Ultrafast Science
A brief history
Too fast to be detected
As a photographer that “freezes” a fast object by taking a picture with a short and bright flash, time-resolved spectroscopy probes matter with ultrashort bunches of photons and detects how it responds on the same time scale.
Nuclei move on temporal scales of tens to hundreds of femtoseconds (1 femtosecond = 10-15 s) hence their evolution can be captured using femtosecond light pulses. Laser sources providing such pulses exist since the ‘80s and enabled the development of femtochemistry, i.e. the discipline studying chemical reactions on their natural temporal scale.
However, the other constituents of matter – electrons – are thousand-times lighter and faster than nuclei.
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) thus eluding femtosecond spectroscopic tools.
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.
Attosecond science
Disclosing the intimate mechanisms of matter
Only on the verge of the new millennium novel optical techniques came to the foreground, enabling the generation of attosecond bursts of extreme ultraviolet light. These tools enabled a novel spectroscopic discipline, Attosecond Science.
Nowadays Attosecond Science discloses the intimate mechanisms of matter by elucidating how electrons behave on their own temporal scale inside atoms, molecules, and condensed matter.
High Harmonic Generation process
Getting faster every day
Over the last few decades, the impressive progress in the 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.