Laboratory for Ultrafast Nanoscale Imaging and Spectroscopy

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Resolution limit of ultrafast electron imaging and spectroscopy

The UEM collaboration (groups of Berz, Duxburym, Ruan) zoomed in on the stochastic space-charge effects in the generation of high-brightness electron beams. Using a multilevel fast multipole method, coupled with the shadow imaging of femtosecond photoelectron pulses for validation, we quantitatively elucidate the photocathode, space charge and virtual cathode physics, which fundamentally limit the spatiotemporal and spectroscopic resolution and throughput of ultrafast electron microscope (UEM) systems[J. Appl. Phys. 111, 044316 (2012)]. A simple microscopic description to capture the nonlinear beam dynamics based on a two-fluid picture and elucidate an unexpected dominant role of image potential pinning in accelerating the emittance growth process. These calculations set theoretical limits on the performance of UEM systems and provide useful guides for photocathode design for high-brightness electron beam systems. electron imaging and spectroscopy [Appl. Phys. Lett. 103, 253115 (2013)]. Further improvement in time resolution may come through improving the source brightness, through for example; laser pulse shaping or tuning of the driving photons to threshold energy from a high eciency cathode; and may make ps macromolecular imaging feasible without the need to outrun the radiation damage effects, as required in the case of imaging using X-rays. Further quantitative details addresses the source-limited performance of ultrafast electron imaging and spectroscopy systems under various acceleration fields, and photo-electron pulse aspect ratios, namely the so-called pancake and cigar scenarios. The simulation results point to the interesting finding that a ps cigar beam indeed excels in coherence length ( up to 20 nm with 10^4 electrons and ~1nm with 10^8 electrons, however the high acceleration field also increases its energy spread, whereas the energy spread of cigar shaped beams is quite unaffected[J. Appl. Phys. 116, 174302 (2014)].

Overall, the fs pancake beam has significantly smaller longitudinal emittance up to the virtual cathode limit, and is more suited for high combined temporal-spectral resolution UEDs, whereas the cigar beam may be suited for ps imaging, close to the single-shot limit. These projections show promising parameter space for conducting ultrafast electron micordiffraction at close to the single-shot level, which is supported by the latest experimental characterization of such a system [Proc. of SPIE, vol. 9198 (2014)].



Decoupling of the electronic and structural phase transitions in VO2

Using optical, TEM and ultrafast electron diffraction experiments we find that single crystal VO2 microbeams gently placed on insulating substrates or metal grids exhibit different behaviors, with structural and metal-insulator transitions occurring at the same temperature for insulating substrates, while for metal substrates a new monoclinic metal phase lies between the insulating monoclinic phase and the metallic rutile phase, which we show may be stabilized by charge doping. Informed by our ultrafast single-beam measurements, the metal-insulator transition in the undoped case can be classified as Mott-driven, Peierls-limited, where the imposed cooperativity is mainly governed by the large spectral weight and the high energy scale inherent to the Peierls transition and that such stabilizing features are subjective to the strong Coulomb interaction predominant only in the insulating state without doping. We also uncover the unique delocalized nature of the interface-mediated charge doping effect that can be rationalized with an active spectral weight transfer from the unfilled band to the Peierls band in the monoclinic structure, which enhances the Peierls interaction supported by its selective effect on the structure phase transition temperature. While the results presented here are very specific, they are generally compatible with the Peierls-Mott scenario proposed by most recent theories, and amazingly reconcilable with an array of different experiments supportive of either Peierls or Mott scenarios.

(Physical Review Letter 109, 166406 (2012) ; Physical Review B 87, 235124 (2013))

Tango dance of electrons and ions in the phase transition of two-dimensional charge-density waves

The coexistence of phases through complex couplings between different degrees of freedom (spin, charge, lattice, orbital) is a hallmark of complex materials with emerging properties such as charge density wave (CDW), superconductivity, and colossal magnetoresistivity.  In particular, CDW is predicted to form in highly anisotropic systems where long- range charge ordering (CO) and periodic lattice distortion (PLD) are strongly coupled, driven by instabilities originated in either the electronic  (Mott) or phononic (Peierls) subsystems.   Recent femtosecond spectroscopy-based pump-probe techniques offered real-time characterizations of nonequilibrium dynamics in the different regimes of electron-phonon coupling, but their properties are elucidated in the temporal responses representative of mostly the evolution of CO state.  Using ultrafast electron crystallography, we provided a complementary view from ultrafast electron crystallography investigation of CeTe3, exploiting its elegant 2D weakly correlated feature, serving as an ideal system to understand the symmetry breaking phase transition in strongly coupled electron-phonon system.  We specifically examined the fluctuational dynamics using the momentum-dependent dynamical diffraction features to address the open questions.  Following a strong perturbation of electron density, we observed clear evidences of nonconcerted suppressions of CDW, expressed in the ionic frame as an unexpected 2D lattice hardening and a short-lived topologically fragmented state in the long-range PLD network resulted from CO suppression, and highly nonuniform fluctuational amplitudes that are signatures of strong anisotropic electron-phonon coupling driving PLD, respectively. The nature of the transition was explained using a three-temperature model, in which the first part of the dynamics can be reconciled with the optical observations of electronic suppression; while the second part manifested in the nonadiabatic ionic responses is highly system-dependent, and informative of the electron-phonon mechanism for CDW formation.   

 (Physical Review B 86, 075145 (2012); 87, 235124 (2013))

Imaging defects growth in silver nanoparticles excited through surface plasmons

The ability to image defect growth processes is central to the understanding of the electronically induced structural phase transitions in solids. The recent emergence of using transient optical doping method to actuate atomic motions by modifying electronic structures of materials has opened up new vistas for exploring novel phases of materials. Whereas optical and photoemission studies have provided significant insights into the initial electronic processes that are strongly coupled to lattice degrees of freedom, the mechanism bridging the femtosecond (fs) optical seeding to the ps-to-ns macroscopic structural changes remains a central topic to be elucidated. Recent developments in ultrafast diffraction techniques have enabled direct probing of atomic dynamics and helped accentuate the important role of electronic excitation in initiating the coherent motions, bond softening  and structural transformations. By employing ultrafast electron nanoscale crystallography , we demonstrate a direct structural study of spatially inhomogeneous processes in Ag nanocrystals (NCs) induced via surface plasmon resonance (SPR) excitation. Contrary to an impulsive process leading to fragmentation, we find that the dominant dynamical feature in the prefragmentation stage is a defect-mediated instability growth, creating sub-nanocrystalline domains with hot surface and relatively cold core. Electronic effects are proposed to account for the incipient creation and subsequent growth of lattice inhomogeneities directly responsible for fragmentation, which are corroborated by the evidences of correlated charge localization and defect percolation on the picosecond timescale following photoexcitation.

(Physical Review Letter 104, 123401 (2010) )

Imaging ultrafast photoelectron dynamics

Understanding the mechanism of vacuum space charge emission and surface charge accumulation is crucial to the development of pulsed laser driven electron technologies, such as time-resolved photoemission, ultrafast electron diffraction and microscopy. Recently, a novel method to directly image the spatiotemporal evolution of the photoemitted electron packet generated over a femtosecond laser excited surface has been developed in UEC group . This method possesses sufficient sensitivity to image electron density as small as 10^10 e/cm^3 and permits quantitative measurement of the instantaneous electron density distribution and its collected translational and expansion speed near the surface. These initial kinetic energy distributions are representative of the hot electron profile within the materials responsible for the photoemission. This technique thus can provide information equivalent to those obtained from time-resolved photoemission spectroscopy, but can be operated at a strongly driven condition with a direct resolution to distinguish the presence of space charge effects. In the study of photoemission from a graphite surface, we found that the dynamical profile of photoelectrons reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and reduction of electron expansion (cooling). The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud. 

(Applied Physics Letter 95, 181108(2009))

August 2009: MSU Strategic Partner Group formed to develop high-brightness femtosecond electron microscope

See PA announcement.

Quantitative modeling of excited state structural dynamics to visualize surface melting of Au nanoparticles

Our group developed a progressive refinement method to treat  excited state ultrafast diffraction patterns by adopting a reverse Monte Carlo approach to refine the excited state structures . The tracking of particle dynamics is constrained by using the inherent dynamical correlations between structural functions obtained from two neighboring time frames. The ability of 'making molecular movies' is tantalizingly close from analyzing the photoinduced structural dynamics for the strong scattering Au and Ag nanoparticles, based on this method.

(Microscopy and Microanalysis Vol. 15, 323-337; Special issue on ultrafast electron microscopy and ultrafast sciences)

Shaping up graphite with light

Can we make diamond from graphite without resorting to extreme heat and pressure as diamond was made in the Earth in geological time? The UEC team raise such a possibility by shining a short burst of intense femtosecond laser pulse on graphite and watching using ultrafast diffraction the movements of some loosely separated carbon atoms in graphene layers undergoing rehybridization, forming more tightly bound diamond-like bonds. Whereas this intriguing intermediate structure is short-lived, approximately for 30 picoseconds, this experiment shows the possibility of a structural transformation purely induced by light in this very versatile class of material. Assisted by density functional theory calculation performed by Tomanek's group at MSU, the cause for such a transformation is attributed to the electronic structure changes and the Coulomb field buildup following the photoexcitation.  We are currently exploring different strategies in optically converting graphite into more permanent form of diamonds on graphite surface and ultrathin slab. 

(PR focus article)

(Phys. Rev. Lett. 101, 077401 (2008))

Measuring photovoltage at ultrafast speed

We detailed a method to directly measure the photovoltage at nanointerfaces using femtosecond electron pulses.  The transient surface voltage is determined by observing Coulomb refraction changes induced by interfacial charging.  Using coherent diffraction signal, we can trace the origin of the refractive beam trajectory in material, thus obtain the nanoscale scale sensitivity of the charge transfer process.   We are currently extending this approach to investigate molecular electronic processes. 

(Phys. Rev. B 77, 245329 (2008)).

Ultrafast Meets Ultrasmall

Our first paper outlining ultrafast electron nanocrystallography and the studies of photomelting of size-selected Au nanoparticles is now published online (Nano Letters: Author ASAP). Download PDF: with subscription, without subscription.

We report in the Nano Letters the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with full-profile atomic density maps determined with sub-picosecond and picometer accuracies. In the ultrasmall environment, the melting process induced by photons is ultrafast and far-from-equilibrium, transforming nanocrystals into shelled nanoliquids.  The energy of the photons is transferred to the atoms within just a few picoseonds. However, it takes tens of picoseond for the atoms to be released from the crystalline motif into a liquid shelled structure. This dynamical transformation is strongly influenced by a coherent shearing motion within fcc structure that is determined as the pathway leading to the photomelting. The nanoliquids is more icosahedral-like, and it reverts to the fcc crystalline state in 100 ps. The structural excitation is displacive which results in a coherent transformation with crystal/liquid coexistence and that differs from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle. The reversible and coherent transformation on the ultrafast time scale demonstrates the directed dynamics on the energy landscape of finite systems. This methodology is general and could be implemented to study a wide class of phenomena pertaining to nanoscaled materials.

Time-resolved melting dynamics of 2nm Au nanoparticles. (Left) Radial distribution function  (RDF) map constructed by stacking RDF of UEC patterns at a sequence of delays between -5-2300 ps at irradiation fluence F=31 mJ/cm2. Surface melting (enclosed by the dashed white line) is visible. (Right) RDF map for F=75 mJ/cm2. Full scale melting is observed. The liquid state (enclosed by dashed white line) is characterized by the drop of 2nd nearest density (at ~ 5) to (1-1/e) of the static value (at negative time).




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