Femtosecond optical doping reveals new macroscopic quantum phases
Characterizing and understanding the emergence of multiple macroscopically ordered electronic phases through subtle tuning of temperature, pressure, and chemical doping has been a long-standing central issue for complex materials research. In a recent Science Advances article, we describe a high-fidelity approache that combine femtosecond electron crystallography and controlled femtosecond photo-doping to reveal the ultrafast structural dynamics of complex quantum phases in response to photonic tuning of interaction energy as an alternative route to chemical doping or applying pressure, but without uncontrolled effects due to strain or disorder. We report a comprehensive study of optical doping–induced emergence of stable phases and metastable hidden phases in charge-density wave tantalum disulfide materials. The electronic phase transitions are triggered by mid-infrared pulses, and a temperature–optical density phase diagram is constructed and substantiated with the dynamics of metastable states, highlighting the cooperation and competition through which the macroscopic quantum orders emerge. These results elucidate key pathways of femtosecond electronic switching phenomena and provide an avenue to comprehensively investigate optical doping–induced transition states. Furthermore, the speed and degree of photodoping substantially exceed that achievable with conventional methods, creating opportunities to generate new phases. The robust nonthermal switching at mesoscales and at ultrafast time scales may oneday provide a platform for designing high-speed low-energy consumption nanophotonics and electronics devices.
This work is a result of collaboration between Ruan, Duxbury, and Mahanti groups at MSU, and Kanatzidis group at Northwestern University.
Resolution limit of ultrafast electron imaging and spectroscopy
The UEM collaboration (groups of Berz, Duxbury, and 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 efficiency 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.
Tango dance of electrons and ions in the phase transition of two-dimensional charge-density waves
Imaging defects growth in silver nanoparticles excited through surface plasmons
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.
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.
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 ~ 5Å) to (1-1/e) of the static value (at negative time).