TWO-PHOTON ABSORPTION AND APPLICATIONS TO BIOLOGICAL SYSTEMS

Fluorescent ions and molecules are important probes for the investigation of biological systems. To initiate the fluorescence, such ions and molecules are usually excited by absorption of a single photon. It is often advantageous, however, to excite the molecule using two or more photons. In this article we examine the multiphoton absorption process, and discuss of the advantages and disadvantages it offers for the investigation of biological systems. The main point of this article is to review the basic physics of two-photon absorption process. We also define the two-photon absorption cross section and discuss how it is measured. Finally, the advantages of two photon absorption are discussed and some applications to biological systems are presented.