Two or more lower-energy photons can be used to excite a molecule into the same higher-energy state that excitation with a single higher-energy photon could. This has led to the imaging techniques now known as two- and three-photon microscopy, which can penetrate deep into living tissue using lower-energy (i.e., longer-wavelength) lasers.

Multiphoton Microscopy — is an imaging technique which uses two or more lower-energy, (i.e., longer-wavelength) lasers to excite molecules to a higher-energy state to emit light. The longer energy wavelengths penetrate tissue much more readily (with less scattering) than higher energy wavelengths, permitting real-time imaging much deeper into living tissue than traditional optical techniques.

The theory behind multiphoton microscopy dates back to Maria Goeppert-Mayer's 1931 doctoral dissertation. Her work showed that a simultaneous combination of lower-energy photons could excite an atom or molecule to a higher energy state just like a single higher energetic photon could.