High harmonic generation (HHG) refers to a nonlinear optical process in which intense laser light interacts with a gaseous medium, typically an atom or a molecule, to produce harmonics of the incident laser frequency. The harmonics generated in this process have frequencies that are multiples of the original laser frequency, and they are in the extreme ultraviolet (XUV) or soft x-ray range of the electromagnetic spectrum.
The HHG process usually involves a strong laser field ionizing the atoms or molecules in a gas. Electrons liberated in this process are accelerated by the laser field and then driven back towards the ionized atom or molecule. When these electrons recombine with the ion, they can emit high-energy photons in the form of harmonic frequencies that are much higher than the original laser frequency. The emitted harmonics can extend into the XUV or even the x-ray region, providing a unique and powerful source of short-wavelength light.
High harmonic generation has found applications in various fields such as attosecond science, where extremely short pulses of light are used to study ultrafast processes in matter, as well as in imaging and spectroscopy applications at the nanoscale.