twophotonmultiphoton

twophotonmultiphoton is a label used to denote the family of nonlinear optical phenomena in which two or more photons participate in a transition or interaction with matter. The term encompasses both two-photon processes and higher-order multiphoton events, including absorption, emission, and various nonlinear conversions driven by high-intensity light.

Two-photon absorption (TPA) is a hallmark example, in which two photons are absorbed simultaneously to reach an excited state. The probability scales roughly with the square of the light intensity, and the process often proceeds via virtual intermediate states. Ultrafast pulsed lasers provide the high peak powers needed for practical TPA. Applications include two-photon fluorescence microscopy, photodynamic therapy, and microfabrication.

Multiphoton processes extend to three-photon and higher-order interactions. Examples include three-photon absorption, multiphoton ionization, and higher-order harmonic generation. The rates typically scale with the intensity to the n−1 power for an n-photon process, and efficiencies depend on pulse duration, spectral bandwidth, and phase matching. At very high intensities, nonperturbative regimes can arise.

Related phenomena include two-photon emission and parametric processes such as second- and third-harmonic generation, as well as spontaneous parametric down-conversion and four-wave mixing. These effects rely on the nonlinear optical susceptibilities chi^(n) and are central to advances in imaging, spectroscopy, and quantum optics.

Historically, the concept began with Maria Göppert-Mayer’s prediction of two-photon absorption in 1931 and its experimental observation in the 1960s. The field has grown with ultrafast laser technology and quantum-light applications, shaping methods across science and engineering. See also nonlinear optics, quantum optics, two-photon microscopy, multiphoton lithography, high-harmonic generation.