Hui-Chun Wu

Self-Organizing GeV, Nanocoulomb, Collimated Proton Beam from Laser Foil Interaction at 7 X 1021 W/cm2

We report on a self-organizing, quasistable regime of laser proton acceleration, producing 1 GeV nanocoulomb proton bunches from laser foil interaction at an intensity of 7 x 10;{21} W/cm;{2}. The results are obtained from 2D particle-in-cell simulations, using a circular polarized laser pulse with Gaussian transverse profile, normally incident on a planar, 500 nm thick hydrogen foil. While foil plasma driven in the wings of the driving pulse is dispersed, a stable central clump with 1-2lambda diameter is forming on the axis. The stabilization is related to laser light having passed the transparent parts of the foil in the wing region and enfolding the central clump that is still opaque. Varying laser parameters, it is shown that the results are stable within certain margins and can be obtained both for protons and heavier ions such as He;{2+}.

Phase-sensitive terahertz emission from gas targets irradiated by few-cycle laser pulses

The effect of the carrier envelope phase (CEP) of few-cycle laser pulses on terahertz (THz) emission from gas targets is investigated by analysis and two-dimensional particle-in-cell simulations. For linearly polarized (LP) light, the THz amplitude depends on the CEP phase sinusoidally. For circularly polarized (CP) light, the THz amplitude is independent of the phase, but its polarization plane rotates with the phase. By measuring the THz amplitude or polarization direction, one can determine the CEP of LP or CP laser pulses, respectively. We find that when the ionization degree of atoms is lower than 10%, the phase dependence of the THz radiation is insensitive to intensity and duration of the laser pulse, which is preferable for the phase determination.

Strong terahertz pulse generation by chirped laser pulses in tenuous gases

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scales linearly with the laser amplitude, which, however, holds only when the latter is at a moderate level. To overcome this limitation, we propose a scheme using chirped laser pulses irradiating on tenuous gas or plasma targets, which can generate THz pulses with amplitude 10-100 times larger than that from the well-known two-color laser scheme, enabling one to obtain THz field up to 10MV/cm with incident laser at approximately 10(16)W/cm(2).

Dense quasi-monoenergetic attosecond electron bunches from laser interaction with wire and slice targets

A scheme is proposed to produce high-quality quasi-monoenergetic attosecond electron bunches based on laser ponderomotive-force acceleration along the surface of wire or slice targets. Two- and three-dimensional particle-in-cell simulations demonstrate that the electron energy depends weakly on the target density. A simple analytical model shows that the electron energy scales linearly with the laser field amplitude, in good agreement with the simulation results. Electron bunches produced by this scheme are suitable for applications such as coherent x-ray radiation, radiography, and injectors in accelerators, etc.

Uniform Laser-Driven Relativistic Electron Layer for Coherent Thomson Scattering

A novel scheme is proposed to generate uniform relativistic electron layers for coherent Thomson backscattering. A few-cycle laser pulse is used to produce the electron layer from an ultra-thin solid foil. The key element of the new scheme is an additional foil that reflects the drive laser pulse, but lets the electrons pass almost unperturbed. It is shown by analytic theory and by 2D-PIC simulation that the electrons, after interacting with both drive and reflected laser pulse, form a very uniform flyer freely cruising with high relativistic γ-factor exactly in drive laser direction (no transverse momentum). It backscatters probe light with a full Doppler shift factor of 4γ. The reflectivity and its decay due to layer expansion is discussed.

Giant half-cycle attosecond pulses

Scientists show that irradiating a double-foil target with intense few-cycle laser pulses can produce single half-cycle 50 as pulses with peak electric fields as high as 1013 V m−1 and pulse energies of up to 0.1 mJ. The findings may stimulate new types of attosecond pump–probe experiments.

Coherent Thomson backscattering from laser-driven relativistic ultra-thin electron layers

AbstractThe generation of laser-driven dense relativistic electron layers from ultra-thin foils and their use for coherent Thomson backscattering is discussed, applying analytic theory and one-dimensional particle-in-cell simulation. The blow-out regime is explored in which all foil electrons are separated from ions by direct laser action. The electrons follow the light wave close to its leading front. Single electron solutions are applied to initial acceleration, phase switching, and second-stage boosting. Coherently reflected light shows Doppler-shifted spectra, chirped over several octaves. The Doppler shift is found ∝ γx2=1/(1-βx2), where βx is the electron velocity component in normal direction of the electron layer which is also the direction of the driving laser pulse. Due to transverse electron momentum py, the Doppler shift by 4γx2=4γ2/(1+(py/mc)2)≈2γ is significantly smaller than full shift of 4γ2. Methods to turn py→0 and to recover the full Doppler shift are proposed and verified by 1D-PIC simulation. These methods open new ways to design intense single attosecond pulses.

Chirped pulse compression in nonuniform plasma Bragg gratings

A nonuniform plasma Bragg grating with a monotonically increasing density-modulation profile can be naturally produced by two Gaussian laser pulses counterpropagating through a homogeneous plasma slab. Such a plasma grating exhibits a nonuniform photonic band gap with a monotonically increasing width. It can be used to compress a positively or negatively chirped pulse. Particle-in-cell simulations show that the compressed pulse has nearly no energy loss and the compression efficiency can exceed 90%.

Manipulating ultrashort intense laser pulses by plasma Bragg gratings

Propagation of ultrashort intense laser pulses in a plasma Bragg grating induced by two intersecting laser pulses is studied. Such a plasma grating exhibits an ultrawide photonic band gap, near which the strong dispersion appears. It is found that the grating dispersion dominates the dispersion of the background plasma by several orders of magnitude. Particle-in-cell (PIC) simulations show the light speed reduction, pulse stretching, chirped-pulse compression, and fast compression of Bragg grating solitons in the plasma grating. The nonlinear coupled-mode theory agrees well with the PIC results. Since the plasma grating can support an ultrahigh damage threshold, it can be a novel photonic device to manipulate extremely intense femtosecond laser pulses.

The reflectivity of relativistic ultra-thin electron layers

AbstractThe coherent reflectivity of a dense, relativistic, ultra-thin electron layer is derived analytically for an obliquely incident probe beam. Results are obtained by two-fold Lorentz transformation. For the analytical treatment, a plane uniform electron layer is considered. All electrons move with uniform velocity under an angle to the normal direction of the plane; such electron motion corresponds to laser acceleration by direct action of the laser fields, as it is described in a companion paper [Eur. Phys. J. D 55, 433 (2009)]. Electron density is chosen high enough to ensure that many electrons reside in a volume λR3, where λR is the wavelength of the reflected light in the rest frame of the layer. Under these conditions, the probe light is back-scattered coherently and is directed close to the layer normal rather than the direction of electron velocity. An important consequence is that the Doppler shift is governed by γx=(1-(Vx/c)2)-1/2 derived from the electron velocity component Vx in normal direction rather than the full γ-factor of the layer electrons.