Xue-Jin Liang

Spontaneous formation of ordered indium nanowire array on Si(001)

Growth of In on the Si(001)-2×n nanostructured surface is investigated by an in situ scanning tunneling microscope (STM). The deposited In atoms predominantly occupy the normal 2×1 dimer-row structure, and develop into a uniform array of In nanowires at a coverage of ∼0.2 ML. High-resolution STM images show that the In atoms form a stable local 2×2 reconstruction that removes surface Si dangling bonds states and saturates all In valency. Since the dimensions of the Si(001)-2×n vacancy line structure depend on impurity concentrations, this study demonstrates that the 2×n surface can be used for spontaneous fabrication of various metal nanowire arrays.

Effects of nanosecond-scale prepulse on generation of high-energy protons in target normal sheath acceleration

A pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes is used to improve the contrast of a laser of peak intensity ∼2 × 1019 W/cm2 to ∼1011 at 100 ps before the peak of the main pulse. A 7 MeV proton beam is observed when a 2.5 μm-thick Al foil is irradiated by this high-contrast laser. The maximum proton energy decreases to 2.9 MeV when a low-contrast (∼108) laser is used. Two-dimensional particle-in-cell simulations combined with MULTI simulations show that the maximum proton energy sensitively relies on the detecting direction. The ns-time-scale prepulse can bend a thin target before the main pulse arrives, which reduces maximum proton energy in the target normal sheath acceleration.

Large-scale proton radiography with micrometer spatial resolution using femtosecond petawatt laser system

An image of dragonfly with many details is obtained by the fundamental property of the high-energy proton source on a femtosecond petawatt laser system. Equal imaging of the dragonfly and high spatial resolution on the micrometer scale are simultaneously obtained. The head, wing, leg, tail, and even the internal tissue structures are clearly mapped in detail by the proton beam. Experiments show that image blurring caused by multiple Coulomb scattering can be reduced to a certain extent and the spatial resolution can be increased by attaching the dragonfly to the RCFs, which is consistent with theoretical assumptions.

Focal spot effects on the generation of proton beams during target normal sheath acceleration

Focal spot effects on the generation of proton beams are investigated by a high-intensity high-contrast laser irradiating on solid foil in target normal sheath acceleration experiments. Different spot size, transverse shape, and intensity of the laser are obtained by appropriately using deformable mirrors and parabolic mirrors. Experiments show that the maximum proton energy is mainly determined by the laser intensity if the focal spot size is not seriously changed. Compared with the previous experimental results, the optimum foil thickness d o is scaled by the laser intensity I as d o ~ I 0.33. The corresponding theoretical estimation is carried out as d o ~ I 0.25 for ultra-high intensity laser systems with similar contrast. MULTI and particle-in-cell simulations are used to interpret the experimental results.

Multi-stage proton acceleration controlled by double beam image technique

A double beam image (DBI) technique is coupled in the two-stage accelerating mechanism to simultaneously improve the spectra and maximum energy of the proton beam. A proton beam with a narrow-spectrum center at 5.4 MeV and a long tail up to 14.4 MeV is generated in the experiment. Experimental and simulation results show that spatial collineation, time synchronization, and real-time monitoring are needed for optimum two-stage proton acceleration and are realized by the DBI technique to a certain extent in our experiment. This DBI technique can be used to achieve optimum two-stage acceleration in a feasible manner and will allow precise manipulation of multistage acceleration to improve the energy and spectra of particle beams.

Crater-like structures induced by intense laser

Crater-like structures are experimentally studied with an ultrashort, ultraintense laser pulse with an intensity of 1.5 × 1018 W/cm2, irradiating borosilicate glass targets, which extends laser-induced craters to the region of relativistic intensities. The morphology of the crater-like structures is measured accurately using a three-dimensional laser scanning confocal microscope and a scanning electron microscope. The experimental results indicate that a circular bowl shape is formed with a depth-to-diameter ratio of about 1/5, which is similar to that of meteorite impact craters. A plasma fireball model is applied to analyze the experimental results. Studies show that catastrophic asteroid strikes may be investigated by irradiating foils with intense laser pulses.