When a femtosecond laser pulse is focused in a gaseous medium at intensities around 10¹³-10¹⁴ W/cm², several tens to hundreds of odd harmonics of the laser frequency can be emitted. This process, known as High-order Harmonic Generation (HHG) was discovered in the early 90's. High harmonic sources have unique properties: they produce bright coherent radiation in the VUV-XUV spectral domain, with pulse durations in the femtosecond or attosecond range.

High harmonic spectrum generated in argon by a 1800 nm laser pulse

The mechanism responsible for the harmonic emission is a three-step process: (1) under the influence of the strong laser field, the Coulomb potential barrier is lowered so that tunnel ionization can occur, releasing an attosecond electron wavepacket in the continuum; (2) the freed electron wavepacket is accelerated by the laser field and can be driven back to the ionic core when the field reverses; (3) it can recombine with the core, emitting a photon whose energy is set by the ionization potential Ip of the considered species and the kinetic energy gained by the electron during step (2).

High harmonic generation can be used in two ways to study molecular dynamics. The most strightforward is to consider it as a source of femtosecond pulses in the VUV which can be used to photoionize excited molecules (fs-VUV Spectroscopy). Alternatively, the high-harmonic generation process itself can be used as a probe of the structure of the emitting medium (ENLOS).