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The effects of the waveguide layer on the band structure of Rayleigh waves are studied in this work based on a one-dimensional acoustic superlattice lithium niobate substrate coated with a waveguide layer. The present phononic structure is formed by the periodic domain-inverted single crystal that is the Z-cut lithium niobate substrate with a waveguide layer on the upper surface. The plane wave expansion method (PWE) is adopted to determine the band gap behavior of the phononic structure and validated by the finite element method (FEM). The FEM is also used to investigate the transmission of Rayleigh waves in the phononic structure with the interdigital transducers by means of the commercial package COMSOL. The results show that, although there is a homogeneous waveguide layer on the surface, the band gap of Rayleigh waves still exist. It is also found that increasing the thickness of the waveguide layer, the band width narrows and the band structure shifts to lower frequency. The present approach can be taken as an efficient tool in designing of phononic structures with waveguide layer. [ABSTRACT FROM AUTHOR]

Proton acceleration from 4 im thick aluminum foils irradiated by 30-TW Ti:sapphire laser pulses is investigated using an angle-resolved proton energy spectrometer. We find that a modulated spectral peak at ~0.82 MeV is presented at 2.5° off the target normal direction. The divergence angle of the modulated zone is 3.8°. Two-dimensional particle-in-cell simulations reveal that self-generated toroidal magnetic field at the rear surface of the target foil is responsible for the modulated spectral feature. The field deflects the low energy protons, resulting in the modulated energy spectrum with certain peaks. [ABSTRACT FROM AUTHOR]

A simple method to enhance ion generation with femtosecond ultraintense lasers is demonstrated experimentally by defocusing laser beams on target surface. When the laser is optimally defocused, we find that the population of medium and low energy protons from ultra-thin foils is increased significantly while the proton cutoff energy is almost unchanged. In this way, the total proton yield can be enhanced by more than 1 order, even though the peak laser intensity drops. The depression of the amplified spontaneous emission (ASE) effect and the population increase of moderate-energy electrons are believed to be the main reasons for the effective enhancement. [ABSTRACT FROM AUTHOR]

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 multipeak characterization is found to be independent of laser 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. [ABSTRACT FROM AUTHOR]

The characteristics of proton beam generated in the interaction of an ultrashort laser pulse with a large prepulse with solid foils are experimentally investigated. It is found that the proton beam emitted from the rear surface is not well collimated, and a “ring-like” structure with some “burst-like” angular modulation is presented in the spatial distribution. The divergence of the proton beam reduces significantly when the laser intensity is decreased. The “burst-like” modulation gradually fades out for the thicker target. It is believed that the large divergence angle and the modulated ring structure are caused by the shock wave induced by the large laser prepulse. A one-dimensional hydrodynamic code, MED103, is used to simulate the behavior of the shock wave produced by the prepulse. The simulation indicates that the rear surface of the foil target is significantly modified by the shock wave, consequently resulting in the experimental observations. [ABSTRACT FROM AUTHOR]

We studied the transport of an intense electron beam produced by high intensity laser pulses through metals and insulators. Targets were irradiated at two different intensities, 1017 W/cm2 and 1019 W/cm2, at the laser facility Xtreme Light XL-III in Beijing, a Ti:Sa laser source emitting 40 fs pulses at 800 nm. The main diagnostic was Cu-Kα fluorescence imaging. Images of Kα spots have been collected for those two laser intensities, for different target thickness, and for different materials. Experimental results are analyzed taking into account both collisional and collective effects as well as refluxing at the edge of the target. The target temperature is evaluated to be Tc ∼ 6 eV for intensity I = 1017 W/cm2 (for all the tested materials: plastic, aluminium, and copper), and Tc ∼ 60 eV in aluminium and 120 eV in titanium for intensity I = 1019 W/cm2. [ABSTRACT FROM AUTHOR]