Two Photon Microscopy and Second Harmonic Generation

Nonlinear microscopy has become an important tool for imaging biological samples such as neurons. In two photon fluorescence, a fluorophore absorbs two photons at a low energy state, one of its electrons briefly converts to a higher energy state, and it emits a single excited photon at an energy level higher than that of the absorbed photons. In second harmonic generation (SHG), laser light is focused on a sample to generate frequency-doubled light: two photons of one wavelength are annihilated and a single photon of half the wavelength is generated.