H. Xu

Improved electrical properties of Ge metal-oxide-semiconductor capacitors with high-k HfO2 gate dielectric by using La2O3 interlayer sputtered with/without N2 ambient

The electrical properties of n-Ge metal-oxide-semiconductor (MOS) capacitors with HfO2/LaON or HfO2/La2O3 stacked gate dielectric (LaON or La2O3 as interlayer) are investigated. It is found that better electrical performances, including lower interface-state density, smaller gate leakage current, smaller capacitance equivalent thickness, larger k value, and negligible C-V frequency dispersion, can be achieved for the MOS device with LaON interlayer. The involved mechanism lies in that the LaON interlayer can effectively block the interdiffusions of Ge, O, and Hf, thus suppressing the growth of unstable GeOx interlayer and improving the dielectric/Ge interface quality.

Propagation of attosecond electron bunches along the cone-and-channel target

Generation and propagation of attosecond electron bunches along a cone-and-channel target are investigated by particle-in-cell simulation. The target electrons are pulled out by the oscillating electric field of an intense laser pulse irradiating a cone target and accelerated forward along the cone walls. It is shown that the energetic electrons can be further guided and confined by a channel attached to the cone tip. The propagation of these electrons along the channel induces a strong quasistatic magnetic field as well as a sheathelectric field since a part of the energetic electrons expands into the surrounding vacuum. The electromagnetic field in turn confines the surface currents. With the cone-and-channel target the energetic electrons can be much better collimated and propagate much farther than that from the classical cone target.

Generation of high-energy-density ion bunches by ultraintense laser-cone-target interaction

A scheme in which carbon ion bunches are accelerated to a high energy and density by a laser pulse (∼1021u2009W/cm2) irradiating cone targets is proposed and investigated using particle-in-cell simulations. The laser pulse is focused by the cone and drives forward an ultrathin foil located at the cones tip. In the course of the work, best results were obtained employing target configurations combining a low-Z cone with a multispecies foil transversely shaped to match the laser intensity profile.

Collimated proton beam generation from ultraintense laser-irradiated hole target

Collimated proton beams from laser interaction with a slab having a hole on its backside are investigated using particle-in-cell simulation. The hot target electrons driven by the laser expand rapidly into the hole. However, at the holes corners the electrons are strongly compressed and an intense electron jet is emitted from each corner, tightly followed by the ions. The plasma jets focus and collimate along the axis of the hole and can propagate without divergence within the hole. The effect of the hole diameter on the collimated proton beam is considered.

Propagation of intense laser pulses in strongly magnetized plasmas

Propagation of intense circularly polarized laser pulses in strongly magnetized inhomogeneous plasmas is investigated. It is shown that a left-hand circularly polarized laser pulse propagating up the density gradient of the plasma along the magnetic field is reflected at the left-cutoff density. However, a right-hand circularly polarized laser can penetrate up the density gradient deep into the plasma without cutoff or resonance and turbulently heat the electrons trapped in its wake. Results from particle-in-cell simulations are in good agreement with that from the theory.

Ion cascade acceleration from the interaction of a relativistic femtosecond laser pulse with a narrow thin target

Particle-in-cell simulations are performed to study the acceleration of ions due to the interaction of a relativistic femtosecond laser pulse with a narrow thin target. The numerical results show that ions can be accelerated in a cascade by two electrostatic fields if the width of the target is smaller than the laser beam waist. The first field is formed in front of the target by the central part of the laser beam, which pushes the electron layer inward. The major part of the abaxial laser energy propagates along the edges to the rear side of the target and pulls out some hot electrons from the edges of the target, which form another electrostatic field at the rear side of the target. The ions from the front surface are accelerated stepwise by these two electrostatic fields to high energies at the rear side of the target. The simulations show that the largest ion energy gain for a narrow target is about four times higher than in the case of a wide target.

Efficient acceleration of a small dense plasma pellet by consecutive action of multiple short intense laser pulses

The acceleration of a micrometer-sized plasma pellet at 100 critical densities (10(23) cm(-3)) by consecutive application of ultra-short ultra-intense laser pulses is studied using two-dimensional particle-in-cell simulation. It is shown that due to the repeated actions of the laser ponderomotive force, a small dense plasma pellet can be efficiently accelerated, with a considerable fraction of the plasma ions accelerated to high speeds. The proposed scheme can provide a high-density flux of energetic ions, which should be valuable in many practical applications.

Generation of quasi-monoenergetic carbon ions accelerated parallel to the plane of a sandwich target

A new ion acceleration scheme, namely, target parallel Coulomb acceleration, is proposed in which a carbon plate sandwiched between gold layers is irradiated with intense linearly polarized laser pulses. The high electrostatic field generated by the gold ions efficiently accelerates the embedded carbon ions parallel to the plane of the target. The ion beam is found to be collimated by the concave-shaped Coulomb potential. As a result, a quasi-monoenergetic and collimated C6+-ion beam with an energy exceeding 10u2009MeV/nucleon is produced at a laser intensity of 5u2009×u20091019u2009W/cm2.

Generation of hemispherical fast electron waves in the presence of preplasma in ultraintense laser-matter interaction

AbstractHemispherical electron plasma waves generated from ultraintense laser interacting with a solid target having a subcriticalpreplasma is studied using particle-in-cell simulation. As the laser pulse propagates inside the preplasma, it becomes self-focused due to the response of the plasma electrons to the ponderomotive force. The electrons are mainly heated viabetatron resonance absorption and their thermal energy can become higher than the ponderomotive energy. The hotelectrons easily penetrate through the thin solid target and appear behind it as periodic hemispherical shell-like layersseparated by the laser wavelength.Keywords: Betatron resonance; Electron plasma waves; Ponderomotive force; Preplasma INTRODUCTIONFast electrons generation in ultraintense laser-solid interactionhave been investigated extensively both theoretically andexperimentally (Sentoku et al., 2006;Kempet al., 2009;Nilson et al., 2011) because of their application in laser-plasma accelerators (Borghesi et al., 2006;Yuet al., 2009;Yang et al., 2010), fast ignition (FI) schemes of inertial con-finement fusion (Tabak et al., 1994; Deutsch & Didelez,2011), bright X-ray sources (Pfeifer et al., 2006), etc. Theenergy spectrum, spatial distribution, and divergence angleof the energetic electrons can be significantly affected by theubiquitous low-density blow-off plasma (the preplasma) cre-ated by the inherent laser prepulse and/or spontaneous emis-sions from the lasing system (Nuteret al., 2008;Davieset al.,2009; MacPhee et al., 2010;Caiet al., 2010;Linet al., 2012;Sakagami et al., 2012). Thus, the effect of the preplasmashould be taken into account in studies of fast electron gener-ation from intense laser-solid target interaction.Interaction of an intense laser pulse with a solid targethaving a large preplasma can involve nonlinear processessuch as self-focusing, filamentation, hole-boring, etc. thatcan play important roles in the generation and propagationof fast electrons (Pukhov & Meyer-ter-Vehn et al., 1996;Friouetal.,2012).Fastelectronsarealsogeneratedinthepre-plasma, and it has been found that they usually have a two-temperature Maxwellian distribution. The temperature of thehigh-energy component is much higher than the ponderomo-tiveenergy(Wilksetal.,1992)anditscaleswiththelengthL

Enhancement of electron injection in laser wakefield acceleration using auxiliary interfering pulses

A relatively simple interfering-pulses-assisted laser wakefield acceleration (IPA-LWFA) scheme is proposed for enhancing the charge of the LWFA electron bunch. Prior to the short intense pump pulse, two long low-intensity auxiliary laser pulses first interact in the plasma and excite a slow electron plasma wave at the beat frequency. The weak but finite-amplitude plasma wave energizes the affected electrons and acts like a slow-moving grating. Particle-in-cell simulations show that electron trapping in the wakefield of the pump laser pulse, which arrives at a later time, can be significantly enhanced. The charge of the IPA-LWFA electron bunch depends mainly on the intensity of the auxiliary pulses and the time delay of the pump laser.