Photoemission-time-delay measurements and calculations close to the 3s-ionization-cross-section minimum in Ar

We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s -> kp channel lead to an induced "Cooper" minimum in the 3s ionization cross section

Probing Electron Correlation on the Attosecond Timescale

This thesis describes how photoemission stimulated by an attosecond pulse train (APT) can be used to extract information on electron correlation in simple quantum systems such as atoms. The emission of the electron by an APT induces a reorganization of the electrons remaining in the ion core. This reorganization causes a change in the trajectory of the emitted electron. An infrared (IR) field is used to probe the delay induced by the ion's potential and the electron's reorganization via an interferometric technique. This thesis focuses on how the delay can be measured in various atomic systems and on how physical information about the electron correlations may be extracted. The first chapter of the thesis presents a brief overview of the attosecond techniques which have been used. It describes how the APTs are generated via a non-linear process called High Order Harmonic Generation (HHG), and how these pulses are characterized temporally using the so-called RABITT technique (reconstruction of attosecond beating by interference in a two-photon transitions). Finally, the different parts of the experimental set-up are described: the laser system, the APT generation chamber and the different detectors. The second part focuses on a theoretical description of the photo-ionization process. The delays measured in the RABITT technique are derived and interpreted using perturbation theory. The influence of electron correlation on the delay is then investigated in the case of a Fano resonance and in double photoionization. The third chapter describes experimental results obtained in various atomic systems. A comparison is made between the photoemission delays from the outer valence shells of argon, neon, and helium; between the inner and outer valence shells of argon; between the on-resonance and off-resonance delays for argon levels interacting in a Fano resonance; and between the delay induced by single and double photoionization in xenon. The experimental results are compared with calculations using several different atomic codes

A novel concept of compact, snapshot hyperspectral camera for ophthalmology.

Hyperspectral imaging is an emerging technique that allows to measure the spectral absorption at each point of a scene, thus offering capability to identify and characterize biomarkers important for clinical practice and therapeutic research as well as enhancing image identification of important structures. So far, few hyperspectral cameras have been used for retinal scanning because of the need to acquire the image in a fraction of a second. Here we present a novel concept of snapshot hyperspectral camera suited for retinal imaging. We demonstrate the technique by presenting the optical density spectrum of a healthy patient’s retina in the 450-700 nm range, together with the spectral response of several retinal features

Effects of pulse chirp on laser-driven proton acceleration

Optimisation and reproducibility of beams of protons accelerated from laser-solid interactions require accurate control of a wide set of variables, concerning both the laser pulse and the target. Among the former ones, the chirp and temporal shape of the pulse reaching the experimental area may vary because of spectral phase modulations acquired along the laser system and beam transport. Here, we present an experimental study where we investigate the influence of the laser pulse chirp on proton acceleration from ultrathin flat foils (10 and 100 nm thickness), while minimising any asymmetry in the pulse temporal shape. The results show a ± 10 % change in the maximum proton energy depending on the sign of the chirp. This effect is most noticeable from 10 nm-thick target foils, suggesting a chirp-dependent influence of relativistic transparency

Attosecond pulse manipulations with XUV mirrors

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Low-divergence femtosecond X-ray pulses from a passive plasma lens

Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy. X-ray pulses with low divergences are produced in a laser-wakefield accelerator by focusing electron bunches in a dense passive plasma lens

Secondary electron imaging of nanostructures using Extreme Ultra-Violet attosecond pulse trains and Infra-Red femtosecond pulses

Surface electron dynamics unfold at time and length scales down to attoseconds and nanometres, making direct imaging with extreme spatiotemporal resolution highly desirable. However, this has turned out to be a major challenge even with the advent of reliable attosecond light sources. In this paper, photoelectrons from Ag nanowires and nanoparticles excited by extreme ultraviolet (XUV) attosecond pulse trains and infrared femtosecond pulses using a PhotoEmission Electron Microscope (PEEM) are imaged. In addition, the samples were investigated using Scanning Electron Microscopy (SEM) and synchrotron based X-ray photoelectron spectroscopy (XPS). To achieve contrast between the nanostructures and the substrate in the XUV images, three different substrate materials were investigated: Cr, ITO and Au. While plasmonic field enhancement can be observed on all three substrates, only on Au substrates do the Ag nanowires appear significantly brighter than the substrate in XUV-PEEM imaging. 3-photon photoemission imaging of plasmonic hot-spots was performed where the autocorrelation trace is observed in the interference signal between two femtosecond Infra-Red (IR) beams with sub-cycle precision. Finally, using Monte Carlo simulations, it is shown how the secondary electrons imaged in the XUV PEEM can potentially reveal information on the attosecond time scale from the near surface region of the nanostructures

spatio-temporal manipulation of attosecond pulses with mirrors

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Multi-purpose two- and three-dimensional momentum imaging of charged particles for attosecond experiments at 1 kHz repetition rate.

We report on the versatile design and operation of a two-sided spectrometer for the imaging of charged-particle momenta in two dimensions (2D) and three dimensions (3D). The benefits of 3D detection are to discern particles of different mass and to study correlations between fragments from multi-ionization processes, while 2D detectors are more efficient for single-ionization applications. Combining these detector types in one instrument allows us to detect positive and negative particles simultaneously and to reduce acquisition times by using the 2D detector at a higher ionization rate when the third dimension is not required. The combined access to electronic and nuclear dynamics available when both sides are used together is important for studying photoreactions in samples of increasing complexity. The possibilities and limitations of 3D momentum imaging of electrons or ions in the same spectrometer geometry are investigated analytically and three different modes of operation demonstrated experimentally, with infrared or extreme ultraviolet light and an atomic/molecular beam