Spatio-temporal couplings (STC) of laser beams are ubiquitous in ultrafast optics. In the femtosecond range, chirped-pulse amplification (CPA), the key technology of amplified ultrashort pulses, relies on the use of massive STCs induced at different locations in laser systems (for instance by gratings or prisms), which should all eventually perfectly cancel out at the laser output. Residual STCs, for example resulting from imperfect compensation, decrease the peak intensity at focus by increasing both the focal spot size and the pulse duration. This is particularly detrimental for ultrahigh-intensity (UHI) lasers, which aim for the highest possible peak intensities. However, it is precisely with these lasers that such uncontrolled defects are most likely to occur, due to the complexity of these systems and the large diameters of the output beams.
Accurately measuring STCs is thus essential in ultrafast optics. Significant progress has been made in the last decade, and several techniques are now available for the partial or complete spatiotemporal characterization of near-visible femtosecond laser beams. However, none of these has yet been applied to UHI femtosecond lasers, due to the difficulty of handling these large and powerful beams. As a result, all UHI lasers are currently characterized under the unjustified and unverified assumption of the absence of STCs, using separate measurements in space and time.
This situation is now becoming a major bottleneck for the development of UHI lasers and their applications. In particular, the optimal and reliable operation of PW-class lasers now available or under construction all around the world will simply not be possible without a proper spatiotemporal metrology. In this talk, we present the first complete spatiotemporal experimental reconstruction of the field E(t,r) for a 100 TW peak-power laser, obtained using self-referenced spatially-resolved Fourier transform spectroscopy [1,2], and thus reveal the spatiotemporal distortions that can affect such beams . This new measurement capability opens the way to in-depth characterization and optimization of ultra-intense lasers and ultimately to the advanced control of relativistic motion of matter with femtosecond laser beams structured in space–time.
In this letter, we propose two techniques capable of spatio-temporally characterizing high-power femtosecond laser chains. We demonstrate a new implementation of SEA TADPOLE. To avoid the problems induced by the the significant spatial jittering of the focal spot on high-power laser chains, our setup is adapted to collimated beams. In addition, a fibered light source is also used to correct the phase fluctuations. This experimental setup allows identifying any spatiotemporal distortions such as the pulse front tilt for instance. In this paper, to the best of our knowledge, we present the very first spatio-temporal characterization done on a TW laser. However, a SEA TADPOLE measurement is not immediate since it requires scanning the beam over the two transverse dimensions which prevent us from studying the shot-to-shot laser fluctuations. This is why, we developed MUFFIN, a single-shot technique capable of spatio-temporally characterizing a laser pulse along its two transverse dimensions. First experimental results obtained with this technique are presented here.
When an intense ultrashort laser pulse impinges on an initially-solid target, it creates a dense plasma at the surface,
which reflects a large fraction of the incident light. At high enough intensities, high-order harmonics of the incident laser
frequency, associated in the time domain to trains of attosecond pulses, are generated in the light beam specularly
reflected by this "plasma mirror". The mechanisms leading to this generation are now relatively well-established, and the
first experimental evidence for attosecond pulses generated on plasma mirrors has recently been reported. An accurate
characterization of the temporal structure of the light reflected by plasma mirrors, down to the attosecond scale, however
remains an experimental challenge. In this paper, we describe three different methods that could be used for such
temporal measurements, from the femtosecond to the attosecond time scale. Two of them are interferometric techniques
which only require measurements of photons, while the third one is a new configuration of a now well-established
method, developed for attosecond pulses generated in gases, and based on photoelectron spectroscopy.
We report the experimental study of the ultra-fast modification of the dielectric function of pure water by an intense
femtosecond laser pulse. Using a time-resolved optical interferometric technique, we measured the variation of the
phase shift, which is proportional to the modification of the real part of the refractive index, as well as the variation of
the fringes contrast, proportional to the modification of the absorption coefficient. We first observe a positive phase
shift due to Kerr effect and immediately followed a negative one. After 200 fs, the phase shift becomes positive and
remains so for at least 3 ps. Using the simple Drude -Lorentz model, we interpret this evolution as the result of
We report the experimental study of the ultra-fast modification of the dielectric function of pure water by an intense femtosecond laser pulse. Using a time-resolved optical interferometric technique, we measured the variation of the phase shift, which is proportional to the modification of the real part of the refractive index, as well as the variation of the fringes contrast, proportional to the modification of the absorption coefficient. We first observe a positive phase shift due to Kerr effect and immediately followed a negative one. After 200 fs, the phase shift becomes positive and
remains so for at least 3 ps. Using the simple Drude - Lorentz model, we interpret this evolution as the result of electron self-trapping.
The interaction of intense femtosecond laser pulse with model samples containing gold nanoparticales embedded in dielectrics is studied in order to understand the role played by nanodefects in optical breakdown of dielectrics. A theoretical study of the conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conuction band of the dielectric, which leads to a strong local increase of the optical
absorption in the initially transparent matrix. This prediction is tested experimentally by using time -resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica and samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
By focusing an intense femtosecond, high temporal contrast, laser on ultra-thin foils (100 nm) in the 10<sup>18</sup>W/cm<sup>2</sup> intensities range, we demonstrate that we create instantaneously a hot solid-density plasma. The use of highorder harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate in such media, enables to study the dynamics of this plasma on the picosecond time-scale. The comparison of the transmission of two successive harmonics permits to determine the electronic density and the temperature with an accuracy better than 15% never achieved up to date in relativistic regimes.
In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
We investigate the harmonics generation from a pure dielectric target when submitted to laser intensities in the 10<sup>18</sup><i>W/cm</i><sup>2</sup>. We demonstrate the negative influence of the prepulses and ASE by addressing the direct comparison of the harmonic spectra with and without the introduction of a perfectly controlled plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free of any plasma expansion.
Interaction of a cluster jet with a very intense laser pulse generates x-rays in the keV domain. Many features of this type of source are well characterized and understood but their temporal structure is still being discussed. We performed experiments on photoelectron emission of a pure copper metallic sample irradiated by such a source of x-rays. We determined the best laser and gas jet parameters in order to enhance the photoemission yield. The maximum signal was obtained with 30 bars of xenon irradiated by 250 fs 75 mJ pulse at 800 nm, corresponding to an intensity of 2.10<sup>16</sup> W/cm<sup>2</sup>. We were able to observe a peak of Auger electrons at 62 eV in the photoelectron spectrum. This suggests that pulse duration measurements based on the Laser Assisted Auger Decay (LAAD) technique are possible.
When an atom is ionized by an x-ray pulse in the presence of a laser field,the drift velocity of photoelectrons shows the phase dependence on the dressing field.We show how to use this effect to characterize single attosecond x-ray pulses.(i)Attosecond streak camera - the distortion of the photoelectron spectra induced by the laser field is used to map the temporal shape of the x-ray pulse to the photoelectron spectra.(ii) Attosecond SPIDER (spectral phase interferometry for direct electric-field reconstruction) - the spectral shearing interferometry of photoelectrons is used to directly retrieve the spectral phase of the x-ray pulse from the photoelectron spectra.
In this paper, we present measurements of the excited carrier density in various wide band gap oxides irradiated by short laser pulses, at intensities below and above breakdown threshold. This is achieved with the help of time resolved interferometry in the frequency domain, a technique which was successfully used to study the dynamics of photoexcited carriers in insulators. The result obtained in different experimental conditions, distance from the surface, pump intensities and duration, during or after the pump pulse, are discussed and compared to the models recently developed to explain optical breakdown.