Multiphoton

Multiphoton refers to processes in which more than one photon participates in a transition between quantum states. In a multiphoton process, energy conservation is achieved by the combined energy of the photons, E_final − E_initial = nħω, where n is the number of photons and ω is the optical frequency. These processes typically require high photon fluxes because they are weaker than single-photon transitions and rely on nonlinear interactions between light and matter.

In atoms and molecules, multiphoton absorption allows access to excited states that are not reachable with a single photon of the same energy. The probability of an n-photon process scales approximately with the light intensity to the n-th power, giving a strong nonlinear dependence on the pulse intensity. Multiphoton ionization occurs when an electron is ejected by absorbing multiple photons, sometimes with additional photons absorbed above the ionization threshold (above-threshold ionization).

In nonlinear optics, many multiphoton phenomena are central, including two-photon and higher-order absorption, as well as processes that involve the combining of photon energies to drive transitions or chemical changes. Multiphoton interactions are also relevant in solid-state contexts, where they can produce nonlinear photoemission or contribute to high-harmonic generation under intense fields.

Applications include two-photon excitation fluorescence, a cornerstone of multiphoton microscopy. This technique uses ultrafast near-infrared pulses to achieve excitation via two-photon absorption, confining excitation to the focal volume and enabling deep-tissue imaging with reduced out-of-focus photodamage.

Historically, the concept of multiphoton absorption was predicted by Maria Goeppert-Mayer in 1931, with later experimental demonstrations as high-intensity light sources became available.