High-order harmonic generation and other intense optical field–matter interactions: review of recent experimental and theoretical advances

An overview of recent experimental and theoretical developments in high-order harmonic generation (HHG), above threshold ionization (ATI), and other phenomena originating in intense optical field-matter interactions, is attempted here. A recurring theme in this review is the common bond between HHG, ATI, laser-assisted Auger decay, and the laser-assisted photoelectric effect, which is attributable solely to the photon field–electron interaction. It is argued that the dominant mechanism for photon production in HHG is not the recombination of an ion with a fast electron but rather is the result of coupling between the optical field and an electron that generates photons having energies that are an integral multiple of the pump photon energy. Characterized and steadily developed over the past decade, HHG and its attendant technology have matured to the extent that it now offers a versatile, table top source of coherent radiation in the vacuum ultraviolet to X-ray spectral regions with the capability of producing pulse widths as short as several hundred attoseconds. Combined with its comparative simplicity, the availability of average powers exceeding 1 mW from a single (plateau) harmonic at pulse repetition frequencies >1 kHz make HHG an attractive alternative to other nonlinear sources requiring tunable dye lasers and metal vapors. Emphasis is placed on the optical and spatial characteristics of the harmonic radiation and their potential impact on quantum electronics.