Performance comparison of Yb:YAG ceramics and crystal gain material in a large-area, high-energy, high average-power diode-pumped laser.

We compare for the first time the influence of different Yb:YAG gain media on the performance of a large-area, high average-power laser system with an output energy of up to 6 J. Monocrystalline slabs grown by a new technique without central growth defect are compared with ceramics. Small signal gain, maximum output energy and thermal lensing are compared for ceramic slabs with co-sintered amplified spontaneous emission (ASE) absorber cladding, monocrystalline slab with and without optically bonded ASE absorber cladding, and surface structured monocrystalline slabs. We show that these large monocrystals with optically bonded absorber cladding have similar performance to cladded ceramics, so far the only material for high-energy Yb:YAG lasers.

Cryogenically-cooled Yb:YGAG ceramic mode-locked laser.

This work reports on a liquid-nitrogen-cooled, SESAM mode-locked Yb:YGAG (Yb:Y(3)Ga(2)Al(3)O(12)) ceramic laser. The Yb:YGAG has a similar structure to Yb:YAG, but its emission spectrum at low temperature remains much broader, which is suitable for ultrashort pulse generation and amplification. A stable pulse train with 119-MHz repetition rate was obtained at a wavelength of 1026 nm. The measured pulse duration is 2.4 ps, which is more than four times shorter than that achieved with a cryogenically-cooled Yb:YAG. Furthermore, laser performance of the Yb:YGAG ceramics in continuous-wave operation and wavelength tunability at 80 K was investigated.

Advanced micro- & nanostructuring for enhanced biocompatibility of stainless steel by multi-beam and beamshaping technology.

Biocompatibility is one of the key issues for implants, especially in the case of stainless steel with medium to low biocompatibility, which may lead to a lack of osseointegration and consequently to implant failure or rejection. To precisely control preferential cell growth sites and, consequently, the biocompatibility of prosthetic devices, two types of surfaces were analyzed, containing periodic nanogrooves laser induced periodic surface structure (LIPSS) and square-shaped micropillars. For the fast and efficient production of these surfaces, the unique combination of high energy ultrashort pulsed laser system with multi-beam and beamshaping technology was applied, resulting in increased productivity by 526% for micropillars and 14 570% for LIPSS compared to single beam methods.In vitroanalysis revealed that micro and nanostructured surfaces provide a better environment for cell attachment and proliferation compared to untreated ones, showing an increase of up to 496% in the number of cells compared to the reference. Moreover, the combination of LIPSS and micropillars resulted in a precise cell orientation along the periodic microgroove pattern. The combination of these results demonstrates the possibility of mass production of functionalized implants with control over cell organization and growth. Thus, reducing the risk of implant failure due to low biocompatibility.

Laser-driven proton acceleration enhancement by nanostructured foils.

Nanostructured thin plastic foils have been used to enhance the mechanism of laser-driven proton beam acceleration. In particular, the presence of a monolayer of polystyrene nanospheres on the target front side has drastically enhanced the absorption of the incident 100 TW laser beam, leading to a consequent increase in the maximum proton energy and beam charge. The cutoff energy increased by about 60% for the optimal spheres' diameter of 535 nm in comparison to the planar foil. The total number of protons with energies higher than 1 MeV was increased approximately 5 times. To our knowledge this is the first experimental demonstration of such advanced target geometry. Experimental results are interpreted and discussed by means of 2(1/2)-dimensional particle-in-cell simulations.

LIPSS-based functional surfaces produced by multi-beam nanostructuring with 2601 beams and real-time thermal processes measurement.

A unique combination of the ultrashort high-energy pulsed laser system with exceptional beam quality and a novel Diffractive Optical Element (DOE) enables simultaneous production of 2601 spots organized in the square-shaped 1 × 1 mm matrix in less than 0.01 ms. By adjusting the laser and processing parameters each spot can contain Laser Induced Periodic Surface Structures (LIPSS, ripples), including high-spatial frequency LIPSS (HFSL) and low-spatial frequency LIPSS (LSFL). DOE placed before galvanometric scanner allows easy integration and stitching of the pattern over larger areas. In addition, the LIPSS formation was monitored for the first time using fast infrared radiometry for verification of real-time quality control possibilities. During the LIPSS fabrication, solidification plateaus were observed after each laser pulse, which enables process control by monitoring heat accumulation or plateau length using a new signal derivation approach. Analysis of solidification plateaus after each laser pulse enabled dynamic calibration of the measurement. Heat accumulation temperatures from 200 to 1000 °C were observed from measurement and compared to the theoretical model. The temperature measurements revealed interesting changes in the physics of the laser ablation process. Moreover, the highest throughput on the area of 40 × 40 mm reached 1910 cm2/min, which is the highest demonstrated throughput of LIPSS nanostructuring, to the best of our knowledge. Thus, showing great potential for the efficient production of LIPSS-based functional surfaces which can be used to improve surface mechanical, biological or optical properties.

Fourier-limited seeded soft x-ray laser pulse.

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

A high-intensity highly coherent soft X-ray femtosecond laser seeded by a high harmonic beam.

Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays--in structural biology, for example--anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Ablation pressure scaling at short laser wavelength.

The ablation pressure at a 0.44-microm laser wavelength has been measured at irradiance up to 2 x 10(14) W/cm(2). The diagnostics consisted in the detection of shock breakout from stepped Al targets. By adopting large focal spots and smoothed laser beams, the lateral energy transport and "drilling effects" have been avoided. The measured scaling shows a fair agreement with analytical models.

New methods for high current fast ion beam production by laser-driven acceleration.

An overview of the last experimental campaigns on laser-driven ion acceleration performed at the PALS facility in Prague is given. Both the 2 TW, sub-nanosecond iodine laser system and the 20 TW, femtosecond Ti:sapphire laser, recently installed at PALS, are used along our experiments performed in the intensity range 10(16)-10(19) W∕cm(2). The main goal of our studies was to generate high energy, high current ion streams at relatively low laser intensities. The discussed experimental investigations show promising results in terms of maximum ion energy and current density, which make the laser-accelerated ion beams a candidate for new-generation ion sources to be employed in medicine, nuclear physics, matter physics, and industry.

Measurements of the highest acceleration gradient for ions produced with a long laser pulse.

Ultrafast plasma light ion streams have been produced using the 300 ps, kJ-class iodine laser, operating at PALS Centre in Prague. Ion detection was performed through standard ion collectors (IC) in time-of-flight configuration (TOF), shielded by thin metallic absorbers. This new diagnostics technique has been theoretically studied and experimentally tested in order to cut the long photopeak contribution and to analyze the ultrafast particle signal. Processing the obtained experimental IC-TOF data, including deconvolution processes of the TOF signals, UV/soft-x-ray photopeak absorption, and ion transmission calculations for different metallic filters, is shown. Mainly amorphous carbon (graphite) targets have been irradiated in order to limit the maximum number of ion charge states and to focus our study on demonstrating the validity of the proposed investigation technique. Maximum ion energy and acceleration gradient estimations as a function of the laser energy and focal spot diameter are reported.

Single-shot soft x-ray laser-induced ablative microstructuring of organic polymer with demagnifying projection.

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a Ne-like Zn 21.2 nm soft x-ray laser. A nickel mesh with a period of 100 microm was approximately 10x demagnified and imprinted on poly(methyl methacrylate) via direct ablation. The quality of the ablated microstructure was found to be mainly dependent on the quality of the projected mask. This first demonstration (to our knowledge) of single-shot projection, single-step lithography illustrates the potential of soft x-ray lasers for the direct patterning of materials with a resolution scalable down to submicrometer domain.

Laser-ablation rates measured using x-ray laser transmission.

The rate of laser ablation at irradiances of approximately 2 x 10{14} W cm{-2} of solid iron has been measured using the transmission of a neonlike zinc x-ray laser at 21.2 nm through thin iron targets. Ablated iron becomes transparent to the x-ray laser flux, enabling the thickness of unablated material and hence the rate of ablation to be measured from time resolved x-ray laser transmission.

Demonstration of a spatial filtering amplifier for high-order harmonics.

We report what is to our knowledge the first demonstration of spatial filtering of a high-order harmonic beam into a soft-x-ray laser plasma amplifier at 32.8 nm. After amplification the seed energy is enhanced by a factor of 50, and the beam profile of the amplified beam exhibits an Airy-like shape due to the spatial filtering by the optical field ionized plasma. Moreover, the transverse coherence of the spatially filtered amplified beam is strongly enhanced, resulting in the generation of a peak coherent power of 0.9 x 10(5) to 1.8 x 10(5) W.

Opacity measurements of a hot iron plasma using an x-ray laser.

The temporal evolution of the opacity of an iron plasma at high temperature (30-350 eV) and high density (0.001-0.2 g cm-3) has been measured using a nickel-like silver x-ray laser at 13.9 nm. The hot dense iron plasma was created in a thin (50 nm) iron layer buried 80 nm below the surface in a plastic target that was heated using a separate 80 ps pulse of 6-9 J, focused to a 100 microm diameter spot. The experimental opacities are compared with opacities evaluated from plasma conditions predicted using a fluid and atomic physics code.

Absolute time-resolved x-ray laser gain measurement.

We present the first direct measurement of the time evolution of the gain of a soft x-ray laser amplifier. The measurement is based on the injection of a seed pulse, obtained by high-order harmonic generation, into an x-ray laser medium. Strong amplification occurs when the seed pulse is synchronized with the gain period. By precisely varying the delay between the x-ray laser plasma creation and the seed pulse injection, the actual temporal evolution of the soft x-ray amplifier gain is obtained with a subpicosecond resolution.

Demonstration of a collisionally excited optical-field-ionization XUV laser driven in a plasma waveguide.

We describe the first demonstration of a collisionally excited optical-field-ionization laser driven within a waveguide. Lasing on the 4d(9)5d-4d(9)5p transition at 41.8 nm in Xe8+ was observed to be closely correlated to conditions under which the pump laser pulses were guided well by a gas-filled capillary discharge waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.

Soft-x-ray emission from small-sized Ne clusters heated by intense, femtosecond laser pulses

Soft-x-ray emission from a cryogenically cooled Ne jet irradiated by intense, 25-fs laser pulses was measured. The Ne spectrum started to drastically change in emitting ions from Ne5+ to Ne7+ below the preexpansion temperature of -120 degrees C. The significant change in the spectrum is attributed to the collisional heating of small-sized Ne clusters formed in the cooled jet. The increase of the laser pulse length from 25 fs to 100 fs resulted in further increase of x-ray emission from Ne7+ states.

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm.

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.

Hugoniot data for carbon at megabar pressures.

We present an experimental point for the carbon equation of state (EOS) at megabar pressures, obtained by laser-driven shock waves. The rear side emissivity of "two-materials two-steps" targets (Al-C) was recorded with space and time resolution and, by applying the impedance mismatch method, allowed a direct determination of relative EOS points. Experiments were performed at the PALS and LULI laboratories using carbon samples with two different values of initial density, in order to explore a wider region of the phase diagram. Previously unreached pressures were obtained. The results are compared with previous experiments and with available theoretical models and seem to show a high compressibility of carbon at megabar pressures.