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We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (V02) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in V02 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales. [ABSTRACT FROM AUTHOR]

Time-resolved measurements of the ionization states of warm dense aluminum via K-shell absorption spectroscopy are demonstrated using betatron radiation generated from laser wakefield acceleration as a probe. The warm dense aluminum is generated by irradiating a free-standing nanofoil with a femtosecond optical laser pulse and was heated to an electron temperature of ~20-25 eV at a close-to-solid mass density. Absorption dips in the transmitted x-ray spectrum due to the Al4+ and Al5+ ions are clearly seen during the experiments. The measured absorption spectra are compared to simulations with various ionization potential depression models, including the commonly used Stewart-Pyatt model and an alternative modified Ecker-Kröll model. The observed absorption spectra are in approximate agreement with these models, though indicating a slightly higher state of ionization and closer agreement for simulations with the modified Ecker-Kröll model. [ABSTRACT FROM AUTHOR]

We report on an experimental observation of the streaking of betatron x rays in a curved laser wakefield accelerator. The streaking of the betatron x rays was realized by launching a laser pulse into a plasma with a transverse density gradient. By controlling the plasma density and the density gradient, we realized the steering of the laser driver, electron beam, and betatron x rays simultaneously. Moreover, we observed an energy-angle correlation of the streaked betatron x rays and utilized it in diagnosing the electron acceleration process in a single-shot mode. Our work could also find applications in advanced control of laser beam and particle propagation. More importantly, the angular streaked betatron x ray has an intrinsic spatiotemporal correlation, which makes it a promising tool for single-shot pump-probe applications.

We have analyzed the coupling of ultraintense lasers (at ∼2×10{19}  W/cm{2}) with solid foils of limited transverse extent (∼10  s of μm) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

Using ultrafast x-ray probing, we experimentally observed a progressive loss of ordering within solid-density aluminum as the temperature raises from 300 K to >10{4} K. The Al sample was isochorically heated by a short ( approximately ps), laser-accelerated proton beam and probed by a short broadband x-ray source around the Al K edge. The loss of short-range ordering is detected through the progressive smoothing of the time-resolved x-ray absorption near-edge spectroscopy (XANES) structure. The results are compared with two different theoretical models of warm dense matter and allow us to put an upper bound on the onset of ion lattice disorder within the heated solid-density medium of approximately 10 ps.

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.