Light at the extremes: From femto- to atto-science for real-time studies of atomic and electronic motions

The progress in the generation of ultrashort pulses has continuously triggered the introduction of new spectroscopic and measurement techniques which offer the opportunity to investigate unexplored research areas with unprecedented time resolution. Few-optical cycle pulses tunable from near-infrared to visible-UV allow to shed light on ultrafast electronic relaxation processes and to achieve real-time detection of molecular vibrations and structural dynamics. High-energy few-optical cycle pulses allow the efficient production of high-order harmonics up to XUV spectral region, leading to the generation of attosecond pulses shedding light on electron wave packet dynamics in complex molecules. perspective Copyright c © EPLA, 2015 Introduction. – Many studies of light-matter interaction require optical pulses with ultrashort duration as well as broad frequency tunability. Both issues are of utmost importance for time-resolved optical spectroscopy and high-field physics. Several technological steps were required to advance ultrafast science from the nanosecond time scale to the femtosecond regime, and eventually to the attosecond regime. The laser provided the basis for all these advances, thanks to its ability to emit perfectly coherent light waves. In parallel, the birth of non-linear optics with the discovery of perturbative nonlinear effects [1,2] allowed to change the optical properties of the materials as a function of the laser radiation intensity. These effects provided light modulators able to vary the phase and/or amplitude of transmitted or reflected light in proportion to its intensity. Insertion of such devices in a laser oscillator allowed the eigenmodes of the laser cavity to be phase locked, turning the laser output into a regular train of light pulses with a duration inversely proportional to the laser bandwidth [3]. The continuous research effort in the development of broadband laser oscillators (a)E-mail: [email protected] (b)E-mail: [email protected] (c)E-mail: [email protected] has allowed the generation of femtosecond pulses down to the few-optical-cycle regime [4,5] thanks to the Kerr-lens mode-locking technique [6] and to the use of chirped mirrors for dispersion control inside the laser cavity [7]. In parallel the invention of the chirped-pulse amplification technique has allowed to increase by orders of magnitude the pulse energy [8]. Nowadays, mainstream ultrashort pulse generation technology is based on Ti:sapphire oscillators operating in Kerr-lens mode-locking followed by chirped-pulse amplification, which provides highly stable and energetic femtosecond pulses. However, the frequency tunability of such sources is limited in a narrow range around the fundamental frequency (800 nm) or its second harmonic. Despite this limitation, the very high peak power and stability level of these sources enabled exploiting a series of non-linear optical effects either in the perturbative and non-perturbative regime generating an ensemble of “secondary sources” able to provide femtosecond light pulses with unprecedented tunability and duration and to enter the attosecond time regime. Femtosecond spectroscopy nowadays is essentially based on the use of broadband optical parametric amplifiers (OPAs) as secondary sources, which exploit second-order perturbative non-linear optical effects. The principle of OPA is quite simple: in a suitable non-linear crystal,