Two-Photon (Nonlinear) Excitation

Why requiring two photons to arrive at the same fluorophore at nearly the same instant confines excitation almost entirely to the single point where a beam is most tightly focused.

References

• So, P.T., Dong, C.Y., Masters, B.R., Berland, K.M. "Two-Photon Excitation." Annual Review of Biomedical Engineering 2 (2000): 399–429.