Spectral filtering of high-order harmonics via optics-free focusing.

Controlling the wavefront of an extreme ultraviolet (XUV) high-order harmonic beam during the generation process offers the capability of modifying the beam properties without resorting to any XUV optics. By characterizing the XUV intensity profile and wavefront, we quantitatively retrieve both the size and the position of the waist of each harmonic generated in an argon jet. We show that optics-free focusing can occur under specific generating conditions leading to XUV focii of micrometer size. We also demonstrate that each focus is located at distinct longitudinal positions. Using this remarkable XUV wavefront control combined with near focus spatial selection, we experimentally demonstrate efficient and adjustable spectral filtering of the XUV beam, along with a strong rejection of the fundamental beam, without using any XUV optics. The experimental results are compared with simulations providing the impact of the filtering on the temporal profile of the XUV field. It shows that the attosecond structure is preserved and that the beam is more homogeneous after the filtering, thereby reducing the longitudinal focii shift. This is a major step to achieve high XUV intensity and probing ultrafast processes with an improved resolution.

High-order parametric generation of coherent XUV radiation.

Extreme ultraviolet (XUV) radiation finds numerous applications in spectroscopy. When the XUV light is generated via high-order harmonic generation (HHG), it may be produced in the form of attosecond pulses, allowing access to unprecedented ultrafast phenomena. However, the HHG efficiency remains limited. Here we present an observation of a new regime of coherent XUV emission which has a potential to provide higher XUV intensity, vital for applications. We explain the process by high-order parametric generation, involving the combined emission of THz and XUV photons, where the phase matching is very robust against ionization. This introduces a way to use higher-energy driving pulses, thus generating more XUV photons.

Optics-less focusing of XUV high-order harmonics.

By experimentally studying high-order harmonic beams generated in gases, we show how the spatial characteristics of these ultrashort extreme-ultraviolet (XUV) beams can be finely controlled when a single fundamental beam generates harmonics in a thin gas medium. We demonstrate that these XUV beams can be emitted as converging beams and thereby get focused after generation. We study this optics-less focusing using a spatially chirped beam that acts as a probe located inside the harmonic generation medium. We analyze the XUV beam evolution with an analytical model and obtain very good agreement with experimental measurements. The XUV foci sizes and positions vary strongly with the harmonic order, and the XUV waist can be located at arbitrarily large distances from the generating medium. We discuss how intense XUV fields can be obtained with optics-less focusing and how the order-dependent XUV beam characteristics are compatible with broadband XUV irradiation and attosecond science.

The GEMpix detector as new soft X-rays diagnostic tool for laser produced plasmas.

Laser produced plasmas lend to several interesting applications. The study of X-ray emission from this kind of plasmas is important not only to characterize plasmas itself but also to study the application of these particular plasmas as intense X-ray sources. In particular several emission configurations can be obtained using different kinds of targets and tuning the characteristics of the laser pulse delivered to the target. Typically, laser pulse duration ranges between a few tens of femtoseconds and tens of nanoseconds, with energies from few mJ to tens of kJ. X-ray photon emissions last for times comparable to the laser pulses and during this time a great number of photons can be emitted. The following paper presents a measure of the soft-X-ray emission on the ECLIPSE laser facility realized with a new triple-GEM gas detector (GEMpix). It is a hybrid gas detector with a C-MOS front-end electronics based on Medipix chips. In the present work, different targets have been used in order to test X-rays of different energies. In this paper, in particular, we present results obtained for copper and iron targets. GEMpix is able to realize a 2D imaging of the X-ray emission from plasma with a signal proportional to the energy released in the gas of the detector active volume. Then through a preliminary single photon equalization realized at the NIXT lab (ENEA), also the number of photons reaching the area of the detector has been estimated.

Validation of modelled imaging plates sensitivity to 1-100 keV x-rays and spatial resolution characterisation for diagnostics for the "PETawatt Aquitaine Laser".

Thanks to their high dynamic range and ability to withstand electromagnetic pulse, imaging plates (IPs) are commonly used as passive detectors in laser-plasma experiments. In the framework of the development of the diagnostics for the Petawatt Aquitaine Laser facility, we present an absolute calibration and spatial resolution study of five different available types of IP (namely, MS-SR-TR-MP-ND) performed by using laser-induced K-shell X-rays emitted by a solid silver target irradiated by the laser ECLIPSE at CEntre Lasers Intenses et Applications. In addition, IP sensitivity measurements were performed with a 160 kV X-ray generator at CEA DAM DIF, where the absolute response of IP SR and TR has been calibrated to X-rays in the energy range 8-75 keV with uncertainties of about 15%. Finally, the response functions have been modeled in Monte Carlo GEANT4 simulations in order to reproduce experimental data. Simulations enable extrapolation of the IP response functions to photon energies from 1 keV to 1 GeV, of interest, e.g., for laser-driven radiography.

Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation.

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20-26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

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.

Controlling the phase-space volume of injected electrons in a laser-plasma accelerator.

To take full advantage of a laser-plasma accelerator, stability and control of the electron beam parameters have to be achieved. The external injection scheme with two colliding laser pulses is a way to stabilize the injection of electrons into the plasma wave, and to easily tune the energy of the output beam by changing the longitudinal position of the injection. In this Letter, it is shown that by tuning the optical injection parameters, one is able to control the phase-space volume of the injected particles, and thus the charge and the energy spread of the beam. With this method, the production of a laser accelerated electron beam of 10 pC at the 200 MeV level with a 1% relative energy spread at full width half maximum (3.1% rms) is demonstrated. This unique tunability extends the capability of laser-plasma accelerators and their applications.

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.

Coherence-based transverse measurement of synchrotron x-ray radiation from relativistic laser-plasma interaction and laser-accelerated electrons.

We observe Fresnel edge diffraction of the x-ray beam generated by the relativistic interaction of a high-intensity laser pulse with He gas. The observed diffraction at center energy 4.5 keV agrees with Gaussian incoherent source profile of full-width-half-maximum (FWHM) < 8 microm. Analysis indicates this corresponds to an upper limit on the transverse profile of laser-accelerated electrons within the plasma in agreement with three-dimensional, particle-in-cell results (FWHM = 4 microm).

High-resolution gamma-ray radiography produced by a laser-plasma driven electron source.

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.

A laser-plasma accelerator producing monoenergetic electron beams.

Particle accelerators are used in a wide variety of fields, ranging from medicine and biology to high-energy physics. The accelerating fields in conventional accelerators are limited to a few tens of MeV m(-1), owing to material breakdown at the walls of the structure. Thus, the production of energetic particle beams currently requires large-scale accelerators and expensive infrastructures. Laser-plasma accelerators have been proposed as a next generation of compact accelerators because of the huge electric fields they can sustain (>100 GeV m(-1)). However, it has been difficult to use them efficiently for applications because they have produced poor-quality particle beams with large energy spreads, owing to a randomization of electrons in phase space. Here we demonstrate that this randomization can be suppressed and that the quality of the electron beams can be dramatically enhanced. Within a length of 3 mm, the laser drives a plasma bubble that traps and accelerates plasma electrons. The resulting electron beam is extremely collimated and quasi-monoenergetic, with a high charge of 0.5 nC at 170 MeV.

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.

Emittance measurements of a laser-wakefield-accelerated electron beam.

The transverse emittance of a relativistic electron beam generated by the interaction of a high-intensity laser with an underdense plasma has been measured with the "pepper-pot" method. For parameters pertaining to the forced laser wakefield regime, we have measured an emittance as low as (2.7+/-0.9) pi mm mrad for (55+/-2) MeV electrons. These measurements are consistent with 3D particle-in-cell simulations of the experiment, which additionally show the existence of a relatively large halo around the beam core.

Electron acceleration by a wake field forced by an intense ultrashort laser pulse.

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.