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Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

The present work reports on Yttrium based photocathodes. A Yttrium (Y) thin film is deposited via pulsed laser deposition (PLD) on the copper (Cu) back flange of a radio frequency (rf) gun for photocathode application. Because of a lower work function with respect to Cu, Y photocathodes are particularly appealing for the possibility to illuminate them with visible laser pulses, with the advantage of a higher energy per pulse, paving the way to high repetition rate photoinjectors, driven by conventional laser sources. In addition, working at $\ensuremath{\lambda}\ensuremath{\sim}400\text{ }\text{ }\mathrm{nm}$ the small energy difference between the Y work function (about 3 eV) and the laser photon energy reduces the contribution of the intrinsic emittance of the material. Photoelectrons, emitted by the thin film Y photocathode driven by the second harmonic of a Ti:Sapphire laser, have been characterized in terms of quantum efficiency and transverse emittance. Results have been compared with the theoretical ones obtained by the three-step model of Spicer for metallic photocathodes.

© 2018 American Physical Society. Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.

In a coherent control experiment, light pulses are used to guide the real-time evolution of a quantum system. This requires the coherence and the control of the pulses' electric-field carrier waves. In this work, we use frequency-domain interferometry to demonstrate the mutual coherence of time-delayed pulses generated by an extreme ultraviolet seeded free-electron laser. Furthermore, we use the driving seed laser to lock and precisely control the relative phase between the two free-electron laser pulses. This new capability opens the way to a multitude of coherent control experiments, which will take advantage of the high intensity, short wavelength, and short duration of the pulses generated by seeded free-electron lasers.

We report on experimental and theoretical investigations of the birefringence of free-standing nanoporous anodic alumina membranes in the optical range. The value of birefringence is analyzed for the samples with different porosities by measuring polarization dependent transmission spectra at different angles of incidence. The experimental data are compared to the results of birefringence simulations in accordance with the modified Bruggeman effective-medium approximation. It is both experimentally and theoretically shown that the birefringence value increases with porosity increases in the low porosity region. The porous alumina samples under investigation possess the greatest value of birefringence (0.062) up to the present.

3rd SPIE Conference on Advances in X-Ray Free-Electron Lasers Instrumentation, Prague, CZECH REPUBLIC, APR 13-16, 2015; International audience; Magnetization control without applying magnetic fields has potential for applications in sensors and devices. In Fe/MnAs/GaAs(001), the Fe magnetization can be modified by acting on the MnAs microstructure via temperature control, without applying external magnetic fields. Here we use an optical laser pulse to vary the local temperature and an x-ray free-electron laser pulse to probe the induced magnetic and structural dynamics in a time-resolved resonant scattering experiment, both pulses having similar to 100 fs duration. Modifications of the MnAs microstructure take place within a few ps, followed by a slower dynamics driven by thermal diffusion. We show that a single optical laser pulse can reverse the Fe magnetization locally, the process being driven not by the fast modifications of the MnAs structure, but rather by its slower return to equilibrium.

In this paper we describe a promising approach to increase the output energy of a laser system emitting infrared radiation in the spectral range 6.78μm based on direct difference frequency generation in non-oxide nonlinear crystals using single-mode Nd:YAG laser and tunable Cr:forsterite laser, which was developed for the muonic-hydrogen experiment. The investigated system is based on lithium thioindate (LiInS2) crystal cut for type II difference frequency generation. The pulses of the Nd:YAG laser (1,064 μm) are combined with the pulses at ~ 1.262 μm of the Cr:forsterite laser through a dichroic mirror and sent to the nonlinear crystals in different optical geometries. The generated radiation reaches an output energy more than 90 μJ in a single pass optical geometry, has 10 ns long pulses at 50 Hz frequency repetition rate and is tunable in the range 6695 - 6870 nm. These results prove the suitability of such an approach for building the laser system for the muonic-hydrogen experiment.

Thin film magnetization reversal without applying external fields is an attractive perspective for applications in sensors and devices. One way to accomplish it is by fine-tuning the microstructure of a magnetic substrate via temperature control, as in the case of a thin Fe layer deposited on a $\mathrm{MnAs}/\mathrm{GaAs}(001)$ template. This work reports a time-resolved resonant scattering study exploring the magnetic and structural properties of the $\mathrm{Fe}/\mathrm{MnAs}$ system, using a 100 fs optical laser pulse to trigger local temperature variations and a 100 fs x-ray free-electron laser pulse to probe the induced magnetic and structural dynamics. The experiment provides direct evidence that a single optical laser pulse can reverse the Fe magnetization locally. It reveals that the time scale of the magnetization reversal is slower than that of the MnAs structural transformations triggered by the optical pulse, which take place after a few picoseconds already.

We present the experimental demonstration of a method for generating two spectrally and temporally separated pulses by an externally seeded, single-pass free-electron laser operating in the extreme-ultraviolet spectral range. Our results, collected on the FERMI@Elettra facility and confirmed by numerical simulations, demonstrate the possibility of controlling both the spectral and temporal features of the generated pulses. A free-electron laser operated in this mode becomes a suitable light source for jitter-free, two-colour pump-probe experiments. (c) 2013 Optical Society of America

In this paper, we report about a numerical and experimental study of a high energy, micro-joule range, supercontinuum (SC) spectrum which is generated in a large mode area (LMA) photonic crystal fiber (PCF). Our experiment consists of launching a train of 100 femtosecond pulses into a 20 cm-long span of a PCF delivered from a Ti:Sapphire pump parametric amplifier. The optical properties of the PCF were accurately calculated using a finite element method mode solver and the real cross section of the fiber. The PCF exhibits a zero dispersion wavelength at 1250 nm and has an effective area of 660 μm2 at λ=1250 nm. We observed an octave-spanning SC with spectral components propagating in the fundamental mode. The physical processes leading to the construction of the continuum spectrum were studied by monitoring the growth of the SC while increasing the input optical power. The main mechanisms behind the spectral broadening are mainly ruled by the effects of self-phase modulation, the stimulated Raman scattering, and the soliton propagation. Our experimental results are compared with the numerical solution of the nonlinear Schrodinger equation and good agreement between experimental and numerical results is found.

We report on experimental and theoretical investigation of birefringence of free-standing nanoporous anodic alumina membranes in the optical range. The value of birefringence is analyzed for the samples with different porosities by measuring polarization dependent transmission spectra at different angles of incidence. The experimental data are compared to the results of birefringence simulations in accordance with three simulation approaches: modified Bruggeman effective-medium approximation, Boundary conditions model, plane-wave expansion method. It is both experimentally and theoretically shown that birefringence value increases with porosity increasing in the low porosity region. The porous alumina samples under investigation possess greatest value of birefringence (0.062) up to date.

Progress was achieved in the last years in the development of multilayer mirrors used in storage ring Free Electron Lasers (FEL) operating in the vacuum ultraviolet spectral range. Based on dense oxide coatings deposited by Ion Beam Sputtering, a stable lasing at 190 nm was demonstrated. The extension towards shorter wavelengths had to overcome severe problems connected to the radiation resistance and the necessary reflectivity of the resonator mirrors. In this context, radiation resistance can be considered as the ability of the mirror materials to withstand the high power laser radiation and the intense energetic background radiation generated in the synchrotron source. The bombardment with high energetic photons leads to irreversible changes and a coloration on the specimen. Reflectivity requirements can be evaluated from the tolerable losses of FEL systems. At ELETTRA FEL the resonator mirror reflectivity must be above 95 %. Evaporated fluoride multilayer mirrors provide sufficient reflectivity, but they do not exhibit an adequate radiation resistance. Pure oxide multilayers show a sufficient radiation resistance, but they cannot reach the necessary reflectivity below 190 nm. A successful approach combines evaporated fluoride multilayer stack with a dense protection layer of silicon dioxide deposited by Ion Beam Sputtering. Such mirror systems were produced reaching a reflectivity of approximately 99 % at 180 nm. Lasing in the storage ring FEL at ELETTRA was realised in the range between 176 - 179 nm. The mirror reflectivity shows only a slight degradation after lasing, which could be fully restored after the lasing experiment.

Fluoride materials like Magnesium fluoride and Lanthanum fluoride exhibit unique properties for applications in mirror and anti reflecting coatings in the VUV spectral range (120- 230 nm). These large band gap materials provide low absorption and a usable refractive index contrast. Common deposition methods are thermal evaporation and electron beam evaporation. A columnar microcrystalline structure with a significant porosity is observed for such coatings. Furthermore, a high sensitivity for contamination processes resulting in an increasing absorption is often perceived. Investigations of mirror systems exposed to a harsh environment like the storage ring free electron laser at ELETTRA show a rapid degradation in respect to a reduced reflectivity, an increased hydrocarbon contamination, and a formation of colour centres. An improved performance of the fluoride coatings could possibly be expected for films, which do not have the polycrystalline columnar structure. Ion beam sputtering deposition of fluoride materials demonstrated its applicability to deposit dense amorphous fluoride coatings down to 193 nm. An IBS deposition plant with a Kaufman ion source, using a reactive fluorine environment, is used to grow fluoride layers with comparably low absorption values. Single and multilayer coatings were optically characterised after deposition, exposed to synchrotron radiation at ELETTRA, and characterized again after irradiation. However, the first set of irradiated multilayer mirrors showed a strong degradation of reflectivity and a strong hydrocarbon contamination. Colour centres were not observed.

In this letter we report experimental observations of transient UV‐induced frustration of the visible light transmission through Ge‐doped single mode fibers. Fiber side exposure to single nanosecond UV pulses at 266 and 248 nm is found to cause an abrupt decrease in transmission immediately following the pulse and a transmission recovery in microsecond time scale. Transmission changes were monitored at several visible wavelengths and for different types of Ge‐doped single mode fibers. The reported effect occurs independently of the growth and saturation of the permanent UV‐induced absorption reported by other authors.

Synchrotron radiation sources and free electron lasers (FEL) represent practical approaches to produce high photon flux down to the VUV wavelength region or the X-ray spectral range. But operating brilliant photon sources requires to overcome severe and crucial damage problems. Optical material are exposed to high power broad band radiation extending from far IR to gamma rays. Typically, materials suffer series of picoseconds pulses at MHz repetition rate with multi photon energy ranging from few eV to several keV. Damages observed on some oxide and fluoride materials irradiated with synchrotron radiation and UV free electron lasers are reviewed. Surface and bulk damages, increased roughness, modification of spectral properties and change of crystalline structures are observed and described. Contamination elements like carbon are detected as major aging factor.

By using a spatially dispersive resonator, three wavelength operation of Nd:Y3Al5O12 laser in pulsed Q‐switched regime at 1064, 1061, and 1052 nm was achieved. The laser output consisted of three pulses corresponding to the three generated wavelengths. The number of simultaneously emitted lines was increased by frequency doubling the laser output pulses in a potassium dihydrogen phosphate crystal.

Operation of pulsed Nd:YAG laser simultaneously at 1.052, 1.061, 1.064, and 1.074 μm is reported. The suppression of the weaker lines by the strongest line (1.064 μm) is avoided by using a cavity scheme which disperses the frequency components transversely in the crystal. The frequency doubling of this radiation in KDP provides several new lines.

An ultrabroadband (up to 35 nm FWHM) generation by a pulsed dye laser is reported. The principle of laser operation is based on a recently proposed cavity configuration, which we have termed “spatially-dispersive” resonator. By means of a prism pair this spreads the various spectral components into different spatial areas in the active medium and thus reduces the competition between them. A generation of a spectrum containing a number of equally spaced lines is also demonstrated.

It is shown, that the Fourier-processing based modulation of short laser pulses can be successfully used for all-optical image transfer. The proposed system contains a transmitter, where the transferred image modulates the frequency profile of the carrier pulse, and a receiver, where after the reverse processing is restored the original image. The main limitations are discussed. The transfer of binary amplitude and phase images is demonstrated.

This paper shows that the Fourier-processing based modulation of short laser pulses can be successfully used for all-optical image transfer. The system under consideration consists of a transmitter, where the transferred image modulates the frequency profile of a carrier pulse, and a receiver, where after the reverse processing an original image is restored. The main limitations are discussed and the information capacity of a single pulse is obtained. The results from preliminary experiments using a CPM-ring dye laser as a source of carrier pulses are presented.

We present a study of the optimum polarization states of the pump and signal beams in two-wave mixing amplification using BaTiO3 where the main objective is to improve the performance by reduction of the beam fanning. We demonstrate numerically and experimentally that at low (less than 50) pump-to-signal ratios it is advantageous to use both pump and signal in mixed polarization states with respect to the principal plane of the crystal.

We present theoretical and experimental studies of the process of two-wave mixing of broadband radiation in BaTiO3. We show that the energy exchange in such a process can be calculated on the basis of a system of coupled-wave equations in which phase-mismatch diffraction processes are taken into account. Good agreement of the model predictions and measurements performed with a variable-bandwidth Ti:sapphire laser is reported. Reasonable amplification of light with a FWHM spectrum of as much as 10 nm is reported in a standard copropagation scheme; above this value a considerable gain decrease and significant spatial profile changes occur.

We report the first generation of coherent, tunable, variable-polarization, soft X-ray femtosecond pulses, generated by a seeded free-electron laser (FEL) operating in the fresh bunch, two-stage harmonic upshift configuration. Characterization of the radiation proves this FEL configuration can produce single-transverse-mode, narrow-spectral-bandwidth output pulses of several tens of microjoules energy and low pulse-to-pulse wavelength jitter at final wavelengths of 10.8 nm and below. The fresh bunch configuration enhances the FEL emission at high harmonic orders by avoiding a gain depression due to the energy spread induced by the first-stage FEL interaction. Coherent signals measured down to 4.3 nm suggest this configuration is directly scalable to photon energies that will enable scientific investigations below the carbon K-edge, including access to the L-edges of many magnetic materials, with an energy per pulse unlocking the gate for experiments in the soft X-ray region with close to Fourier-transform-limited pulses. © 2013 Macmillan Publishers Limited. All rights reserved.