D. Charalambidis

Laser-induced field-free alignment of the OCS molecule

We investigate the dynamical alignment of jet-cooled OCS molecules induced by a short laser pulse. The alignment is measured through the orientational contribution of the optical Kerr effect using a second weak laser pulse as a probe. Maximum alignment is observed at conditions close to saturation of ionization. The results are analysed with a quantum mechanical model solving for the rotational dynamics.

Direct two-XUV-photon double ionization in xenon

We report the observation of the direct two-XUV-photon double ionization of xenon by energetic coherent XUV continuum radiation. The spectrum of the XUV continuum spans from ?15 to ?23 eV, for which the two-photon sequential double ionization channel is partially open. The two-XUV-photon process is exploited in a second-order autocorrelation measurement of the temporal width of the continuum radiation, revealing energetic XUV pulses at the border between the atto- and femto-second scales.

Multiphoton ionization saturation intensities and generalized cross sections from ATI spectra

A simple method for the determination of saturation intensities and in some cases generalized cross sections in multiphoton ionization is presented. It utilizes the dependence of the ponderomotive shift on the laser intensity above the saturation limit. An application to He and Ar interacting with 500 fs pulses at 248 nm is demonstrated. Experimental results are compared with theoretical calculations.

Observation of two-XUV-photon ionization using harmonic generation from a short, intense laser pulse

We report the observation of a two-photon ionization process in the XUV wavelength regime. In a near-resonant 1 + 1 ionization scheme, Ar atoms are ionized absorbing the 15 eV third harmonic photons produced in a gas jet by the 0.5 ps intense laser pulses of a KrF excimer laser emitting at 248.6 nm. The present demonstration of a non-linear process in the XUV regime reveals feasibility of high-intensity applications utilizing the uniquely high peak power of non-conventional short wavelength radiation sources based on harmonic generation.

Two-XUV-Photon Processes: A Key Instrument in Attosecond Pulse Metrology and Time Domain Applications

Attosecond pulses today are generated at pulse energies leading to intensities sufficient to induce two-photon transitions in the extreme ultraviolet (XUV) spectral region. Recently, ultra-broadband coherent XUV continua fulfill also the requirements in inducing such processes. Two-XUV-photon ionization is a pivotal tool in attosecond pulse metrology, as well as for XUV-pump-XUV-probe applications targeting the tracking of ultrafast dynamics, providing at the same time spatial selectivity.Based on these developments, this chapter (a) reviews approaches leading to high intensities of attosecond pulse trains and coherent XUV continua; (b) reviews metrology approaches based on two-XUV-photon ionization, showing their importance through comparative studies with existing XUV-IR cross-correlation approaches; and (c) reports the feasibility of XUV-pump-XUV-probe applications at the 1fs resolution level, in an experiment, where atomic coherence is induced in a rich manifold of doubly excited and inner-shell excited autoionizing states, the evolution of which is tracked through double ionization.

On the Generation of Intense Isolated Attosecond Pulses by Many-Cycle Laser Fields

Real-time observation of ultrafast dynamics in all states of matter requires temporal resolution on the atomic unit of time (24.189 asec) (\(1\,\mathrm{asec} = 1{0}^{-18}\,\mathrm{s}\)). Tools for tracking such ultrafast dynamics are ultrashort light pulses. During the last decade, continuous efforts in ultrashort pulse engineering led to the development of light pulses width duration close to the atomic unit of time. Attosecond (asec) pulses have been synthesized by broadband coherent extreme ultraviolet (XUV) radiation generated by the interaction of gases or solids with an intense IR fs pulse. Asec pulse trains can be generated when the medium interacts with many-cycle driving IR fs laser fields. In this case, a broadband XUV frequency comb is emitted from the medium. The Fourier synthesis of a part of the comb results in an asec pulse train. Isolated asec pulses are generated when the medium is forced to emit XUV radiation only during few cycles of the driving laser field. This leads to the emission of a broadband quasicontinuum XUV radiation. The Fourier synthesis of the continuum part of the spectrum results in an isolated asec pulse. For the realization of studies of ultrafast dynamics, intense asec pulses are preferable. If the pulses are intense enough to induce a nonlinear process in a target system, they can be used for ultrafast dynamic studies in an XUV pump-probe configuration. Although trains of intense asec pulses are commonly produced nowadays, the generation of intense isolated asec pulses remains a challenge. Here, we review a recently developed approach for the generation of intense asec pulses using high-peak-power many-cycle laser fields. The approach is based on controlling, with asec precession, the response of the atomic dipole to an external many-cycle driving field in such a way as to emit an isolated asec XUV burst. This approach has been implemented by using the inteferometric polarization gating (IPG) technique. The bandwidth of the generated XUV radiation is large enough to enable the synthesis of isolated XUV pulses with durations of a few hundred asec. The technique paves the way for the generation of intense isolated asec pulses, tuneable in duration and frequency, for carrier-envelope phase (CEP) variation studies of many-cycle driving fields, and it offers exciting opportunities for multiphoton XUV-pump-XUV-probe experiments.

XUV Attosecond Pulses: Generation, Metrology and Potential Applications

We report on the implementation and detailed assessment of the first 2nd order autocorrelation measurement of an XUV attosecond pulse train. Spatiotemporal effects on the generated radiation and measured quantities have been evaluated.

Determination of higher-order harmonic generation `cut-offs' through high-resolution time-domain spectroscopy

A time-resolved spectroscopic technique, employing two time-delayed 60 fs long laser pulses, has been used for spectroscopic studies in higher-order harmonic generation (HOHG) in the gas phase. The frequency spectrum of the coherent components of the measured XUV radiation is obtained through the Fourier transform of the measured temporal traces. The cut-off of the HOHG has been determined for three different noble gases at several laser intensities. The cut-off is found to follow its well known theoretical formula.