Tatiana A. Pikuz

Submicrometer-resolution in situ imaging of the focus pattern of a soft x-ray laser by color center formation in LiF crystal.

We demonstrate high quality, single-shot in situ imaging of the focused Ag x-ray laser (XRL) at 13.9 nm with 700 nm spatial resolution by color center formation in LiF. The flux and intensity for the color center formation in LiF are evaluated from the experimental data. Comparisons with previous reports show that the threshold x-ray flux for the color center formation in LiF for the 13.9 nm, 7 ps Ag XRL is 3 orders of magnitude less than that with the 46.9 nm, 2 ns capillary discharge Ar XRL.

Acceleration of highly charged GeV Fe ions from a low-Z substrate by intense femtosecond laser

Almost fully stripped Fe ions accelerated up to 0.9 GeV are demonstrated with a 200 TW femtosecond high-intensity laser irradiating a micron-thick Al foil with Fe impurity on the surface. An energetic low-emittance high-density beam of heavy ions with a large charge-to-mass ratio can be obtained, which is useful for many applications, such as a compact radio isotope source in combination with conventional technology.

Scanning near-field optical microscopy images of microradiographs stored in lithium fluoride films with an optical resolution of λ∕12

Here we show a new, simple method to observe soft x-ray microradiographs of biological material. A thin film of lithium fluoride (LiF) works as image detector, storing the microradiograph obtained exposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the stored image, collecting the optically stimulated visible luminescence emitted by the LiF active color centers locally produced by the x rays, a scanning near-field optical microscope is used with an optical aperture of 50nm, i.e., λ∕12, where λ is the wavelength of the collected photoluminescence.

Nanoscale surface modifications and formation of conical structures at aluminum surface induced by single shot exposure of soft x-ray laser pulse

We irradiated the soft x-ray laser (SXRL) pulses having a wavelength of 13.9 nm, a duration time of 7 ps, and fluences of up to 27 mJ/cm2 to aluminum (Al) surface. After the irradiation process, the modified surface was observed with the visible microscope, the scanning electron microscope, and the atomic force microscope. The surface modifications caused by the SXRL pulses were clearly seen, and it was found that the conical structures having about 70–150 nm in diameters were formed under a single pulse shot. The conical structures were formed in the features with the average depth of about 40 nm, and this value was in accordance with the attenuation length of the SXRL beam for Al. However, those conical structures were deconstructed under the multiple pulse shots exposure. Thermomechanical modeling of SXRL laser interaction with Al surface, which explains nanostructure surface modification, was provided.

Efficient generation of Xe K-shell x rays by high-contrast interaction with submicrometer clusters

The interaction between a 25 TW laser and Xe clusters at a peak intensity of 1 × 10¹⁹ W/cm² has been investigated. Xe K-shell x rays, whose energies are approximately 30 keV, were clearly observed with a hard x-ray CCD at 3.4 MPa. Moreover, we studied the yield of the Xe K-shell x rays by changing the pulse duration of the laser at a constant laser energy and found that the pulse duration of 40 fs is better than that of 300 fs for generating Xe K-shell x rays.

Elemental sensitivity in soft x-ray imaging with a laser-plasma source and a color center detector

Elemental sensitivity in soft x-ray imaging of thin foils with known thickness is observed using an ultrafast laser-plasma source and a LiF crystal as detector. Measurements are well reproduced by a simple theoretical model. This technique can be exploited for high spatial resolution, wide field of view imaging in the soft x-ray region, and it is suitable for the characterization of thin objects with thicknesses ranging from hundreds down to tens of nanometers.

Nonlinear increase of X-ray intensities from thin foils irradiated with a 200 TW femtosecond laser

We report, for the first time, that the energy of femtosecond optical laser pulses, E, with relativistic intensities I > 1021  W/cm2 is efficiently converted to X-ray radiation, which is emitted by “hot” electron component in collision-less processes and heats the solid density plasma periphery. As shown by direct high-resolution spectroscopic measurements X-ray radiation from plasma periphery exhibits unusual non-linear growth ~E4–5 of its power. The non-linear power growth occurs far earlier than the known regime when the radiation reaction dominates particle motion (RDR). Nevertheless, the radiation is shown to dominate the kinetics of the plasma periphery, changing in this regime (now labeled RDKR) the physical picture of the laser plasma interaction. Although in the experiments reported here we demonstrated by observation of KK hollow ions that X-ray intensities in the keV range exceeds ~1017  W/cm2, there is no theoretical limit of the radiation power. Therefore, such powerful X-ray sources can produce and probe exotic material states with high densities and multiple inner-shell electron excitations even for higher Z elements. Femtosecond laser-produced plasmas may thus provide unique ultra-bright X-ray sources, for future studies of matter in extreme conditions, material science studies, and radiography of biological systems.

Submicron ionography of nanostructures using a femtosecond-laser-driven-cluster-based source

An intense isotropic source of multicharged carbon and oxygen ions with energy above 300 keV and particle number >108 per shot was obtained by femtosecond Ti:Sa laser irradiation of submicron clusters. The source was employed for high-contrast contact ionography images with 600 nm spatial resolution. A variation in object thickness of 100 nm was well resolved for both Zr and polymer foils.

Electron acceleration via high contrast laser interacting with submicron clusters

We experimentally investigated electron acceleration from submicron size argon clusters-gas target irradiated by a 100 fs, 10 TW laser pulses having a high-contrast. Electron beams are observed in the longitudinal and transverse directions to the laser propagation. The measured energy of the longitudinal electron reaches 600 MeV and the charge of the electron beam in the transverse direction is more than 3 nC. A two-dimensional particle-in-cell simulation of the interaction has been performed and it shows an enhancement of electron charge by using the cluster-gas target.

Bright, point X-ray source based on a commercial portable 40 ps Nd:YAG laser system

AbstractWe present some experimental results on X-ray spectra obtained from plasmas produced using a compact Nd:YAG lasersystem. The beam was focused on different targets~Cu,Al,Ge,...!and both high resolution and low resolution X-rayspectra were recorded.Keywords: ps laser; X-ray plasma sources; X-ray spectroscopy 1. INTRODUCTIONLaser-plasma sources are very powerful laboratory-scaleX-raydeviceswherewavelengthtunabilityisfeasiblebecauseof the comparatively free choice of target material, laserwavelength, and intensity. They have already been success-fully applied for radiobiology ~Turcu et al., 1994a; Masiniet al., 1999; Milani et al., 1999; Bortolotto et al., 2000!,X-ray microscopy~Batani et al., 2000, 2002; Desai et al.2003;Polettietal.,2004!,micro-lithography~Bijkerketal.,1992; Turcu et al., 1994b!. For all these applications theknowledgeofX-rayspectraisveryimportant,eitherdirectlyorbecauseenergydepositionandradiationdosearestronglyenergy dependent.For instance in micro-lithography;1-keVX-ray photonsare required to achieve efficient radiation transfer throughthe X-ray mask membrane and sufficient energy depositiononto the resist coated Si wafer.In radiobiology, laser-plasma X-ray sources were used toirradiate biological specimens: V-79 Chinese hamster cells~Turcu et al., 1994a!, and study DNA damage, DNA repairand repair inhibition, or Saccharomices Cerevisiae yeastcells ~Masini et al., 1999; Milani et al., 1999! and studymetabolic damages. In the first case, X-rays at hn’1.2 keVwere produced using L-shell emission from Cu targets,which could guaranty penetration of X-rays to the cellnucleus, so that absorbed dose was really related to DNAdamage. In the second case, it was essential to produce lowenergy X-ray photons, which would mainly deposit the doseatthecellwall,inordertoproducedamagesatthemetaboliclevel without touching the cell nucleus.Finally in X-ray microscopy, the spectrum is importantbecause only X-rays in the “water window” ~between 22and 44 A ! will contribute to the formation of a clean image,being absorbed by biological material but not by water. Thepresence of other X-ray lines will diminish the image con-trast dramatically.Besides all this, the study of X-ray spectra from plasmasis very important in itself and it is a well developed field inphysics. Indeed on one side, X-ray spectra are very impor-tant for atomic physics because by recording X-ray emis-sion lines, it is possible to access the energy levels in atomsand in multi-charged ions~Rosmej et al., 1997; Stepanovet al., 1997; Vergunova et al., 1997; Biemont et al., 2000!.On the other side, X-ray spectroscopy is a key technique inplasma diagnostics ~Koenig et al., 1997; Magunov et al.,1998; Batani et al., 1999! since from measurements ofX-ray spectra we can calculate the plasma parameters: elec-tron density, electron temperature, ion charges, and evendeduce important information on plasma opacities and onthe exact shape of the electron distribution function~includ-ing the presence of fast electrons!A key problem for applications and for X-ray spectros-copy, would be the development of very compact X-raysources which would be cheap ~and be reproduced in many