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Results for “Ultrafast laser spectroscopy”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Phonon-Assisted Intervalley Scattering Determines Ultrafast Exciton Dynamics in MoSe 2 Bilayers

While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster timescale in MoSe 2 bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the K valley to other lower-energy Γ and Λ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to the manipulation of spin-valley degrees of freedom in TMDC bilayers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Attosecond inner-shell lasing at ångström wavelengths

Since the invention of the laser, nonlinear effects such as filamentation, Rabi cycling and collective emission have been explored in the optical regime, leading to a wide range of scientific and industrial applications. X-ray free-electron lasers (XFELs) have extended many optical techniques to X-rays for their advantages of ångström-scale spatial resolution and elemental specificity. An example is XFEL-driven inner-shell Kα 1 (2p 3/2 → 1s 1/2 ) X-ray lasing in elements ranging from neon to copper, which has been used for nonlinear spectroscopy and development of new X-ray laser sources. Here, in this study, we show that strong lasing effects similar to those in the optical regime can occur at 1.5–2.1 Å wavelengths during high-intensity (>10 19 W cm −2 ) XFEL-driven Kα 1 lasing of copper and manganese. Depending on the temporal XFEL pump pulse substructure, the resulting X-ray pulses (about 10 6 −10 8 photons) can exhibit strong spatial inhomogeneities and spectral splitting, inhomogeneities and broadening. Three-dimensional Maxwell–Bloch calculations show that the observed spatial inhomogeneities result from X-ray filamentation and that the broad spectral features are driven by sub-femtosecond Rabi cycling. Our simulations indicate that these X-ray pulses can have pulse lengths of less than 100 attoseconds and coherence properties that provide opportunities for quantum X-ray optics applications.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Streaked Sub-ps-resolution X-ray Line Shapes and Implications for Solid-density Plasma Dynamics

A high-resolution x-ray spectrometer was coupled with an ultrafast x-ray streak camera to produce time-resolved line shape spectra measured from hot, solid-density plasmas. A Bragg crystal was placed near a laser-produced plasma to maximize throughput; alignment tolerances were established by raytracing. The streak camera produced single-shot time-resolved spectra, heavily sloped due to photon time-of-flight differences, with sufficient reproducibility to accumulate photon statistics. The images are time-calibrated by the slope of streaked spectra and dewarped to generate spectra emitted at different times defined at the source. The streaked spectra demonstrate the evolution of spectral shoulders and other features on ps timescales, showing the feasibility of plasma parameter measurements on the rapid timescales necessary to study high-energy-density plasmas.

High-energy-density plasmas↗
DOE OSTI2022

A machine learning photon detection algorithm for coherent x-ray ultrafast fluctuation analysis

X-ray free electron laser experiments have brought unique capabilities and opened new directions in research, such as creating new states of matter or directly measuring atomic motion. One such area is the ability to use finely spaced sets of coherent x-ray pulses to be compared after scattering from a dynamic system at different times. This enables the study of fluctuations in many-body quantum systems at the level of the ultrafast pulse durations, but this method has been limited to a select number of examples and required complex and advanced analytical tools. By applying a new methodology to this problem, we have made qualitative advances in three separate areas that will likely also find application to new fields. As compared to the “droplet-type” models, which typically are used to estimate the photon distributions on pixelated detectors to obtain the coherent x-ray speckle patterns, our algorithm achieves an order of magnitude speedup on CPU hardware and two orders of magnitude improvement on GPU hardware. We also find that it retains accuracy in low-contrast conditions, which is the typical regime for many experiments in structural dynamics. Finally, it can predict photon distributions in high average-intensity applications, a regime which up until now has not been accessible. Our artificial intelligence-assisted algorithm will enable a wider adoption of x-ray coherence spectroscopies, by both automating previously challenging analyses and enabling new experiments that were not otherwise feasible without the developments described in this work.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Autoionizing Polaritons in Attosecond Atomic Ionization

Light-induced states and Autler-Townes splitting of laser-coupled states are common features in the photoionization spectra of laser-dressed atoms. The entangled light-matter character of metastable Autler-Townes multiplets, which makes them autoionizing polaritons, however, is still largely unexplored. We employ attosecond transient-absorption spectroscopy in argon to study the formation of polariton multiplets between the 3s –1 4p and several light-induced states. We measure a controllable stabilization of the polaritons against ionization, in excellent agreement with ab initio theory. Using an extension of the Jaynes-Cummings model to autoionizing states, we show that this stabilization is due to the destructive interference between the Auger decay and the radiative ionization of the polaritonic components. Furthermore, these results give new insights into the optical control of electronic structure in the continuum and unlock the door to applications of radiative stabilization in metastable polyelectronic systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Multipolariton control in attosecond transient absorption of autoionizing states

Tunable attosecond transient absorption spectroscopy is an ideal tool for studying and manipulating autoionization dynamics in the continuum. We investigate near-resonant two-photon couplings between the bright 3s –1 4p and dark 3s –1 4f autoionizing states of argon that lead to Autler-Townes-like interactions, forming entangled light-matter states or polaritons. We observe that one-photon couplings with intermediate dark states play an important role in this interaction, leading to the formation of multiple polaritonic branches whose energies exhibit avoided crossings as a function of the dressing-laser frequency. Our experimental measurements and theoretical essential-state simulations show good agreement and reveal how the delay, frequency, and intensity of the dressing pulse govern the properties of autoionizing polariton multiplets. Furthermore, these results demonstrate new pathways for quantum control of autoionizing states with optical fields.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Solid state core-exciton dynamics in NaCl observed by tabletop attosecond four-wave mixing spectroscopy

Nonlinear wave mixing in solids with ultrafast x-rays can provide insight into complex electronic dynamics of materials. Here, tabletop-based attosecond noncollinear four-wave mixing (FWM) spectroscopy using one extreme ultraviolet (XUV) pulse from high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses characterizes the dynamics of the Na + L 2,3 edge core-excitons in NaCl around 33.5 eV. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the Na+ Γ point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear wave mixing spectroscopy. In addition, other dark excitonic states that are coupled to the XUV-allowed levels by the NIR pulses are characterized spectrally and temporally. Approximately <10fs coherence lifetimes of the core-exciton states are observed. Furthermore, the core-excitonic properties are discussed in the context of strong electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid state measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Time-resolved X-ray diffraction diagnostic development for the National Ignition Facility

Here we present the development of an experimental platform that can collect four frames of x-ray diffraction data along a single line of sight during laser-driven, dynamic-compression experiments at the National Ignition Facility. The platform is comprised of a diagnostic imager built around ultrafast sensors with a 2-ns integration time, a custom target assembly that serves also to shield the imager, and a 10-ns duration, quasi-monochromatic x-ray source produced by laser-generated plasma. We demonstrate the performance with diffraction data for Pb ramp compressed to 150 GPa and illuminated by a Ge x-ray source that produces ∼7 × 10 11 , 10.25-keV photons/ns at the 400 μm diameter sample.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Creation of Multi-Principal Element Alloy NiCoCr Nanostructures via Nanosecond Laser-Induced Dewetting

The multi-principal element alloy nanoparticles (MPEA NPs), a new class of nanomaterials, present a highly rewarding opportunity to explore new or vastly different functional properties than the traditional mono/bi/multimetallic nanostructures due to their unique characteristics of atomic-level homogeneous mixing of constituent elements in the nanoconfinements. Here, in this paper, the successful creation of NiCoCr nanoparticles, a well-known MPEA system is reported, using ultrafast nanosecond laser-induced dewetting of alloy thin films. Nanoparticle formation occurs by spontaneously breaking the energetically unstable thin films in a melt state under laser-induced hydrodynamic instability and subsequently accumulating in a droplet shape via surface energy minimization. While NiCoCr alloy shows a stark contrast in physical properties compared to individual metallic constituents, i.e., Ni, Co, and Cr, yet the transient nature of the laser-driven process facilitates a homogeneous distribution of the constituents (Ni, Co, and Cr) in the nanoparticles. Using high-resolution chemical analysis and scanning nanodiffraction, the environmental stability and grain arrangement in the nanoparticles are further investigated. Thermal transport simulations reveal that the ultrashort (≈100 ns) melt-state lifetime of NiCoCr during the dewetting event helps retain the constituent elements in a single-phase solid solution with homogenous distribution and opens the pathway to create the unique MPEA nanoparticles with laser-induced dewetting process.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Streaked optical pyrometer for proton-driven isochoric heating experiments of solid and foam targets

We have designed, built, and calibrated an ultrafast streaked optical pyrometer (SOP) with ~5 ps resolution for measuring the time-resolved surface blackbody temperature of an isochorically heated plasma. The pyrometer is calibrated to measure temperatures from 0.5 eV to 25 eV, with the option of adding calibrated neutral density filters to observe hotter plasmas. We demonstrated the use of this pyrometer on isochoric heating experiments using laser-accelerated proton beams as an energy source at the $f$ /40 beamline of the Texas Petawatt Laser. We used the large focal spot size (~80 μm radius) of the $f$ /40 pulses to accelerate high numbers of ~MeV protons off 5 μm thick solid gold targets. During our experimental campaign, we heated 10 μm thick aluminum foil targets to 1–10 eV over a duration of 50 ps and also observed heating to ~2 eV in 100 μm thick 60 mg/cm 3 carbon foams using the SOP to measure both the temperature and heating timescale.

Equations of state↗
DOE OSTI2026

Pursuing Heteroleptic Ligand Design Principles for Photoactive Fe Complexes with Ultrafast X-ray Emission and Variable-Temperature Optical Spectroscopies

Understanding the key parameters that govern the photophysical and photochemical properties of transition metal complexes is essential for the development of efficient photosensitizers for photocatalytic applications. Achieving this objective necessitates clear and detailed investigations of their electronic excited states, for which time-resolved metal Kβ X-ray emission spectroscopy (XES) has proven highly effective. Here, we present a time-resolved Fe Kβ XES study of a heteroleptic Fe(II) polypyridyl carbene complex, [Fe(phen) 2 (C 4 H 10 N 4 )] 2+ (1; phen = 1,10-phenanthroline), utilizing both the valence-to-core and Kβ mainline spectral regions, complemented by variable-temperature transient optical absorption (VT-TA) spectroscopy. Detailed analysis of the time-resolved Kβ XES data, supported by density functional theory (DFT) calculations and an Eyring analysis of the VT-TA data, reveals parallel excited state relaxation dynamics that support an assignment of the long-lived excited state to a triplet metal-centered state. Placing these results in the context of prior studies of heteroleptic Fe(II) polypyridyl cyanide complexes motivated a series of DFT calculations to investigate the effects of ligand structural flexibility and arrangement. These calculations reinforce the experimentally derived conclusion that constraining structural flexibility with multidentate ligands significantly impacts the excited state relaxation dynamics. Furthermore, our study emphasizes that the arrangement of strong field ligands in heteroleptic complexes substantially affects the energy of Jahn–Teller active triplet metal-centered states in low-spin d 6 metal complexes. Together, these findings provide synthetic design principles for extending metal-to-ligand charge transfer excited state lifetimes of heteroleptic Fe complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Influence of substitution pattern on the dynamics of internal conversion and intersystem crossing in thiopyridone isomers

We report a combined experimental and theoretical investigation of the ultrafast internal conversion (IC) and intersystem crossing (ISC) dynamics of two thiopyridone (TP) isomers in solution. Our study used ultrafast transient X-ray absorption spectroscopy (XAS) at the sulfur K-edge, in conjunction with electronic excited state surface hopping molecular dynamics and simulations of the excited state XAS, to investigate the impact of the functional group substitution pattern and solvent on the dynamics of IC and ISC. The combination of the localized X-ray probe and the simulation results enables, in part, the differentiation between ππ* and nπ* character excited states, as well as singlet and triplet states. Access to nπ* character excitations has particular value since they often prove challenging to assess with optical spectroscopy. For 2-TP, the photoexcited S 2 (ππ*) state rapidly undergoes IC to the S 1 (nπ*) state below the instrument response time, followed by ISC to the T 1 (ππ*) state on a timescale of 600 fs in acetonitrile. For 4-TP, the timescale of S 2 to S 1 IC increases to 330 fs and the timescale of ISC increases to more than 10 ps. The differences between isomers are rationalized by considering the key role of the, nπ* intermediates in mediating the intersystem crossing of these systems. Varying the substitution pattern of the molecule can stabilize or destabilize these intermediates leading to the increase in ISC rate in the ortho isomer as compared to the para isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.

Garratt, Douglas [SLAC National Accelerator Labora↗
DOE OSTI2022

Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii

Abstract Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of $$\sim $$ ∼ 35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Two-dimensional retrieval methods for ultrafast imaging of molecular structure using laser-induced electron diffraction

Molecular structural retrieval based on electron diffraction has been proposed to determine the atomic positions of molecules with sub-angstrom spatial and femtosecond temporal resolutions. Given its success on small molecular systems, in this work, we point out that the accuracy of structure retrieval is constrained by the availability of a wide range of experimental data in the momentum space in all molecular systems. To mitigate the limitations, for laser-induced electron diffraction, here we retrieve molecular structures using two-dimensional (energy and angle) electron momentum spectra in the laboratory frame for a number of small molecular systems, which have previously been studied with 1D methods. Compared to the conventional single-energy or single-angle analysis, our 2D methods effectively expand the momentum range of the measured data. Besides utilization of the 2D data, two complementary methods are developed for consistency check on the retrieved results. Further, the 2D nature of our methods also offers a way of estimating the error from retrieval, which has never been explored before. Comparing with results from prior experiments, our findings show evidence that our 2D methods outperform the conventional 1D methods. Paving the way to the retrieval of large molecular systems, in which their tunneling ionization rates are challenging to obtain, we estimate the error of using the isotropic model in place of including the orientation-dependent ionization rate.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Ultrafast Suppression of the Ferroelectric Instability in KTaO 3

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266 nm, 50 fs pulse duration) of the incipient ferroelectric potassium tantalate, KT a O 3 . By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3 Å and < 10 fs pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along Γ to X and Γ to M. Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. Additionally, the density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen 2p derived π-bonding orbitals to Ta 5d derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Distinct Optical Excitation Mechanisms of a Coherent Magnon in a van der Waals Antiferromagnet

The control of antiferromagnets with ultrashort optical pulses has emerged as a prominent field of research. Tailored laser excitation can launch coherent spin waves at terahertz frequencies, yet a comprehensive description of their generation mechanisms is still lacking despite extensive efforts. Using terahertz emission spectroscopy, we investigate the generation of a coherent magnon mode in the van der Waals antiferromagnet NiPS 3 under a range of photoexcitation conditions. By tuning the pump photon energy from transparency to resonant with a 𝑑−𝑑 transition, we reveal a striking change in the coherent magnon’s dependence on the pump polarization, indicating two distinct excitation mechanisms. Furthermore, our findings provide a strategy for the manipulation of magnetic modes via photoexcitation around subgap electronic states.

Light-induced magnetic effects↗
DOE OSTI2021

Electron thermalization and relaxation in laser-heated nickel by few-femtosecond core-level transient absorption spectroscopy

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel M 2,3 edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening (σ) and a red shift (ω s ) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase (ΔT ) is linearly proportional to the Gaussian broadening factor σ, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of 640 ± 80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ω s ∝ ΔT 1.5 . Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. Here, the study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗