 High harmonic generation

High Harmonic Generation (HHG) is a nonlinear optical process in which high-frequency photons are produced by the interaction of intense laser light with atoms, molecules, or solid surfaces. This phenomenon occurs when a strong laser field ionizes an atom, freeing electrons. These freed electrons can then be accelerated by the laser field and, upon recombining with their parent ions, emit photons at integer multiples (harmonics) of the original laser frequency.