Observation of K-Shell Soft X Ray Emission of Nitrogen Irradiated by XUV-Free Electron Laser FLASH at Intensities Greater than 10(16) W/cm(2)

In the past few years, the development of light sources of the 4(th) generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of similar to 20 fs, micro focusing leading to intensities larger than 10(16) W/cm(2). Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Search for Rapid Changes in the Visible-Light Corona during the 21 June 2001 Total Solar Eclipse

Some 8000 images obtained with the Solar Eclipse Coronal Imaging System (SECIS) fast-frame CCD camera instrument located at Lusaka, Zambia, during the total eclipse of 21 June 2001 have been analysed to search for short-period oscillations in intensity that could be a signature of solar coronal heating mechanisms by MHD wave dissipation. Images were taken in white-light and Fe xiv green-line (5303 ) channels over 205 seconds (frame rate 39 s(-1)), approximately the length of eclipse totality at this location, with a pixel size of four arcseconds square. The data are of considerably better quality than those that we obtained during the 11 August 1999 total eclipse (Rudawy et al.: Astron. Astrophys. 416, 1179, 2004), in that the images are much better exposed and enhancements in the drive system of the heliostat used gave a much improved image stability. Classical Fourier and wavelet techniques have been used to analyse the emission at 29 518 locations, of which 10 714 had emission at reasonably high levels, searching for periodic fluctuations with periods in the range 0.1 -aEuro parts per thousand 17 seconds (frequencies 0.06 -aEuro parts per thousand 10 Hz). While a number of possible periodicities were apparent in the wavelet analysis, none of the spatially and time-limited periodicities in the local brightness curves was found to be physically important. This implies that the pervasive Alfv,n wave-like phenomena (Tomczyk et al.: Science 317, 1192, 2007) using polarimetric observations with the Coronal Multi-Channel Polarimeter (CoMP) instrument do not give rise to significant oscillatory intensity fluctuations.

Application of a picosecond laser plasma continuum light source to a dual-laser plasma photoabsorption experiment

The characteristics of an extreme-ultraviolet (XUV) continuum light source and its application to a dual-laser plasma (DLP) photoabsorption experiment are described. The continuum emitting plasma was formed by focusing a 7 ps, 248 nm, 15 mJ laser pulse onto a number of selected targets known to be good XUV continuum emitters (Sm, W, Au and Pb), while the second absorbing plasma was produced by a 15 ns, 1064 nm, 300 mi pulse. The duration of the continuum emission for these plasmas has a mean value of similar to 150 ps, but depends on both the target material and the picosecond laser pulse energy. Using this picosecond DLP set-up we have been able to measure the photoabsorption spectrum of an actinide ion (thorium) for the first time.

Measurements of the hole boring velocity from Doppler shifted harmonic emission from solid targets

The fast ignitor scheme for inertial confinement fusion requires forward driving of the critical density surface by light pressure (hole boring) to allow energy deposition close to the dense fuel. The recession velocity of the critical density surface has been observed to be nu/c = 0.015 at an irradiance of 1.0 x 10(19) W cm(-2) at a wavelength of 1.05 micron, in quantitative agreement with modeling. (C) 1996 American Institute of Physics.

2ND-HARMONIC LIGHT GENERATION IN THE INTERACTION OF LASER-BEAMS WITH UNDERCRITICAL PLASMAS

The process of second harmonic generation (SHG) in undercritical plasmas is studied. It is shown that filamentation and self-focusing of the laser beam in the plasma can break the plasma density symmetry and lead to SHG by free electrons. In turn, second harmonic emission may be used to investigate the plasma parameters and to diagnose the process of laser beam filamentation itself.

SPECTRALLY MODULATED 2ND-HARMONIC EMISSION FROM LASER-PLASMA FILAMENTS

Spectra of forward emitted second harmonic light from laser interaction with filamentary plasmas have been experimentally studied. Rather regular modulations in the frequency domain have been observed into overall red-shifted spectra. The observed spectral features are consistent with self-phase-modulation of the intense laser light in growing filaments. A model accounts for this effect.

TRANSITION TO FILAMENTARY REGIME IN LASER-PLASMA INTERACTION EVIDENCED BY 2-OMEGA-EMISSION

The level of second-harmonic light emitted from a laser-exploded foil plasma at nominal irradiance up to 3.10(13) W/cm2 was found to be extremely sensitive to both target position and irradiance on target. Either a small target displacement or a small increase in irradiance resulted in a jump of the 2omega level of more than three orders of magnitude. Correspondingly, a transition was observed from a 2omega source pattern clearly signed by the original laser spot pattern to unstable patterns of filaments whose size is consistent with the maximum growth of the instability.

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.

On the γ-ray emission of Type Ia supernovae

A multidimension, time-dependent Monte Carlo code is used to compute sample ?-ray spectra to explore whether unambiguous constraints could be obtained from ?-ray observations of Type Ia supernovae. Both spherical and aspherical geometries are considered and it is shown that moderate departures from sphericity can produce viewing-angle effects that are at least as significant as those caused by the variation of key parameters in 1D models. Thus, ?-ray data could, in principle, carry some geometrical information, and caution should be applied when discussing the value of ?-ray data based only on 1D explosion models. In light of the limited sensitivity of current ?-ray observatories, the computed theoretical spectra are studied to revisit the issue of whether useful constraints could be obtained for moderately nearby objects. The most useful ?-ray measurements are likely to be of the light curve and time-dependent hardness ratios, but sensitivity higher than currently available, particularly at relatively hard energies (~2-3 MeV), is desirable. © 2008 The Authors. Journal compilation © 2008 RAS.

Focusing of light by a nanohole array

The authors demonstrate a mechanism for focusing at optical frequencies based on the use of nanohole quasiperiodic arrays in metal screens. Using coherent illumination at 660 nm and scanning aperture optical microscopy, similar to 290 nm "hot spots" were observed at a distance of similar to 12.5 mu m from the array. Even smaller hot spots of about similar to 200 nm in waist were observed closer to the plane of the array.(c) 2007 American Institute of Physics.

Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein's seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency >10 4 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure. 

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.

OPTIMIZATION OF THE SLOW-MODE PLASMON POLARITON IN LIGHT-EMITTING TUNNEL-JUNCTIONS

Light emitted from metal/oxide/metal tunnel junctions can originate from the slow-mode surface plasmon polariton supported in the oxide interface region. The effective radiative decay of this mode is constrained by competition with heavy intrinsic damping and by the need to scatter from very small scale surface roughness; the latter requirement arises from the mode's low phase velocity and the usual momentum conservation condition in the scattering process. Computational analysis of conventional devices shows that the desirable goals of decreased intrinsic damping and increased phase velocity are influenced, in order of priority, by the thickness and dielectric function of the oxide layer, the type of metal chosen for each conducting electrode, and temperature. Realizable devices supporting an optimized slow-mode plasmon polariton are suggested. Essentially these consist of thin metal electrodes separated by a dielectric layer which acts as a very thin (a few nm) electron tunneling barrier but a relatively thick (several 10's of nm) optically lossless region. (C) 1995 American Institute of Physics.

BROAD-BAND EXCITATION AND EMISSION OF SURFACE-PLASMONS

The well known advantages of using surface plasmons, in particular the high sensitivity to surface adsorbates, are nearly always compromised in practice by the use of monochromatic excitation and the consequent lack of proper spectroscopic information. This limitation arises from the angle/wavelength selective nature of the surface plasmon resonance. The work described here uses an elegant broadband excitation/decay scheme in a substrate(silica)-grating profiled photoresist-Ag film geometry. Laser radiation of wavelength 488 nm, incident through the silica substrate, excites by near-field coupling a broad band of surface plasmons at the photoresist-Ag interface within the spectral range of the photoresist fluorescence. With a judicious choice of grating period this mode can cross-couple to the mode supported at the Ag-air interface. This latter mode can, in turn, couple out to light by virtue of the same grating profile. The spectral distribution of the light emitted due to this three-step process has been studied as a function of the angle of emission and depth of the grating profiled surface for each polarization. It is found that the optimum emission efficiency occurs with a groove depth in the region of 65 nm. This is considerably greater than the optimum depth of 40 nm required for surface plasmon-photon coupling at a Ag-air interface or, in other words, for the last step of the process in isolation.

DEPENDENCE OF LIGHT OUTPUT FROM NOMINALLY SMOOTH AL-OX-AU TUNNEL-JUNCTIONS ON ELECTRODE MORPHOLOGY

The light output from nominally smooth Al-Ox-Au tunnel junctions is observed to be substantially independent of the deposition rate of the Au film electrode. Films deposited quickly (2 nm s-1) and those deposited slowly (0.16 nm s-1) have similar spectral dependences and intensities. (This is in contrast to roughened films where those deposited quickly give out less light, especially towards the blue end of the spectrum.) The behaviour can be interpreted in terms of the ratio l(ph)/l(em) where l(ph) and l(em) are the mean free paths of surface plasmons between external photon emissions and internal electromagnetic absorptions respectively. Once l(ph)/l(em) exceeds 100, as it does on smooth films, grain size has little further effect on the spectral shape of the light output. In fast-deposited films there are two compensating effects on the output intensity: grain boundary scattering decreases it and greater surface roughness increases it.