Electron refluxing and K-shell line emission from Ti foils irradiated with sub-picosecond laser pulses at 527nm

We present data on emission of K-shell radiation from Ti foils irradiated with subpicosecond pulses of second harmonic radiation (527 nm) from the TARANIS laser system at intensities of up to 1018 Wcm-2. The data are used to demonstrate that a resonance absorption type mechanism is responsible for absorption of the laser light and to estimate fast electron temperatures of 30–60 keV that are in broad agreement with expectation from models of absorption for a steep density gradient. Data taken with resin-backed targets are used to demonstrate clear evidence of electron refluxing even at the modest fast electron temperatures inferred.

Fast electron penetration in laser-irradiated solids

In this letter we present data comparing the yield of Cu K-alpha radiation for foils of differing thickness irradiated with a Ti:Sapphire laser generating 40 fs pulses at 800 nm and incident at 45 degrees. At tight focus, the yields for all thicknesses are similar, whilst away from best focus there are clear differences. We discuss the origin of these similarities and differences in terms of the penetration of fast electrons into the foil and the possible importance of refluxing of fast electrons as they reach the non-irradiated side of the foil.

Monoenergetic Ion Beam Generation by Driving Ion Solitary Waves with Circularly Polarized Laser Light

Experimental data from the Trident Laser facility is presented showing quasimonoenergetic carbon ions from nm-scaled foil targets with an energy spread of as low as 15% at 35 MeV. These results and high resolution kinetic simulations show laser acceleration of quasimonoenergetic ion beams by the generation of ion solitons with circularly polarized laser pulses (500 fs, ¼ 1054 nm). The conversion ef?ciency into monoenergetic ions is increased by an order of magnitude compared with previous experimental results, representing an important step towards applications such as ion fast ignition.

Radiation-Pressure Acceleration of Ion Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration from ultrathin diamondlike carbon foils irradiated by ultrahigh contrast laser pulses of energy 0.7 J focused to peak intensities of 5×1019  W/cm2. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell simulations reveal that those C6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Rayleigh-Taylor Instability of an Ultrathin Foil Accelerated by the Radiation Pressure of an Intense Laser

We report experimental evidence for a Rayleigh-Taylor-like instability driven by radiation pressure of an ultraintense (1021W/cm2) laser pulse. The instability is witnessed by the highly modulated profile of the accelerated proton beam produced when the laser irradiates a 5 nm diamondlike carbon (90% C, 10% H) target. Clear anticorrelation between bubblelike modulations of the proton beam and transmitted laser profile further demonstrate the role of the radiation pressure in modulating the foil. Measurements of the modulation wavelength, and of the acceleration from Doppler-broadening of back-reflected light, agree quantitatively with particle-in-cell simulations performed for our experimental parameters and which confirm the existence of this instability. © 2012 American Physical Society.

Laser - Ion Acceleration in the Laser Transparency Regime

Experiments on laser-induced ion acceleration from ultra-thin (nm) foil targets reveal a dramatic increase in the conversion efficiency and the acceleration of C6$+$ions in a phase stable way by the laser radiation pressure.

Mono-energetic ion beam acceleration in solitary waves during relativistic transparency using high-contrast circularly polarized short-pulse laser and nanoscale targets

In recent experiments at the Trident laser facility, quasi-monoenergetic ion beams have been obtained from the interaction of an ultraintense, circularly polarized laser with a diamond-like carbon target of nm-scale thickness under conditions of ultrahigh laser pulse contrast. Kinetic simulations of this experiment under realistic laser and plasma conditions show that relativistic transparency occurs before significant radiation pressure acceleration and that the main ion acceleration occurs after the onset of relativistic transparency. Associated with this transition are a period of intense ion acceleration and the generation of a new class of ion solitons that naturally give rise to quasi-monoenergetic ion beams. An analytic theory has been derived for the properties of these solitons that reproduces the behavior observed in kinetic simulations and the experiments. © 2011 American Institute of Physics.

Investigation of magnesium laser ablated plumes with Thomson scattering

Optical Thomson scattering has been implemented as a diagnostic of laser ablated plumes generated with second harmonic Nd:YAG laser radiation at 532 nm. Thomson scattering data with both spatial and temporal resolution has been collected, giving both electron density, and temperature distributions within the plume as a function of time. Although the spatial profiles do not match very well for simple models assuming either isothermal or isentropic expansion, consideration of the measured ablated mass indicates an isothermal expansion fits better than an isentropic expansion and indeed, at late time, the spatial profile of temperature is almost consistent with an isothermal approximation.

XUV spectroscopic characterization of warm dense aluminum plasmas generated by the free-electron-laser FLASH

We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 mu J are focused to intensities ranging between 10(13) and 10(17) W/cm(2). We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations.

Direct Photodetachment of F‾ by mid-infrared few-cycle femtosecond laser pulses

The recent adiabatic saddle-point approach of Shearer et al. [ Phys. Rev. A 84 033409 (2011)] is extended to multiphoton detachment of negative ions with outer p-state electrons. This theory is applied to investigate the strong-field photodetachment dynamics of F- ions exposed to few-cycle femtosecond laser pulses, without taking into account the rescattering mechanism. Numerical calculations are considered for mid-infrared laser wavelengths of 1300 and 1800 nm at laser intensities of 7.7 × 1012, 1.1 × 1013, and 1.3 × 1013 W/cm2. Two-dimensional momenta saddle-point spectra exhibit a distinct distribution in the shape of a “smile” in the complex-time plane. Electron momentum distribution maps of direct electrons are investigated. These produce a distinct pattern of above-threshold detachment (ATD) concentric rings due to constructive and destructive quantum interference of electrons detached from their parent ions. Probability detachment distributions presented, capturing the influence of saturation effects that are found to become more significant with increasing laser intensity at a fixed wavelength. ATD photoangular distributions as functions of laser intensity and wavelength near channel closings are also investigated and found to be sensitive to initial-state symmetry. Nonmonotonic structures observed in the ejected photoelectron energy spectra are attributed to interference effects from coherent electronic wave packets. Additionally the profiles of all the photoelectron emission spectra show strong dependence on the carrier-envelope phase, indicating that it is a reliable parameter for characterizing the wave form of the pulse.

Multichannel interference in high-order-harmonic generation from Ne+ driven by an ultrashort intense laser pulse

We apply the time-dependent R-matrix method to investigate harmonic generation from Ne+ at a wavelength of 390 nm and intensities up to 1015 W cm−2. The 1s22s22p4 (3Pe,1De, and 1Se) states of Ne2+ are included as residual-ion states to assess the influence of interference between photoionization channels associated with these thresholds. The harmonic spectrum is well approximated by calculations in which only the 3Pe and 1De thresholds are taken into account, but no satisfactory spectrum is obtained when a single threshold is taken into account. Within the harmonic plateau, extending to about 100 eV, individual harmonics can be suppressed at particular intensities when all Ne2+ thresholds are taken into account. The suppression is not observed when only a single threshold is accounted for. Since the suppression is dependent on intensity, it may be difficult to observe experimentally.

Contrasting damage characteristics in direct incidence and surface plasmon mediated single-shot laser ablation of aluminium films

Thin, oxidised Al films grown an one face of fused silica prisms are exposed. tinder ambient conditions, to single shots from an excimer laser operating at wavelength 248 nm. Preliminary characterisation of the films using attenuated total reflection yields optical and thickness data for the Al and Al oxide layers; this step facilitates the subsequent, accurate tuning of the excimer laser pulse to the: surface plasmon resonance at the Al/(oxide)/air interface and the calculation of the fluence actually absorbed by the thin film system. Ablation damage is characterised using scanning electron, and atomic force microscopy. When the laser pulse is incident, through the prism on the sample at less than critical angle, the damage features are molten in nature with small islands of sub-micrometer dimension much in evidence, a mechanism of film melt-through and subsegment blow-off due to the build up of vapour pressure at the substrate/film interface is appropriate. By contrast, when the optical input is surface plasmon mediated, predominately mechanical damage results with the film fragmenting into large flakes of dimensions on the order of 10 mu m. It is suggested that the ability of surface plasmons to transport energy leads to enhanced, preferential absorption of energy at defect sites causing stress throughout the film which exceeds the ultimate tensile stress for the film: this in turn leads to film break-up before melting can onset. (C) 1998 Elsevier Science B.V.

SURFACE-PLASMON ENHANCED LASER-ABLATION OF THIN METAL-FILMS

Surface plasmon enhancement of laser ablation of thin Al films is examined with a view to its application in metal film patterning and nano-structuring. Al films, deposited on silica prisms, are first characterized by attenuated total reflection using a broadband UV source and appropriate interference filter. The films are subsequently subjected to excimer laser radiation of wavelength 248 nm under conditions both of direct incidence from the air side of the film, and of surface plasmon excitation in which light is incident through the prism at greater than critical angle. For a given level of ablation damage in a particular film the fluence required using the surface plasmon technique is 3-5 times less than that needed when direct incidence is used. This is roughly in line with the energy absorbed in the film. From a practical standpoint it is clear that ablation of metal films can be achieved with much lower fluences than has hitherto been possible, thus reducing the requirements on laser output and relaxing the power handling constraints on any input optical elements.

Investigating the dynamics of laser induced sparks in atmospheric helium using Rayleigh and Thomson scattering

We have used optical Rayleigh and Thomson scattering to investigate the expansion dynamics of laser induced plasma in atmospheric helium and to map its electron parameters both in time and space. The plasma is created using 9 ns duration, 140 mJ pulses from a Nd:YAG laser operating at 1064 nm, focused with a 10 cm focal length lens, and probed with 7 ns, 80 mJ, and 532 nm Nd:YAG laser pulses. Between 0.4 μs and 22.5 μs after breakdown, the electron density decreases from 3.3 × 1017 cm−3 to 9 × 1013 cm−3, while the temperature drops from 3.2 eV to 0.1 eV. Spatially resolved Thomson scattering data recorded up to 17.5 μs reveal that during this time the laser induced plasma expands at a rate given by R ∼ t0.4 consistent with a non-radiative spherical blast wave. This data also indicate the development of a toroidal structure in the lateral profile of both electron temperature and density. Rayleigh scattering data show that the gas density decreases in the center of the expanding plasma with a central scattering peak reemerging after about 12 μs. We have utilized a zero dimensional kinetic global model to identify the dominant particle species versus delay time and this indicates that metastable helium and the He2 + molecular ion play an important role.

Demonstration of laser pulse amplication by stimulated Brillouin scattering

The energy transfer by stimulated Brillouin backscatter from a long pump pulse (15 ps) to a short seed pulse (1 ps)has been investigated in a proof-of-principle demonstration experiment. The two pulses were both amplified in differentbeamlines of a Nd:glass laser system, had a central wavelength of 1054 nm and a spectral bandwidth of 2 nm, and crossedeach other in an underdense plasma in a counter-propagating geometry, off-set by 10◦. It is shown that the energy transferand the wavelength of the generated Brillouin peak depend on the plasma density, the intensity of the laser pulses, and thecompetition between two-plasmon decay and stimulated Raman scatter instabilities. The highest obtained energy transferfrom pump to probe pulse is 2.5%, at a plasma density of 0.17ncr, and this energy transfer increases significantly withplasma density. Therefore, our results suggest that much higher efficiencies can be obtained when higher densities (above0.25ncr) are used.