Figure 2.

Transition diagrams of 2PF and SHG. (a) Two-photon excitation occurs through the absorption of two lower-energy photons via short-lived intermediate states. After excitation, the fluorophore first relaxes to the lowest energy level of the excited electronic states via fast vibrational processes, and then relaxes back to the ground state, emitting a spontaneous fluorescence photon. (b) In the process of SHG, two photons are annihilated and a single photon with doubled energy (half the wavelength) is generated. No real-state transition is required in SHG.