twophoton

Two-photon refers to processes in which matter interacts with two photons either simultaneously or in a strongly correlated way. These are typically nonlinear optical or quantum electrodynamic phenomena that cannot be explained by single-photon interactions alone. The term is used across spectroscopy, microscopy, and quantum information to describe events where two photons contribute to an excitation, emission, or scattering process.

Two-photon absorption is the best-known example. In this process, a molecule or atom transitions to a higher energy level by absorbing two photons at once, or within a timescale short enough that the intermediate state is virtual. The likelihood of this process grows with the square of the light intensity, making it highly nonlinear. Practically, near-infrared or infrared light is used to excite visible or ultraviolet transitions, enabling localized excitation in deep tissue with reduced photodamage. This principle underpins two-photon excitation fluorescence microscopy and related spectroscopies.

Two-photon emission occurs when an excited system relaxes by emitting two photons, often with correlated properties. In quantum optics, two-photon sources frequently exploit spontaneous parametric down-conversion to generate pairs of entangled photons, which are crucial for experiments in quantum communication and computation.

Direct photon–photon interactions predicted by quantum electrodynamics (photon-photon scattering) are extremely weak under ordinary conditions, but they illustrate the broader concept of twophoton processes. In practice, twophoton phenomena provide powerful tools for probing matter, enabling noninvasive imaging, precision spectroscopy, and advances in quantum technologies.