We present the first measurement of the vectorial response of strongly dressed helium atoms probed by an attosecond pulse train (APT) polarised either parallel or perpendicular to the dressing field polarisation. The transient absorption is probed as a function of delay between the APT and the linearly polarised 800 nm field of peak intensity 1.3 x 10(14) W cm(-2). The APT spans the photon energy range 16-42 eV, covering the first ionisation energy of helium (24.59 eV). With parallel polarised dressing and probing fields, we observe modulations with periods of one half and one quarter of the dressing field period. When the polarisation of the dressing field is altered from parallel to perpendicular with respect to the APT polarisation we observe a large suppression in the modulation depth of the above ionisation threshold absorption. In addition to this we present the intensity dependence of the harmonic modulation depth as a function of delay between the dressing and probe fields, with dressing field peak intensities ranging from 2 x 10(12) to 2 x 10(14) W cm(-2). We compare our experimental results with a full-dimensional solution of the single-atom time-dependent (TD) Schrodinger equation obtained using the recently developed abinitio TDB-spline ADC method and find good qualitative agreement for the above threshold harmonics.

We present transient absorption spectra of an extreme ultraviolet attosecond pulse train in helium dressed by an 800 nm laser field with intensity ranging from $2\times10^{12}$ W/cm$^2$ to $2\times10^{14}$ W/cm$^2$. The energy range probed spans 16-42 eV, straddling the first ionisation energy of helium (24.59 eV). By changing the relative polarisation of the dressing field with respect to the attosecond pulse train polarisation we observe a large change in the modulation of the absorption reflecting the vectorial response to the dressing field. With parallel polarized dressing and probing fields, we observe significant modulations with periods of one half and one quarter of the dressing field period. With perpendicularly polarized dressing and probing fields, the modulations of the harmonics above the ionisation threshold are significantly suppressed. A full-dimensionality solution of the single-atom time-dependent Schr\"odinger equation obtained using the recently developed ab-initio time-dependent B-spline ADC method reproduce some of our observations.

We present a technical review of a beamline built to perform pump-probe experiments with a temporal resolution of < 200 attosecond. This is designed specifically to use the technique of attosecond transient absorption spectroscopy (ATAS) to resolve ultra-fast electron dynamics in atoms and molecules. A non-collinear, interferometrically stable geometry is adopted to allow us to individually control the characteristics of each of the pump and probe arms independent from each other. With the use of an auxiliary interferometer to correct for long-term drifts between the pump and probe arms we measure better than 150as resolution for our time-corrected delay despite having separated beam paths of over 4m in length. In our first experiment we have focused on the time dependence of the electronic states of an atom in a strong laser field. An extreme ultra-violet (XUV) attosecond pulse train (APT) and a precisely synchronized 30fs IR pulse are used in this work. Delay-dependent absorption modulations are observed at even multiples (2 and 4) of the IR dressing field frequency as the pump-probe delay is scanned. We investigate the dependence of the 2ω order absorption modulation amplitude from the transient absorption of laser-dressed helium as the IR dressing field ellipticity is varied, and we discuss the issues in obtaining such results. We present qualitative data indicating a clear anisotropy in the response of the atom to an IR dressing field, and discuss how we will improve this measurement in future experiments.

We report on our recent development of a femtosecond infrared laser system based on self-phase-stabilized seed by difference-frequency generation of supercontinuum obtained with so-called hollow fiber compression technique and two-stage optical parametric amplification. After the final amplifier, we obtained pulses with duration of < 40 fs, tunable center wavelength from 1.2 to 1.8 μm, maximum pulse energy of > 500 μJ and as a first demonstration of this new laser system, we generated high-harmonics up to 47th order in a tube target filled with Xe gas.