spin-orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene-copper(I)-carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher-lying triplet state of metal-to-ligand charge transfer character (3 MLCT), which enables large spin-orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S1 state, where the complex can return to a co-planar coordination geometry that presents high fluorescence efficiency.

Measuring ultrafast time-bin qudits

Time-bin qudits have emerged as a promising encoding platform in many quantum photonic applications. However, the requirement for efficient single-shot measurement of time-bin qudits instead of reconstructive detection has restricted their widespread use in experiments. Here, we propose an efficient method to measure arbitrary superposition states of time-bin qudits and confirm it up to dimension 4. This method is based on encoding time bins at the picosecond time scale, also known as ultrafast time bins. By doing so, we enable the use of robust and phase-stable single spatial mode temporal interferometers to measure time-bin qudit in different measurement bases.

Tunable Photoinduced Charge Transfer at the Interface between Benzoselenadiazole-Based MOF Linkers and Thermally Activated Delayed Fluorescence Chromophore.

Structural modifications to molecular systems that lead to the control of photon emission processes at the interfaces between photoactive materials play a key role in the development of fluorescence sensors, X-ray imaging scintillators, and organic light-emitting diodes (OLEDs). In this work, two donor-acceptor systems were used to explore and reveal the effects of slight changes in chemical structure on interfacial excited-state transfer processes. A thermally activated delayed fluorescence (TADF) molecule was chosen as the molecular acceptor. Meanwhile, two benzoselenadiazole-core MOF linker precursors, Ac-SDZ and SDZ, with the presence and absence of a C≡C bridge, respectively, were carefully chosen as energy and/or electron-donor moieties. We found that the SDZ -TADF donor-acceptor system exhibited efficient energy transfer, as evidenced by steady-state and time-resolved laser spectroscopy. Furthermore, our results demonstrated that the Ac-SDZ-TADF system exhibited both interfacial energy and electron transfer processes. Femtosecond-mid-IR (fs-mid-IR) transient absorption measurements revealed that the electron transfer process takes place on the picosecond timescale. Time-dependent density functional theory (TD-DFT) calculations confirmed that photoinduced electron transfer occurred in this system and demonstrated that it takes place from C≡C in Ac-SDZ to the central unit of the TADF molecule. This work provides a straightforward way to modulate and tune excited-state energy/charge transfer processes at donor-acceptor interfaces.