Two-photon interaction

Two-photon interaction occurs when an atom or molecule simultaneously absorbs or emits two photons—particles of light—rather than just one. This process typically requires high-intensity light, like laser beams. In absorption, the combined energy of the two photons matches an energy level in the atom, exciting it to a higher state; in emission, the atom releases two photons as it relaxes. This phenomenon is useful in advanced imaging and spectroscopy because it allows precise control of light-matter interactions, enabling applications like deep tissue imaging with minimal damage and improved resolution.