Attosecond technology, enabling the generation of attosecond light pulses, is the core of Attosecond Science; it exploits an extremely nonlinear optical effect called High-order Harmonic Generation (HHG).
When an intense and short laser pulse is focused in any material, the electric field component of light strongly bends the atomic potential well that traps the outermost electrons. These electrons escape through the potential barrier – a purely quantum effect called tunnel photoionization – and continue their evolution driven by the oscillating electric field. As soon as this field reverses, the freed electrons are driven back towards the parent ions; those electrons that recombine release their excess energy in a light burst that lasts few tens to few hundreds of attoseconds.
Even in case of ultrashort laser pulses, several attosecond light bursts are emitted by the driven atoms during the laser pulse; this leads to the generation of a periodic train of attosecond pulses in the temporal domain that appears in the spectral domain as a comb of numerous harmonics of the impinging laser light (hence the term HHG).
Numerous techniques have been conceived to generate an isolated attosecond pulse despite the longer duration of the driving laser pulse; most of them, termed temporal gating techniques, are based on the manipulation of the laser pulse’ electric field.
A stringent requirement for the generation of reliable and stable attosecond pulses is that the driving laser source provides pulses with a stable Carrier-Envelope Phase offset; this condition ensures that the electric field of the pulses, rather than the envelope, is reproducible and leads always to the same electron ionization and recollision dynamics.
Until the ‘90s this aspect was completely neglected in ultrafast laser technology; once taken into consideration, a novel class of laser sources emerged with applications in very diverse fields like, for instance, metrology with optical frequency combs.