Zhongyu Zheng

Imaging the homogeneous nucleation during the melting of superheated colloidal crystals

Homogeneous Melting The nucleation and melting of crystals are primarily driven by surfaces and defects, which can lower the thermodynamic barrier to a phase transition. A harder problem to study is when the transition occurs uniformly. Wang et al. (p. 87; see the Perspective by Weeks) imaged the homogeneous melting of superheated colloidal crystals using a laser to initiate the melting at the interior of the crystal. The authors were then able to track nucleation precursors and nucleus evolution and to find where defects and instabilities limited the homogeneous melting process. Uniform colloidal crystals are used to study the effects of superheating on homogeneous melting. The nucleation process is crucial to many phase transitions, but its kinetics are difficult to predict and measure. We superheated and melted the interior of thermal-sensitive colloidal crystals and investigated by means of video microscopy the homogeneous melting at single-particle resolution. The observed nucleation precursor was local particle-exchange loops surrounded by particles with large displacement amplitudes rather than any defects. The critical size, incubation time, and shape and size evolutions of the nucleus were measured. They deviate from the classical nucleation theory under strong superheating, mainly because of the coalescence of nuclei. The superheat limit agrees with the measured Born and Lindemann instabilities.

Glass Transitions in Quasi-Two-Dimensional Suspensions of Colloidal Ellipsoids

We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as they approach the glass transitions. The clusters of translational and rotational fastest-moving ellipsoids formed mainly within pseudonematic domains and around the domain boundaries, respectively.

Self-diffusion in two-dimensional hard ellipsoid suspensions.

We studied the self-diffusion of colloidal ellipsoids in a monolayer near a flat wall by video microscopy. The image processing algorithm can track the positions and orientations of ellipsoids with subpixel resolution. The translational and rotational diffusions were measured in both the laboratory frame and the body frame along the long and short axes. The long-time and short-time diffusion coefficients of translational and rotational motions were measured as functions of the particle concentration. We observed the nondiffusive crossover region in the intermediate time regime due to the caging of neighboring particles. Both the beginning and the ending times of the intermediate regime exhibit power-law dependence on concentration. The long-time and short-time diffusion anisotropies change nonmonotonically with concentration and reach minima in the semidilute regime because the motions along long axes are caged at lower concentrations than the motions along short axes. The time derivatives of mean-square displacements change linearly with the inverse of time in the intermediate time regimes at various particle densities. This indicates that their relaxation functions decay as 1/t which provides new challenges in theory. The effects of coupling between rotational and translational Brownian motions were demonstrated and the two time scales corresponding to anisotropic particle shape and anisotropic neighboring environment were measured.

Effective fast electron acceleration along the target surface

The dependence of angular distributions of fast electrons generated in the interaction of p-polarized femtosecond laser pulses with foil targets on laser intensities is investigated. A novel fast electron beam along the front target surface is observed for high laser intensity. It is found that the electron acceleration along the target surface is more efficient than those in other directions.

Oval-like hollow intensity distribution of tightly focused femtosecond laser pulses in air.

The propagation of a tightly focused femtosecond laser pulse in air has been investigated. Unlike long-distance self-guided propagation of short laser pulses, a novel oval-like hollow distribution of the laser intensity is observed in the experiments and reproduced by the numerical simulations. The formation of the hollow structures can be explained by the interplay between ionization-induced refraction and Kerr self-focusing.

Homogeneous melting of 3D superheated colloidal crystals

We directly observed the kinetics of homogeneous melting in 3D superheated colloidal crystals at single-particle resolution by video microscopy. We found that the precursor of nuclei is in fact local particle exchanges surrounded by particles with large displacement amplitudes rather than any types of defects. The critical size, incubation time, shape evolution of nucleus were measured under different degrees of superheating.

Multi-peak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of < 60°. This multi-peak characteristic is found to be independent of the polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Effect of target shape on fast electron emission

Fast electrons emission from the interaction of femtosecond laser pulses with shaped solid targets has been studied. It is found that the angular distributions of the forward fast electrons are closely dependent upon the target shape. The important role played by the electrostatic fields built up near the target surfaces in the confinement of fast electrons is identified. Our two-dimensional particle-in-cell simulations can reproduce the main observations.

Relativistic Laser Acceleration Of Electrons Along Solid Surfaces

Recent experimental and theoretical studies on surface electron emission will be presented. A collimated fast electron beam was observed along the target surface irradiated by intense laser pulses up to 20TW when the laser is incident with large angles such as over 45 degree. Numerical simulations suggest that such an electron beam is formed due to the confinement of the surface quasistatic electric and magnetic fields. Meanwhile, an acceleration process similar to the inverse‐free‐electron‐laser is found to occur and is responsible for the generation of the most energetic electrons. A general formula for electron angular distributions accounting for the quasistatic electric and magnetic fields is given. In certain conditions, quasi‐monoenergetic electron beams are also produced. These results are of interest for potential applications of laser‐produced electron beams and helpful to the undersanding of the cone‐target physics in the fast ignition related experiments.