The spatial distribution and the temporal evolution of electrons in matter play a key role in defining the chemical and physical properties of atoms, molecules, and solids. By inspecting the behavior of electrons at a fundamental level, it comes out that their motion occurs on an extremely fast timescale, from a few-femtoseconds (1 fs= 10^-15 s) down to the attosecond domain (1 as= 10^-18 s). Ultrashort temporal probes are then required to grasp the electron processes.

Over the last few decades, the impressive progress in femtosecond laser technology has driven the development of advanced experimental methods for generating light bursts with attosecond temporal durations. In particular, the generation of attosecond pulses relies on a strongly non-linear process, the High-order Harmonic Generation process (HHG). Through HHG, a broadband spectrum of harmonics of the fundamental laser pulse frequency can be achieved, that covers the Extreme UltraViolet (EUV) up to the soft-X ray regions, supporting attosecond temporal durations. To date, a wide range of experimental techniques exploiting HHG-based EUV pulses has been demonstrated, including attosecond photoelectron spectroscopy, attosecond transient absorption, and reflectivity, high-order harmonic spectroscopy, etc., marking a promising route for facing the compelling challenge of understanding and controlling ultrafast electron dynamics.

Our research activities in the field of attosecond science cover the following areas:

Attosecond spectroscopy

Attosecond pulse (AP) generation marks a paramount achievement in ultrafast technology and is at the core of Attosecond Science. APs are obtained by temporally confining the HHG emission within an ultrashort temporal window, at a sub-optical cycle of the driving laser field, through optical gating schemes. In time-resolved experiments, APs are synchronized with another ultrashort optical pulse in a pump-probe scheme, for exciting and probing electron dynamics with a temporal resolution ultimately defined by the duration of the attosecond burst.

In our laboratories, APs are routinely generated and applied to the investigation of electron processes of fundamental interest for a wide number of fields. Specifically, the research is oriented toward the understanding and control of chemical properties, photo-induced reactions, and charge transfer processes in molecules and biologically relevant systems. Moreover, we exploit the attosecond pump-probe approach to explore fundamental physical mechanisms at the basis of charge carrier motion in materials appealing for the development of ultrafast optoelectronics and spintronic devices.

High harmonic spectroscopy

High Harmonic Spectroscopy (HHS) allows studying fundamental processes occurring on time scales from tens of femtoseconds to hundreds of attoseconds by exploiting the intrinsically ultrafast electronic nature of the HHG process. HHG originates from the quiver motion of electrons in matter driven by an ultrashort external laser field. The electron oscillations are associated with a time-dependent emission of coherent EUV and Soft-X photons. In particular, each photon energy of the harmonic spectrum can be associated with a precise clocking in the time frame of the external laser field. For this reason, HHG provides a self-probing time-resolved spectroscopy technique for the generation medium. Additionally, the harmonics spectrum encodes spatial information on the system’s electronic structure.

In our laboratories, we apply HHS for exploring complex electronic and molecular dynamics excited by strong-laser fields, including electron-hole dynamics, collective electron dynamics, charge dynamics upon molecular vibrations, and atomic rearrangements in the formation or breaking of chemical bonds.

New sources of EUV and X-ray pulses

The successful application of HHG-based EUV and Soft-X sources to ultrafast spectroscopy and X-ray science has been widely demonstrated but its exploitation is significantly hindered, even today, by the technological complexity of the required setups and the low generation efficiency, particularly when moving towards higher photon energies. This limit underlies the challenging goal of exploring novel approaches to enhance the application perspectives of these sources.

Within our facilities, we are currently working on the development of innovative miniaturized microfluidic platforms for EUV generation and manipulation. The microfluidic approach offers the unique advantage of enabling accurate control and tailoring of the harmonic generation conditions at a micrometer scale. Moreover, following this route, we foresee the potential for integrating multiple optical functionalities within a single monolithic chip, thus paving the way to a novel generation of palm-top-size EUV and X-ray experimental stations.