Attosecond electron pulses for 4D diffraction and microscopy

In this contribution, we consider the advancement of ultrafast electron diffraction and microscopy to cover the attosecond time domain. The concept is centered on the compression of femtosecond electron packets to trains of 15-attosecond pulses by the use of the ponderomotive force in synthesized gratings of optical fields. Such attosecond electron pulses are significantly shorter than those achievable with extreme UV light sources near 25 nm ( approximately 50 eV) and have the potential for applications in the visualization of ultrafast electron dynamics, especially of atomic structures, clusters of atoms, and some materials.     

Breaking resolution limits in ultrafast electron diffraction and microscopy

Ultrafast electron microscopy and diffraction are powerful techniques for the study of the time-resolved structures of molecules, materials, and biological systems. Central to these approaches is the use of ultrafast coherent electron packets. The electron pulses typically have an energy of 30 keV for diffraction and 100-200 keV for microscopy, corresponding to speeds of 33-70% of the speed of light. Although the spatial resolution can reach the atomic scale, the temporal resolution is limited by the pulse width and by the difference in group velocities of electrons and the light used to initiate the dynamical change. In this contribution, we introduce the concept of tilted optical pulses into diffraction and imaging techniques and demonstrate the methodology experimentally. These advances allow us to reach limits of time resolution down to regimes of a few femtoseconds and, possibly, attoseconds. With tilted pulses, every part of the sample is excited at precisely the same time as when the electrons arrive at the specimen. Here, this approach is demonstrated for the most unfavorable case of ultrafast crystallography. We also present a method for measuring the duration of electron packets by autocorrelating electron pulses in free space and without streaking, and we discuss the potential of tilting the electron pulses themselves for applications in domains involving nuclear and electron motions.     

Imaging conical intersection dynamics during azobenzene photoisomerization by ultrafast X-ray diffraction

X-ray diffraction is routinely used for structure determination of stationary molecular samples. Modern X-ray photon sources, e.g., from free-electron lasers, enable us to add temporal resolution to these scattering events, thereby providing a movie of atomic motions. We simulate and decipher the various contributions to the X-ray diffraction pattern for the femtosecond isomerization of azobenzene, a textbook photochemical process. A wealth of information is encoded besides real-time monitoring of the molecular charge density for the cis to trans isomerization. In particular, vibronic coherences emerge at the conical intersection, contributing to the total diffraction signal by mixed elastic and inelastic photon scattering. They cause distinct phase modulations in momentum space, which directly reflect the real-space phase modulation of the electronic transition density during the nonadiabatic passage. To overcome the masking by the intense elastic scattering contributions from the electronic populations in the total diffraction signal, we discuss how this information can be retrieved, e.g., by employing very hard X-rays to record large scattering momentum transfers.

Diffraction signals reveal the momentum transfer experienced by an incident photon through interference with one or more scatterers. Traditionally, X-ray diffraction is used to determine the structure of crystalline matter. A momentum-space image, provided by the scattered photons, is recorded, allowing for the reconstruction of the real-space crystal structure. Diffraction patterns are dominated by the Bragg peaks, arising from the interaction of multiple scatterers in a long-range crystalline order. The relevant material quantity is the electronic charge density, from which the X-ray photons elastically scatter.The advent of free-electron X-ray light sources (1, 2) has added two game-changing ingredients to diffraction experiments. The first is a peak brilliancy that can exceed third-generation synchrotron sources by nine orders of magnitude (3). The high number of photons in the beam allows for a significant reduction of the sample size, while still achieving the necessary ratio of scattered photons per object to record diffraction patterns. Structure determination of nanocrystals (4, 5), aligned gas-phase molecular samples (6), and macromolecular structures (7, 8) were reported. Attempts toward imaging single molecules at free-electron lasers were made (9), although some challenges remain (8). The second ingredient is temporal resolution, enabled by subfemtosecond pulse durations (1 fs = 10−15 s), also available from tabletop setups through, e.g., high harmonic generation (10) or laser-driven plasma sources (11). Time-resolved diffraction movies can monitor the electronic charge density evolution during a chemical process (12–19). A complementary technique is ultrafast electron diffraction, where the X-rays are replaced by bright electron pulses that scatter from the total (nuclear + electronic) charge density (20–24).Diffraction from matter in nonstationary states, such as molecules undergoing a chemical transformation, involves physical processes that go beyond elastic scattering from instantaneous snapshots of the electronic ground state density. This is especially true for excited molecules, being in a time-evolving superposition of many-body states. Inelastic scattering from different electronic states (25–27), as well as electronic and vibrational coherences (27–30), contributes to the signal.Here we study theoretically the diffractive imaging of vibronic coherences that emerge at conical intersections (CoIns). These are regions of degeneracy in electronic potential energy surfaces (PESs), where the movement of electrons and nuclei become strongly coupled, enabling ultrafast radiationless relaxation channels back to the ground state (31, 32). Processes like the primary event of vision (33) or the outcome of photochemical reactions (23), among many others, are determined by the occurrence and properties of CoIns. A textbook example of CoIn dynamics is the optical switching between cis and trans azobenzene (Fig. 1A): azobenzene can be switched selectively on a femtosecond timescale between both isomers (34–36) with high quantum yield, it exhibits interesting photophysics like a violation of Kasha’s rule (37, 38), and it has found broad application as the photoactive unit in switching the activity of pharmaceutical compounds (39) or neurons (optogenetics) (40). There is a large body of experimental and theoretical literature on azobenzene, and the previous references are by no means exhaustive. For a deeper overview of scientific progress about azobenzene photophysics, we refer the reader to ref. 38 and section 8 of its supplement.Open in a separate windowFig. 1.Molecular structures and electron densities of azobenzene. (A) Isomerization scheme with the (Left) trans and (Right) cis geometry. Both processes can be selectively achieved with high quantum yield. In this work, we simulate the cis→trans process. (B) Real-space electronic densities at a structure close to the CoIn at 90° of CNNC torsion, with the ground state (96 electrons; Left), the transition state (1 electron; Middle), and the excited state (96 electrons; Right) density.Three-vibrational-mode PESs for the photoisomerization of azobenzene were reported in ref. 41. The nuclear space consists of the carbon–nitrogen–nitrogen–carbon (CNNC) torsion angle and the two CNN bending angles between the azo unit and the benzene rings. CNNC torsion is the reactive coordinate that connects the cis and trans minima at 0° and 180°, respectively, while the two CNN angles are responsible for the symmetry breaking that leads to the minimum energy CoIn seam (41). We perform nuclear wave packet simulations of the cis→trans isomerization, yielding the total nuclear + electronic molecular wave function. Using the quantum-electrodynamical formalism for time-resolved diffraction signals developed in refs. 27 and 42, we follow the isomerization via its diffraction snapshots. The focus of this paper is on the coherence contributions to the diffraction signal and their distinctly different shapes and magnitudes compared to the other contributions. This results in a real-space movie of the molecular transition charge density at the CoIn.     

Observing (non)linear lattice dynamics in graphite by ultrafast Kikuchi diffraction

In materials, the nature of the strain–stress relationship, which is fundamental to their properties, is determined by both the linear and nonlinear elastic responses. Whereas the linear response can be measured by various techniques, the nonlinear behavior is nontrivial to probe and to reveal its nature. Here, we report the methodology of time-resolved Kikuchi diffraction for mapping the (non)linear elastic response of nanoscale graphite following an ultrafast, impulsive strain excitation. It is found that the longitudinal wave propagating along the c-axis exhibits echoes with a frequency of 9.1 GHz, which indicates the reflections of strain between the two surfaces of the material with a speed of ∼4 km/s. Because Kikuchi diffraction enables the probing of strain in the transverse direction, we also observed a higher-frequency mode at 75.5 GHz, which has a relatively long lifetime, on the order of milliseconds. The fluence dependence and the polarization properties of this nonlinear mode are entirely different from those of the linear, longitudinal mode, and here we suggest a localized breather motion in the a-b plane as the origin of the nonlinear shear dynamics. The approach presented in this contribution has the potential for a wide range of applications because most crystalline materials exhibit Kikuchi diffraction.Materials of graphite-type structure are prototype models for studies in solid-state physics (1–3), mainly because of their highly anisotropic bonding characteristics and their consequential unique properties (4, 5). Although the elastic properties of graphite and other carbon allotropes have been investigated for decades (6), a comprehensive and complete picture (7–9) still is lacking, especially when describing the nonlinear elastic properties, whose measurement requires homogeneous and large specimens (8).Using parallel-beam X-ray or electron illumination, the atomic structure of materials can be determined by measuring the position and intensity of Bragg spots. Time-resolved Bragg diffraction has been incorporated successfully to study longitudinal elastic motion, which involves compression/expansion of the unit cell along the direction perpendicular to the specimen’s surface. In graphite, which has an anisotropic thermal expansion, both longitudinal acoustic waves (10) and shear (transverse) waves may be generated as a result of the breaking of the translational symmetry at the level of thermoelastic forces (11).With convergent electron beam illumination, time-integrated Kikuchi diffraction provides a sensitive and precise measure of static lattice plane orientation (12). When time resolved, the evolution of Kikuchi diffraction patterns makes possible the investigation of both the longitudinal and transverse dynamics of the strain and associated lattice deformations (13, 14). This is because Kikuchi diffraction results from the scattering of electrons in the material and a large cone of wave vectors permits reflections that fulfill Bragg condition from different lattice planes; hence, it is possible to monitor the dynamics as the planes tilt and the lattice changes transversely. In contrast, these types of motions will not change the position (only the intensity) of Bragg spots in conventional diffraction experiments; for an illustration, see figure 2 in ref. 13.Open in a separate windowFig. 2.Kikuchi diffraction dynamics. (A) Oscillation dynamics of the ZOLZ plane (220): a single 75.5-GHz mode is detected. The black solid line is a monoexponential fit to mimic the incoherent thermal stress buildup. Note that the behavior in the negative time is a result of the long lifetime (milliseconds) of the 75.5-GHz mode (see text for details). (B) Oscillation dynamics of the HOLZ plane (142): a 9.1-GHz mode superimposed on the 75.5-GHz mode is detected. (C) Oscillation dynamics of the ZOLZ plane (220) and the HOLZ plane (31): echoes at 9.1-GHz frequency were observed for the HOLZ plane (31) (data are shifted by −0.05 mrad for clarity). For the ZOLZ plane (220), no transients were observed, as discussed in the text.Here, we use the Kikuchi diffraction approach in a convergent-beam ultrafast electron crystallography (CB-UEC) setup to investigate the acoustic wave dynamics of single crystals of graphite. By monitoring the temporal evolution of the Kikuchi pattern, it was possible to separate the longitudinal and transverse atomic motions. We observe longitudinal acoustic echoes propagating along the c-axis of the graphite unit cell, which represent the linear response and out-of-plane motion of the graphite lattice. The transverse lattice motion is observed as a higher-frequency mode at 75.5 GHz, and this mode exhibits a relatively long lifetime, on the order of milliseconds, and a polarization direction that rotates with the excitation fluence. The mode is an in-plane shear deformation of the graphite unit cell, and we suggest a localized breather motion in the a-b plane as its origin. In this work, the specimen was obtained from a natural graphite single crystal (naturally graphite) by mechanical cleavage (see Fig. 1B), and the thickness used is 220 nm.Open in a separate windowFig. 1.Kikuchi diffraction of graphite. (A) A representative pattern together with the indexed Kikuchi lines. Dashed and solid lines represent “deficiency” and “excess” lines (12), respectively. (B) Free-standing single crystalline graphite sample on a TEM grid. The white + marks the probed position. (C) Gray and colored dashed lines denote, respectively, lattice planes at equilibrium and when lattice deformation occurs. The blue lines represent the tilting of a ZOLZ plane (Left) and of a HOLZ plane (Right) as induced by an in-plane shear motion along the a-axis. The red lines represent the effect of a longitudinal motion along the c-axis; only for the HOLZ plane (Right) is there an effective tilting.The CB-UEC experiments were performed in a transmission geometry (15, 16), and the electron beam was focused down to a micrometer-length scale with a convergent angle of ∼3 mrad. Fig. 1A shows the measured CB diffraction pattern together with the indexed Kikuchi lines for graphite (17). The electron beam direction was set to 3.45° off the [0001] zone axis (Fig. 1A) to minimize the background from multiple electron scatterings.As a function of time, the shift of the Kikuchi line position with respect to the electron beam is proportional to the tilting of the corresponding lattice plane in the specimen; thus, lattice motion may be correlated directly with changes in the measured CB pattern. Lattice planes with different orientations respond differently to a certain lattice modulation (14). For instance, a lattice plane belonging to the zero-order Laue zone (ZOLZ), which is parallel to the c-axis, is sensitive to any tilt or shear motion developing in the a-b plane, whereas it is completely unaffected by motion along the c-axis (see schematics in Fig. 1C, Left). This is not the case for a lattice plane in high-order Laue zones (HOLZs) intersecting with the c-axis, which turns out to also be tilted by a lattice expansion/contraction along the c-axis (see schematics in Fig. 1C, Right).The experimental setup and the methodology for ultrafast dynamics measurements are detailed elsewhere (18–20). Briefly, structural dynamics are initiated by femtosecond laser pulses (120 fs, 800 nm, 45° incidence, p-polarized, 1.1 × 1.6-mm² spot size, 1 kHz) and probed by ultrashort electron pulses with kinetic energy of 30 keV, at different delay times between the arrival time of the excitation laser pulse and the electron pulse at the sample. The dynamics were recorded with time steps of 1 ps, or longer if needed. For each delay time, more than 50 patterns, each averaged over 10⁴ electron pulses, were recorded to achieve a high signal-to-noise level. It is worth noting that the use of the pump laser and the pulsed electron probe with a repetition rate of 1 kHz enables us to follow the dynamics under investigation up to 1 ms from time zero, which is defined as the moment at which the dynamics are initiated and both the optical and electron pulses overlap in space and time. This is possible to establish by monitoring the temporal response at the negative delay times (19). By using active cavity length control of the femtosecond laser oscillator, the repetition rate is synchronized to an external clock, yielding a frequency stability better than 1 Hz with a synchronization jitter below 1 ps.After determining the position of all Kikuchi lines (Supporting Information), we construct the temporal evolution of the tilting motion for each corresponding lattice plane by monitoring the change in position as a function of the delay time. Representative dynamics for the ZOLZ planes (2 20) and (22 0) and the HOLZ planes (1 42) and ( 31) are shown in Fig. 2, with time steps of 3 ps. Positive values of the tilt indicate that the lattice plane is inclining outward relative to the direct electron beam, whereas negative values indicate an inward inclination. Noted in Fig. 2A is the exponential fit (black solid curve), which describe the average incoherent thermal stress buildup (14). Experiments performed at the same sample position with a parallel electron beam illumination in a [0001]-zone axis result in a diffraction pattern that exhibits sharp Bragg spots, and the intensity of the spots is altered as the result of thermal-induced changes of the lattice, i.e., no oscillations are observed.Besides the thermal stress buildup, two major oscillations with distinct envelopes are observed in the Kikuchi pattern: a fast oscillation with a period of ∼13 ps and a slower one superimposed on the fast oscillation with a period of ∼110 ps. The amplitude of the fast oscillation is sensitive to the particular crystallographic direction (see Fig. 2 A and C for an example), whereas the slow one remains nearly the same for all associated lattice planes (Fig. 2 B and C).From fast Fourier transform (FFT) analysis, we determine the frequencies to be 75.5 GHz and 9.1 GHz, as displayed in Fig. 3A.^* To visualize the lattice motion from the measured Kikuchi pattern, we use the method of Fourier maps introduced by Yurtsever and Zewail (13). This method permits us to follow changes in both the amplitude and phase as a function of time. For each pixel of the diffraction pattern, we determine the amplitude and the phase in the Fourier spectra at the resonance frequencies of 9.1 GHz and 75.5 GHz. The images for the phase change are presented in Fig. 4 for both frequencies and for the three investigated excitation fluences. They clearly show the relative shift of the Kikuchi lines, from which details of the lattice motion can be deduced. From these Fourier maps, it is evident that the mode at 9.1 GHz involves only Kikuchi lines associated with the HOLZ planes, whereas the mode at 75.5 GHz affects the whole pattern, although with different amplitudes for the different planes. It also is clear from the maps that the polarization of the 75.5-GHz mode lies close to the direction [1 00] and exhibits a clear rotation with the excitation fluence. A more quantitative evaluation of these observations will be discussed below.Open in a separate windowFig. 3.FFT spectra and oscillation period. (A) FFT spectra of the dynamics after time zero for the ZOLZ plane (220) (Left) and for the HOLZ plane (31) (Right). The spectral resolution of the FFT is 0.65 GHz. (B) Close view of the oscillations at 75.5 GHz from the ZOLZ plane (220), measured with time steps of 1 ps (red) and 3 ps (blue). Both profiles exhibit the same sinusoidal behavior with identical periods and matching phase (see text).Open in a separate windowFig. 4.Fourier maps for the phases at the indicated resonance frequency. Shown are the phase images at 9.1 GHz (A) and 75.5 GHz (B) for the three investigated fluences, as obtained by Fourier transform of the time-resolved Kikuchi patterns. It is evident that the mode at 9.1 GHz involves only Kikuchi lines associated with HOLZ planes, whereas the mode at 75.5 GHz affects the whole pattern, with different amplitudes for different planes. The orange and red arrows in B are shown to follow the evolution of the Kikuchi lines (211) and (030), respectively. The yellowish region in B close to each Kikuchi line is a qualitative measure of the relative change. As the fluence increases, this region evolves preferentially toward the right part of the diffraction pattern and, hence, the direction of polarization, as shown by the white double arrows; a more quantitative evaluation of the amplitude is given in Fig. 5. The scale bar is in degrees.Careful control experiments were performed to verify the intrinsic origin of the 75.5-GHz mode. First, we compared the transients obtained using 3-ps steps with those recorded with smaller time steps of 1 ps, but at the same acquisition time per point. For both cases, we obtained the same 13.2-ps period with a precise phase matching (as shown in Fig. 3B). This rules out the effect of external periodic artifacts because the period, although the same in “real time,” must change in the “pump–probe” recording when the step duration changes; a more detailed discussion is presented in Supporting Information. Second, for the same diffraction pattern, the mode amplitude depends on the planes involved, i.e., the modulation is not the same across the entire pattern, as shown in Fig. 2. Third, the oscillations were absent when the excitation laser was blocked. Finally, to further verify the robustness of the mode across the specimen, we carried out the same measurements at another position on the graphite surface (with a thickness of 100 nm instead of 220 nm) and more than 1 mm away from the current one. We obtained the same oscillation dynamics with the identical frequency of 75.5 GHz.It follows that the weak-intensity oscillations observed in the negative delay-time region (Fig. 2) are a result of the long-lived 75.5-GHz mode, which survives until a new excitation from a following pump pulse (in a millisecond) resets the lattice in motion. When examining the phase of oscillations, we found no correlation between the phase observed before (small amplitude) and after (large amplitude) time zero, thus excluding the possibility of a one-frequency artifact oscillation. It is worth mentioning that direct monitoring of the output of our amplified laser shows no additional pulse between the pump pulses separated by 1 ms at a sensitivity level greater than 1%.In contrast to the long-lived mode, the 9.1-GHz mode and its higher-order harmonics (observed at 18.2, 27.3, and 36.5 GHz; Fig. 3A, Right) vanish at long delay times; the decay time is a few nanoseconds, typical of heat dissipation in solid systems. The observation that only the HOLZ planes exhibit the 9.1-GHz mode indicates that this mode is that of a wave with polarization and propagation direction along the c-axis, a direction that does not induce orientational modulation of the ZOLZ planes. This is supported further by the close resemblance of the temporal lineshape for this mode to the successive acoustic echoes created by an impulsively excited picosecond acoustic wave traveling between the two boundaries of a metal thin film along its normal (21–23).In this picture, a traveling sound wave is created, and its frequency fulfills the relation f = v/2l, where v is the speed of sound and l is the film thickness. With the measured thickness of l = 220 nm, which we determined from electron energy-loss spectroscopy, and knowing the frequency f = 9.1 GHz from FFT plots, we obtained the sound velocity of 4.0 km/s, which is in very good agreement with the value reported for the sound velocity of longitudinal acoustic waves propagating along the c-axis in graphite [v_LA[001] = 4.14 km/s (8)]. The behavior of this traveling longitudinal wave along the [0001] direction reflects the linear response of the lattice to the impulsive excitation, and a linear chain model (24) can describe the atomic motions involved.The lattice strain created by the excitation is within a region of the absorption length [33 nm at λ = 800 nm in graphite (25)] close to the front surface. From this point, this lattice deformation begins to propagate as a sound wave into the depth of the sample along the c-axis and undergoes multiple reflections by the front and back surfaces of the graphite plate. Because of the inhomogeneous excitation, this wave induces large surface displacement and considerable energy dissipation every time it reaches the boundaries.^† Because the laser absorption length initially makes the longitudinal spatial extension smaller than the sample thickness, the sound wave at the boundaries approaches a delta-like functional behavior, which results in several high-order harmonics in the FFT spectra.For both types of modes, we investigated the fluence dependence of the amplitude. This dependence can be visualized directly in the Fourier maps shown in Fig. 4. Whereas the amplitude of the 9.1-GHz mode exhibits a linear dependence on the excitation fluence, the mode at 75.5 GHz shows a much different behavior. For instance, the amplitude of the HOLZ plane (2 11) shows no discernable change as a function of the fluence in the range of 2.0–2.9 mJ/cm², whereas for the ZOLZ plane (03 0), the amplitude exhibits a definite nonlinear trend (at 2.0 mJ/cm², the mode is barely visible, whereas at 2.4 mJ/cm² and 2.9 mJ/cm², it has a well-defined amplitude).From these fluence-dependence experiments, it was possible to systematically monitor the oscillation amplitude along different directions of the Brillouin zone, represented in Fig. 5 for the excitation fluence of 2.0 mJ/cm² (Fig. 5 A and D), 2.4 mJ/cm² (Fig. 5 B and E), and 2.9 mJ/cm² (Fig. 5 C and F). The oscillation amplitude exhibits the same temporal evolution, within the time scale of 1 ns from the initial excitation, for all Kikuchi lines with a well-defined frequency of 75.5 GHz. Moreover, the maximum value remains approximately constant, within the measurement sensitivity, to 0.10 mrad for all fluences. By performing a temporal average of the amplitude profiles for each plane, we could determine the amplitude distribution along different crystallographic directions projected on the a-b plane of the graphite unit cell (Fig. 5 D–F).Open in a separate windowFig. 5.Temporal evolution and plane-specific projection of amplitude. (A–C) Temporal evolutions of the amplitudes at excitation fluences of 2.0 mJ/cm², 2.4 mJ/cm², and 2.9 mJ/cm², respectively, for the 75.5-GHz mode. Light green regions mark the time interval over which the oscillation amplitude has been averaged. (D–F) The average amplitudes derived from the temporal profiles shown in A–C are plotted along different crystallographic directions in the a-b plane of the graphite lattice. The intensity of the oscillation amplitude is represented by a shade modulation from red (intense) to blue (weak), and quantitatively described by the length of the black double-way arrows. The plane perpendicular to the deduced polarization is denoted by a green solid line. The orange lozenge in each panel outlines the graphite unit cell in the a-b plane.The direction of polarization can be obtained from the distribution of amplitudes. This is because lattice planes perpendicular to the polarization direction have strong oscillation amplitudes, whereas those lying close to the polarization direction have weak amplitudes. As evident from Fig. 5 A–C, both the ZOLZ and the HOLZ planes exhibit a synchronous temporal evolution with similar amplitudes, suggesting that the 75.5-GHz mode is a shear motion with a polarization direction preferentially lying in the a-b plane, as schematically depicted in Fig. 1C (blue colored).^‡ We note that an out-of-plane component would introduce a different temporal behavior for the ZOLZ and HOLZ planes, especially for those close to the polarization direction. We also note that as the excitation fluence increases, the 75.5-GHz mode changes its polarization direction rather than its total amplitude (Figs. 4 and ​and55).The above studies of ultrafast Kikuchi diffraction, dependence on the fluence, and the polarization properties provide the following picture for lattice dynamics. The linear mode at 9.1 GHz has its origin in a longitudinal wave, and the atomic motions can be understood simply by considering a linear chain of atoms, each of which represents a plane perpendicular to the c-axis. Because the absorption length of the optical pulse is smaller than the sample thickness, a delta function-like stress excitation is created, and in this regime, the presence of higher harmonics is expected, as observed experimentally and is evident in the FFT spectra.The nonlinear mode at 75.5 GHz is an in-plane shear deformation, as discussed above, created by lateral strain confinement (27) (see also Supporting Information). At a microscopic level, several authors (28–30) theoretically showed that elastically deformed carbon structures can support the existence of high-frequency and long-lived nonlinear localized modes in the form of breathers (31). These nonlinear modes are robust to local perturbations of the lattice and for a graphene sheet, are represented by the out-of-phase oscillation of neighboring atoms that are polarized within the a-b plane (32). Given the observed period of 13.2 ps and using the shear “speed” of ∼14.66 km/s (8), the length scale is ∼200 nm. The long lifetime of the breathers is a result of the fact that their frequency lies inside the gap of the phonon spectrum (29, 32); thus, their decay to other modes essentially is forbidden. The excitation of breathers has been reported experimentally in several systems, such as spin waves (33), optical lattices (34), and micromechanical cantilever arrays (35), and they also are expected for atomic systems because of the discreteness of the lattice structure, the presence of lattice defects, and the nonlinearity of the interaction between atoms.In conclusion, with CB ultrafast Kikuchi diffraction, it is shown that both the longitudinal and transverse (shear) elastic dynamics of the material can be observed and separated by following changes of selective diffraction reflections. The longitudinal motion along the c-axis is the result of a linear acoustic wave propagation, whereas the transverse motion in the a-b planes is associated with a nonlinear shear deformation. From knowledge of the fluence dependence, the polarization properties, and the long lifetime, we suggest a motion involving a localized breather on a length scale of a few hundred nanometers. The reported Fourier maps at the frequency of an individual mode display the atomic planes involved in the motion, and this feature is unique to Kikuchi diffraction dynamics. It is the basis for the potential use of the approach in a wide range of applications.     

Spatiotemporal reaction kinetics of an ultrafast photoreaction pathway visualized by time-resolved liquid x-ray diffraction

We have studied the reaction dynamics for HgI(2) in methanol by using time-resolved x-ray diffraction (TRXD). Although numerous time-resolved spectroscopic studies have provided ample information about the early dynamics of HgI(2), a comprehensive reaction mechanism in the solution phase spanning from picoseconds up to microseconds has been lacking. Here we show that TRXD can provide this information directly and quantitatively. Picosecond optical pulses triggered the dissociation of HgI(2), and 100-ps-long x-ray pulses from a synchrotron probed the evolving structures over a wide temporal range. To theoretically explain the diffracted intensities, the structural signal from the solute, the local structure around the solute, and the hydrodynamics of bulk solvents were considered in the analysis. The results in this work demonstrate that the determination of transient states in solution is strongly correlated with solvent energetics, and TRXD can be used as an ultrafast calorimeter. It also is shown that a manifold of structural channels can be resolved at the same time if the measurements are accurate enough and that global analysis is applied. The rate coefficients for the reactions were obtained by fitting our model against the experimental data in one global fit including all q-values and time delays. The comparison between all putative reaction channels confirms that two-body dissociation is the dominant dissociation pathway. After this primary bond breakage, two parallel channels proceed. Transient HgI associates nongeminately with an iodine atom to form HgI(2), and I(2) is formed by nongeminate association of two iodine atoms.     

Scanning ultrafast electron microscopy

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the space-charge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsed-electron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability.     

Four-dimensional ultrafast electron microscopy

Electron microscopy is arguably the most powerful tool for spatial imaging of structures. As such, 2D and 3D microscopies provide static structures with subnanometer and increasingly with ångstrom-scale spatial resolution. Here we report the development of 4D ultrafast electron microscopy, whose capability imparts another dimension to imaging in general and to dynamics in particular. We demonstrate its versatility by recording images and diffraction patterns of crystalline and amorphous materials and images of biological cells. The electron packets, which were generated with femtosecond laser pulses, have a de Broglie wavelength of 0.0335 Å at 120 keV and have as low as one electron per pulse. With such few particles, doses of few electrons per square ångstrom, and ultrafast temporal duration, the long sought after but hitherto unrealized quest for ultrafast electron microscopy has been realized. Ultrafast electron microscopy should have an impact on all areas of microscopy, including biological imaging.     

Plasmonic antennas as design elements for coherent ultrafast nanophotonics

Broadband excitation of plasmons allows control of light-matter interaction with nanometric precision at femtosecond timescales. Research in the field has spiked in the past decade in an effort to turn ultrafast plasmonics into a diagnostic, microscopy, computational, and engineering tool for this novel nanometric–femtosecond regime. Despite great developments, this goal has yet to materialize. Previous work failed to provide the ability to engineer and control the ultrafast response of a plasmonic system at will, needed to fully realize the potential of ultrafast nanophotonics in physical, biological, and chemical applications. Here, we perform systematic measurements of the coherent response of plasmonic nanoantennas at femtosecond timescales and use them as building blocks in ultrafast plasmonic structures. We determine the coherent response of individual nanoantennas to femtosecond excitation. By mixing localized resonances of characterized antennas, we design coupled plasmonic structures to achieve well-defined ultrafast and phase-stable field dynamics in a predetermined nanoscale hotspot. We present two examples of the application of such structures: control of the spectral amplitude and phase of a pulse in the near field, and ultrafast switching of mutually coherent hotspots. This simple, reproducible and scalable approach transforms ultrafast plasmonics into a straightforward tool for use in fields as diverse as room temperature quantum optics, nanoscale solid-state physics, and quantum biology.The intriguing prospect of resolving and using nanoscale and quantum-mechanical processes in large, complex, and disordered systems pushes physics, biology, chemistry, and engineering to ever smaller length scales and ever shorter time scales (1–3). A promising route to unlocking this regime is the marriage of ultrafast spectroscopy with nanoplasmonics (4–6), as evidenced by various experiments aimed at controlling localization or measuring ultrafast dynamics of hotspots, such as polarization control of localization (7, 8), measurements of plasmon dephasing (9–11), and adiabatic compression of pulses at plasmonic tips (12). However, for ultrafast nanoplasmonics to find widespread application in physics, biology, and material sciences, the ability to engineer a plasmonic system at will to provide a desired ultrafast response in a predetermined nanoscale hotspot is crucial: only then will the technique reach the necessary reproducibility, flexibility, and simplicity to be broadly usable. The achievement of this goal requires three conditions be met: localization of a broadband pulse in a nanoscale volume, deterministic near-field dynamics for a given plasmonic structure, and the ability to tune the near-field dynamics by plasmonic design. To prove the achievement of these first three goals, a fourth ability, measuring the ultrafast field dynamics in a given hotspot, is also required.Large inroads have been made toward achieving those goals individually. Nanoscale field dynamics have been measured using photoelectron emission microscopy (PEEM), two-photon photoemission, or second harmonic spectroscopy (12–14); closed-loop coherent control has had large successes in creating (time-dependent) localization on nanostructures (7); nanofocusing of shaped ultrafast pulses has been achieved using near-field tips (12); and plasmonic structures have been used to, for example, design vortex beams or create controllable nonlinear emission (15, 16). However, the achievement of all goals simultaneously, let alone in a simple, flexible, and reproducible manner, has proved elusive. This has caused ultrafast plasmonics to remain a challenging topic of research, rather than fulfilling its potential as a tool that can unlock the fascinating regime of nano and quantum phenomena in complex physical, chemical, and biological systems.We endeavor to experimentally reach this desired regime by coupling calibrated nanoantennas into plasmonic structures that deterministically shape the spectral amplitude and phase, and therefore the ultrafast dynamics, of the near field in a predetermined hotspot.Plasmonic antennas are widely explored for applications in sensing and imaging owing to their ability to confine far-field illumination to near-field hotspots, with properties determined by antenna geometry (17–20), material (21, 22), and the excitation scheme (23–25). Furthermore, plasmonic antennas sustain coherent excitation (4, 8) and the antenna resonances exhibit broad bandwidths. Based on Fourier’s principle, this renders plasmonic antennas inherently suited for the investigation of ultrafast processes and coherent control (16, 26): A wide bandwidth in the frequency domain offers the potential for an ultrashort pulse in the time domain.With a nonlinear measurement, we determine the amplitude and phase response of single nanoantennas to ultrafast excitation, and we use calibrated antennas as building blocks to engineer two examples of deterministic ultrafast nanoscale pulse shaping by a plasmonic system: a subwavelength resolution phase modulator and an ultrafast hotspot switch. With this, we show that it is possible to create tunable, deterministic, ultrafast hotspot dynamics based on plasmonic design and an a priori defined, simple pulse.     

Cardiovascular ultrafast computed tomographic angiography

Ultrafast computed tomography can substitute for angiography and provide answers to clinical and therapeutic questions. Because of superior definition of vessels and myocardium, current applications include imaging the aorta, carotids, vena cava, and other venous structures, congenital and acquired pulmonary artery abnormalities, coronary arteries, coronary artery saphenous, and internal mammary artery bypass grafts, right and left ventricles, cardiac tumors and thrombi, cardiomyopathies (ischemic, dilated, hypertrophic, and restrictive), congenital heart disease, and a variety of other acquired abnormalities. Parallel contiguous tomographic levels provide excellent structural definition including inner vessel wall and endocardium.     

Magnetic resonance imaging vs. ultrafast computed tomography for cardiac diagnosis

Ultrafast computed tomography (CT) and magnetic resonance imaging (MRI) generate high resolution tomographic cardiac images. Ultrafast CT requires intravenous injection of x-ray contrast combined with an image acquisition time of 50 msec. MRI requires no contrast injection, but has relatively long acquisition times due to gating. Both technologies can be used to evaluate cardiac chamber and great vessel dimensions, intracardiac and extracardiac masses, ventricular hypertrophy, left ventricular mass, congenital heart disease, regional and global left ventricular function, right ventricular function and pericardium. MRI is highly useful for detection and semi-quantitation of valvular regurgitation while ultrafast CT is not. Aortic and mitral valve stenosis can be detected by both, but MRI is the preferred study. Though both techniques can be used to assess coronary artery bypass graft status, ultrafast CT is the preferred method. It is concluded that ultrafast CT and MRI have broad applications for cardiac diagnosis.     

4D multiple-cathode ultrafast electron microscopy

Four-dimensional multiple-cathode ultrafast electron microscopy is developed to enable the capture of multiple images at ultrashort time intervals for a single microscopic dynamic process. The dynamic process is initiated in the specimen by one femtosecond light pulse and probed by multiple packets of electrons generated by one UV laser pulse impinging on multiple, spatially distinct, cathode surfaces. Each packet is distinctly recorded, with timing and detector location controlled by the cathode configuration. In the first demonstration, two packets of electrons on each image frame (of the CCD) probe different times, separated by 19 picoseconds, in the evolution of the diffraction of a gold film following femtosecond heating. Future elaborations of this concept to extend its capabilities and expand the range of applications of 4D ultrafast electron microscopy are discussed. The proof-of-principle demonstration reported here provides a path toward the imaging of irreversible ultrafast phenomena of materials, and opens the door to studies involving the single-frame capture of ultrafast dynamics using single-pump/multiple-probe, embedded stroboscopic imaging.In 4D ultrafast electron microscopy (UEM), ultrafast light pulses generate electron packets by photoemission at the cathode of an electron microscope, and these are used to probe a dynamic process initiated by heating or exciting the microscopic specimen with a second, synchronized ultrafast light pulse (1, 2). In conventional implementations, each pump pulse on the specimen is accompanied by one suitably delayed laser pulse on the cathode to generate one packet of electrons probing a single time point in the evolution of the specimen. A record of the full course of temporal evolution of the specimen is then constructed by repeating the experiment multiple times with variation of the delay time between the two light pulses, reading out a separate CCD image for each delay time. Thus, information about different time points in the dynamic response of the specimen is obtained from different excitation events. This implementation is ideally suited for a specimen that undergoes irreversible but sufficiently well-defined dynamics to allow a new specimen area to be used for each time point (Fig. 1A), or for a specimen that recovers fully to allow repeated identical excitations of the same area (Fig. 1B); see also Methodology. The applications of these two approaches are numerous, as highlighted in a recent review account of the work (3).Open in a separate windowFig. 1.Variant implementations of UEM. (A) Single-pulse UEM, which enables single-shot imaging of homogeneous specimens. (B) Stroboscopic UEM. (D) Multiple-cathode UEM. For comparison, we include, in C, the single-cathode, deflection method. See text for details.For the study of completely nonrepetitive dynamics, for example, a stochastic process in a heterogeneous sample that does not return to its initial configuration, a series of snapshots following a single excitation event can provide the only direct and detailed view of the evolution. Observing the effects of a single excitation pulse with video-mode imaging can currently reach millisecond-scale time resolution, far short of the time scale for many phenomena of interest in nanoscale materials science, chemistry, and physics. Nanosecond resolution has been reached (4) by combining one excitation pulse with a train of light pulses on a single cathode, with deflection of the imaging electrons after passing the specimen plane to direct each successive pulse to a new region of the detector (Fig. 1C). This nanosecond method has been successfully used with a high-speed electrostatic deflector array to obtain time sequences of irreversible and stochastic processes (5, 6).Here we demonstrate a technique that removes any limit on time resolution imposed by image deflection and in a single frame enables the capture of ultrafast phenomena. With this approach, it is possible to probe and distinctly record multiple time points in a dynamic process following a single initiation pulse. The probing electron packets are all generated by a single light pulse that impinges on multiple, spatially distinct, cathode surfaces (Fig. 1D). Time separations between packets in the electron-pulse train are adjusted by the cathode spatial and electrostatic configuration. In the present application, two electron packets, generated from two source locations at the same potential and separated in time by 19 picoseconds (ps), are recorded on each CCD frame after undergoing diffraction in a gold film following femtosecond heating. The packets originate from different cathode locations, pass through the same area of the specimen, and are recorded at distinct locations on the detector, thereby encoding two different time points in the evolution of the specimen. The results obtained provide the basis for exploration of expanded application of the multiple-cathode concept.     

Exercise ultrafast computed tomography for the detection of coronary artery disease

Ultrafast computed tomography permits the assessment of global and regional left ventricular function during exercise. To evaluate the feasibility of using this new technique for the diagnosis of coronary artery disease, 27 patients undergoing cardiac catheterization for diagnosis of chest pain were evaluated. Fifteen patients had significant (greater than 50%) coronary artery stenosis by quantitative coronary angiography. One vessel disease was found in 12 patients and multivessel disease in 3. Fourteen (93%) of the 15 patients with significant coronary stenosis had a decrease in ultrafast computed tomographic ejection fraction during exercise from (mean +/- SD) 65 +/- 7% to 60 +/- 7% (p less than 0.001). The tomographic ejection fraction increased greater than 5% units during exercise in 10 (83%) of the 12 patients with normal coronary arteries. The mean tomographic ejection fraction in this group was 68 +/- 6% at rest and 75 +/- 6% at peak exercise (p less than 0.001). Regional wall motion was quantified by analyzing the segmental ejection fraction of 12 30 degree pie segments at each tomographic level of the left ventricle. A new regional wall motion abnormality developed during exercise in 12 (86%) of 14 patients with coronary artery disease; one patient was excluded because of a technical problem in data storage. Eleven (93%) of the 12 patients with normal coronary arteries had normal wall motion during exercise. In no patient with ischemic heart disease were both variables, ejection fraction response and regional wall motion, normal. Exercise ultrafast computed tomography appears to be a useful technique for the evaluation of coronary artery disease in patients with chest pain and predominant single vessel coronary artery disease.(ABSTRACT TRUNCATED AT 250 WORDS)     

Temporal lenses for attosecond and femtosecond electron pulses

Here, we describe the “temporal lens” concept that can be used for the focus and magnification of ultrashort electron packets in the time domain. The temporal lenses are created by appropriately synthesizing optical pulses that interact with electrons through the ponderomotive force. With such an arrangement, a temporal lens equation with a form identical to that of conventional light optics is derived. The analog of ray diagrams, but for electrons, are constructed to help the visualization of the process of compressing electron packets. It is shown that such temporal lenses not only compensate for electron pulse broadening due to velocity dispersion but also allow compression of the packets to durations much shorter than their initial widths. With these capabilities, ultrafast electron diffraction and microscopy can be extended to new domains,and, just as importantly, electron pulses can be delivered directly on an ultrafast techniques target specimen.     

Intravenous pulses of methylprednisolone for systemic lupus erythematosus

BACKGROUND: Intravenous (IV) pulses of methylprednisolone (MEP) commonly are used to treat severe manifestations of systemic lupus erythematosus (SLE). However, despite wide use of this treatment the best dose, timing, and the situations in which this treatment should be used remain largely anecdotal. AIM: To review the mechanisms of action and evidence for clinical use of IV MEP in the treatment of SLE. Method: The literature on MEP use in SLE from 1966 to 2002, using PubMed from the National Library of Medicine, was reviewed. RESULTS: As with other modes of corticosteroid administration, IV MEP has significant anti-inflammatory and immunosuppressive actions. These actions have been shown to be effective in treating SLE in clinical trials, for lupus nephritis. The studies are mainly uncontrolled and retrospective. Long-term observations from a few double-blind prospective trials suggest that monthly pulses of MEP, in addition to IV cyclophosphamide, may be useful. Pulse MEP is beneficial for several serious manifestations of SLE, such as neuro-psychiatric lupus, pulmonary hemorrhage, severe blood dyscrasias, cardiomyopathy, and vasculitis. However, significant side effects may occur, mostly infections, which are worse in patients with hypoalbuminemia. CONCLUSION: IV pulses of MEP rapidly immunosuppress patients with organ and/or life-threatening manifestations of SLE. However, the gold standard 1 g/day for 3 consecutive days is associated with significant infectious complications and lower doses may be just as useful.     

Four-dimensional ultrafast electron microscopy of phase transitions

Reported here is direct imaging (and diffraction) by using 4D ultrafast electron microscopy (UEM) with combined spatial and temporal resolutions. In the first phase of UEM, it was possible to obtain snapshot images by using timed, single-electron packets; each packet is free of space-charge effects. Here, we demonstrate the ability to obtain sequences of snapshots ("movies") with atomic-scale spatial resolution and ultrashort temporal resolution. Specifically, it is shown that ultrafast metal-insulator phase transitions can be studied with these achieved spatial and temporal resolutions. The diffraction (atomic scale) and images (nanometer scale) we obtained manifest the structural phase transition with its characteristic hysteresis, and the time scale involved (100 fs) is now studied by directly monitoring coordinates of the atoms themselves.     

Coronary artery visualization using ultrafast computed tomography

The advantages and limitations of ultrafast computed tomography in the imaging of normal and pathologic conditions of the coronary artery are discussed. The scanner's speed, resolution, and lack of significant motion artifact enhance the visualization of coronary arteries. Coronary artery calcification also is well visualized, and coronary artery fistuli, coronary bypass graft patency, and Kawasaki disease can be assessed accurately using contrast-enhanced flow studies. The inability to image stenoses and the lack of longitudinal images detract from its usefulness. Future scanner upgrades to provide increased resolution and thinner slices should improve the scanner's ability to evaluate the coronary artery.     

Photon gating in four-dimensional ultrafast electron microscopy

Ultrafast electron microscopy (UEM) is a pivotal tool for imaging of nanoscale structural dynamics with subparticle resolution on the time scale of atomic motion. Photon-induced near-field electron microscopy (PINEM), a key UEM technique, involves the detection of electrons that have gained energy from a femtosecond optical pulse via photon–electron coupling on nanostructures. PINEM has been applied in various fields of study, from materials science to biological imaging, exploiting the unique spatial, energy, and temporal characteristics of the PINEM electrons gained by interaction with a “single” light pulse. The further potential of photon-gated PINEM electrons in probing ultrafast dynamics of matter and the optical gating of electrons by invoking a “second” optical pulse has previously been proposed and examined theoretically in our group. Here, we experimentally demonstrate this photon-gating technique, and, through diffraction, visualize the phase transition dynamics in vanadium dioxide nanoparticles. With optical gating of PINEM electrons, imaging temporal resolution was improved by a factor of 3 or better, being limited only by the optical pulse widths. This work enables the combination of the high spatial resolution of electron microscopy and the ultrafast temporal response of the optical pulses, which provides a promising approach to attain the resolution of few femtoseconds and attoseconds in UEM.In ultrafast electron microscopy (UEM) (1–3), electrons generated by photoemission at the cathode of a transmission electron microscope are accelerated down the microscope column to probe the dynamic evolution of a specimen initiated by an ultrafast light pulse. The use of femtosecond lasers to generate the electron probe and excite the specimen has made it possible to achieve temporal resolution on the femtosecond time scale, as determined by the cross-correlation of the optical and electron pulses. One important method in the UEM repertoire is photon-induced near-field electron microscopy (PINEM) (4, 5), in which the dynamic response detected by the electron probe is the pump-induced charge density redistribution in nanoscale specimens (6).Photon–electron coupling is the basic building block of PINEM, which takes place in the presence of nanostructures when the energy-momentum conservation condition is satisfied (4, 5). This coupling leads to inelastic gain/loss of photon quanta by electrons in the electron packet, which can be resolved in the electron energy spectrum (5, 7, 8). This spectrum consists of discrete peaks, spectrally separated by multiples of the photon energy (n?ω), on the higher and lower energy sides of the zero loss peak (ZLP) (4) (Fig. 1). The development of PINEM enables the visualization of the spatiotemporal dielectric response of nanostructures (9), visualization of plasmonic fields (4, 5) and their spatial interferences (10), imaging of low atomic number nanoscale materials (11), characterization of ultrashort electron packets (12, 13), and imaging of different biological structures (14).Open in a separate windowFig. 1.Concept of photon gating in 4D electron microscopy. (A) The microscope column with one electron (dark blue) and two optical (red) pulses focused onto the specimen. The wavefunctions of the three pulses are schematically shown at the top. One optical pulse is coincident with the electron pulse at the specimen to generate a PINEM signal. The resulting light blue PINEM pulse is sliced out from other electrons for detection as an energy spectrum, an image, or a diffraction signal (see the text). The second optical pulse initiates the dynamics to be probed. (B) Electron energy spectrum generated at the specimen plane when optical and electron pulses arrive simultaneously. The gain energy range is shaded light blue. (C) Illustration for the temporal pulse sequence, two optical and one electron pulse for ultrafast time-resolved PINEM measurements.As shown by Park et al. (5), the PINEM intensity (I_PINEM) is given by the square modulus of the field integral F˜0 (i.e., IPINEM∝|F˜0|2), in the weak interaction limit. The near field of a nanoparticle leads to the scattering of the electron packet, which can be treated rigorously using the Schrödinger equation/Mie scattering theory. It follows that PINEM images the object and displays its field characteristics depending on its shape, the polarization and wavelength of optical excitation, and the width of pulses used. For a spherical nanoparticle, the field integral at point (x, y) in the specimen plane is simplified to give (6)F˜0≈−iE˜0⁡cos⁡ϕχs 23 a3(Δk)2K[Δkb],[1]where E˜0 is the electric field amplitude of the incident light, ? the light polarization angle, a the particle radius, b=x2+y2 the impact parameter, K the modified Bessel function of the second type, Δk the momentum change of the electron, and χ_s = 3(ε ? 1)/(ε + 2), where χ_s is the material susceptibility and ε the dielectric function.In previous studies of the parameters in Eq. 1, only E˜0 was time dependent. The PINEM intensity, at a given point in space, was a function only of the time delay between the optical and electron pulses, providing, for the pulse lengths currently used, a cross-correlation profile when this delay was scanned across the time of temporal coincidence, or t = 0 (4, 5, 9, 13). Hitherto, PINEM has not been used to study the ultrafast dynamics of matter. Here, we follow the strategy of using the PINEM gain electrons generated by a first optical pulse, whose delay relative to the electron pulse is maintained at t = 0, to probe dynamics initiated by introduction of a second optical pulse on the specimen, as proposed theoretically in ref. 15. By this approach, we were able to optically gate the electron pulse (i.e., create an electron pulse that only lasts for the duration of the optical pulse) and achieve significant enhancement of the temporal resolution (see the second paragraph below).The concept of the experiment is illustrated by Fig. 1A, in which the electron pulse in blue and one optical pulse (P₁) in red are shown arriving at the specimen plane simultaneously. Interaction between photon and electron in the presence of the specimen “slices out” the light blue pulse of gain electrons, which are separated from all other electrons by energy dispersion or filtering to be detected according to microscope settings in spectroscopy, imaging, or diffraction mode, as illustrated schematically at the bottom of the column. Note, it is possible to obtain PINEM diffraction, but this is not the subject of this paper. A second, or pump, optical pulse (P₂) is shown below the specimen, having already triggered the dynamics of interest. A series of time axes is plotted in Fig. 1C showing examples of characteristic sequences of pulse arrival times at the specimen plane during the experiment, with the pump arrival defining the zero of time.A striking feature of this technique that was alluded to above is the potential for high temporal resolution, unlimited by the electron pulse duration, because the optical pulse acts as a temporal gate for a longer electron pulse. In the weak interaction limit, the duration of the pulse of PINEM electrons emulates that of the optical pulse that created it (15), as clearly shown in Fig. 1A. When these photon-gated electrons are used to probe dynamics triggered by a second ultrafast optical pulse, the time resolution is determined by the cross-correlation of the two optical pulses. This paves the way for the realization of attosecond electron microscopy, as done in all-optical spectroscopy (16) but with the spatial resolution being that of atomic motions. As suggested in Fig. 1A, we envisage the use of the photon-gated electron pulses, in imaging or in diffraction mode, for the study of a variety of optically initiated material processes, either of the nanostructure or of its surrounding media.The PINEM signal can be directly monitored to detect changes in any of the specimen optical or physical properties expressed in Eq. 1. Here, we demonstrate the use of the time-resolved PINEM technique where it is shown that the photoinduced dielectric response of VO₂—which is strongly related to the lattice symmetry (17)—manifests itself in a change in PINEM intensity. We relate the changes in optical properties of the polycrystalline VO₂ nanoparticles to the phase transition dynamics from initial (monoclinic) insulator phase to (tetragonal) metal phase, the subject of numerous previous studies.Vanadium dioxide has been discussed as an active metamaterial (18) and one of the best candidates for solid-state ultrafast optical switches in photonics applications (19, 20) due to its unique structural photoinduced phase transition behavior (21). This phase transition has been examined by investigating the change in the heat capacity through thermal excitation (22, 23), whereas its ultrafast dynamics has been studied by optical spectroscopy (24, 25), THz spectroscopy (26, 27), X-ray diffraction (28, 29), ultrafast electron crystallography (30), and electron microscopy (31).