Maurizio Reduzzi

Coherent control with a short-wavelength free-electron laser

Researchers demonstrate correlation of two colours (63.0 and 31.5 nm wavelengths) in a free-electron laser and control photoelectron angular distribution by adjusting phase with 3 attosecond resolution.

Observation of autoionization dynamics and sub-cycle quantum beating in electronic molecular wave packets

The coherent interaction with ultrashort light pulses is a powerful strategy for monitoring and controlling the dynamics of wave packets in all states of matter. As light presents an oscillation period of a few femtoseconds (T = 2.6 fs in the near infrared spectral range), an external optical field can induce changes in a medium on the sub-cycle timescale, i.e. in a few hundred attoseconds. In this work, we resolve the dynamics of autoionizing states on the femtosecond timescale and observe the sub-cycle evolution of a coherent electronic wave packet in a diatomic molecule, exploiting a tunable ultrashort extreme ultraviolet pulse and a synchronized infrared field. The experimental observations are based on measuring the variations of the extreme ultraviolet radiation transmitted through the molecular gas. The different mechanisms contributing to the wave packet dynamics are investigated through theoretical simulations and a simple three level model. The method is general and can be extended to the investigation of more complex systems.

Complete analog control of the carrier-envelope-phase of a high-power laser amplifier

In this work we demonstrate the development of a complete analog feedback loop for the control of the carrier-envelope phase (CEP) of a high-average power (20 W) laser operating at 10 kHz repetition rate. The proposed method combines a detection scheme working on a single-shot basis at the full-repetition-rate of the laser system with a fast actuator based either on an acousto-optic or on an electro-optic crystal. The feedback loop is used to correct the CEP fluctuations introduced by the amplification process demonstrating a CEP residual noise of 320 mrad measured on a single-shot basis. The comparison with a feedback loop operating at a lower sampling rate indicates an improvement up to 45% in the residual noise. The measurement of the CEP drift for different integration times clearly evidences the importance of the single-shot characterization of the residual CEP drift. The demonstrated scheme could be efficiently applied for systems approaching the 100 kHz repetition rate regime.

Generating high-contrast, near single-cycle waveforms with third-order dispersion compensation

Femtosecond laser pulses lasting only a few optical periods hold the potential for probing and manipulating the electronic degrees of freedom within matter. However, the generation of high-contrast, few-cycle pulses in the high power limit still remains nontrivial. In this Letter, we present the application of ammonium dihydrogen phosphate (ADP) as an optical medium for compensating for the higher-order dispersion of a carrier-envelope stable few-cycle waveform centered at 735 nm. The ADP crystal is capable of removing the residual third-order dispersion present in the spectral phase of an input pulse, resulting in near-transform-limited 2.9 fs pulses lasting only 1.2 optical cycles in duration. By utilizing these high-contrast, few-cycle pulses for high-harmonic generation, we are able to produce nanojoule-scale, isolated attosecond pulses.

Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters

Ne clusters (∼5000  atoms) were resonantly excited (2p→3s) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Two-photon resonant excitation of interatomic coulombic decay in neon dimers

The recent availability of intense and ultrashort extreme ultraviolet sources opens up the possibility of investigating ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by ab initio electronic structure and nuclear dynamics calculations.

Attosecond absorption spectroscopy in molecules

We present results on attosecond transient absorption in small molecules. Ultrafast relaxation dynamics in nitrogen can be temporally resolved by combining an isolated attosecond pulse and an intense synchronized carrier-envelope-phase stable infrared pulse. The lifetime of the different Fano resonances can be retrieved.

Attosecond electronic recollision as field detector

We demonstrate the complete reconstruction of the electric field of visible-infrared pulses with energy as low as a few tens of nanojoules. The technique allows for the reconstruction of the instantaneous electric field vector direction and magnitude, thus giving access to the characterisation of pulses with an arbitrary time-dependent polarisation state. The technique combines extreme ultraviolet interferometry with the generation of isolated attosecond pulses.

Complete photoionization experiment and autoionizing states in Ne II

The interaction of intense extreme ultraviolet (XUV) pulses with an atom can lead to multi-photon absorption and multiple ionization of the target. By using intense, linearly and circularly polarized XUV pulses of the Free Electron Laser (FEL) FERMI [1], we realized the first experimental demonstration of a quantum mechanically complete experiment (CE) in an ionic system. The quest for a CE, as the strongest test of theory and as minimal source of information from which any observable can be predicted, was first formulated at the end of the 1960s, [2], for electron-atom scattering, and then extended to atomic and molecular photoionization (PI). The complete information cannot be obtained by measuring only the photo-electron angular distribution (PAD) and the cross sections, but additional variables are required. Thus, CEs in atomic PI were realized by measuring the angle-resolved PAD, combined with the observation of other variables or, in case of atoms with open shells, by controlling the initial polarization of the target. All the CEs on PI so far have been performed with neutral targets. As atomic PI leads to polarized residual ions, CE in positive ions can be realized within the two photon double sequential ionization (TPDI) process, now available thanks to the intense XUV pulses of FELs.

Vectorial optical field reconstruction by attosecond spectral interferometry

An electrical pulse is completely defined by its time-dependent amplitude, phase and polarization state. For optical and near-infrared pulses the manipulation and characterization of the last one is fundamental due to its relevance in several scientific and technological fields. Although the complete characterization of optical waveform has been already demonstrated [1, 2], a technique both capable to fully characterize also weak probe pulses, with energy in the 10–100nJ, and, at the same time, free of systematic distortions, would be highly desirable. In this work we report on new theoretical and experimental results to demonstrate a novel approach for the complete characterization of the electric field of an optical pulse. Our method is based on the combination of two elements: the implementation of extreme ultraviolet (XUV) interferometry [3], with time resolution in the attosecond domain, and the demonstration that the motion of an attosecond electronic wave packet, created by an intense laser pulse, allows to sample an unknown electric field along a controllable, fixed direction. Combining these elements, we demonstrate the full reconstruction of electric fields with intensities as low as I∼109 W/cm2 and with a generic time-dependent polarization state, with an all-optical method.