Two-Photon Microscopy

Images deep within living tissue by relying on a fluorophore absorbing two lower-energy photons simultaneously, a rare event confined naturally to the tightly focused point of the beam.

References

• Denk, W., Strickler, J.H., Webb, W.W. "Two-Photon Laser Scanning Fluorescence Microscopy." Science 248, no. 4951 (1990): 73–76.