Multiphoton microscopy: An introduction to gastroenterologists

Figure 1 Multiphoton excitation. A: Single-photon excitation. Individual photons of high-energy blue light (wavelength, λ = 480 nm) excite fluorophores in the sample. After an electron in the fluorophore is transferred to the excited state (blue arrow), it loses energy rapidly owing to non-radiative relaxation (dashed arrow). Subsequently, fluorescence emission (yellow curved arrow) occurs at a longer wavelength than the excitation light as the electron falls back to the ground state (green arrow); B: Two-photon excitation. Two infrared photons (λ = 780 nm) are absorbed simultaneously (red arrows) to excite the fluorophore and light is emitted in the same manner as for single-photon excitation (green arrow) with emission of fluorescein.