Two-photon absorption (TPA) is the simultaneous absorption of two-photons of the same or different wavelenth by a material. In physics two-photon absorption is categorized as a third-order nonlinear optical phenomenon.
The image on the left of Figure 1 shows fluorescence emitted through out the path of a laser in a photoactive solution. This fluorescence results from one-photon absorption which is the most common light absorption phenomenon in nature. One-photon absorption can occur at low intensities of light leading to excited molecules which then retract to ground state by emitting fluorescence. In this particular case the wavelength of light absorbed is 488 nm (blue). It is to be noted that entire path of the laser through the photoactive solution is illuminated by the green fluorescence of the emitted light.In contrast the image on the right of Figure 1 features an almost invisible green dot of fluorescence at the middle of the focus (see inset). This latter case uses an pulsed infrared laser instead of the blue laser employed in the former case. Here the energy of the infrared photon is lower than that of the blue laser, but the pulsed lasers can deliver very high intensities. At such high intensities a molecule can simultaneously absorb two-photon with about half the energy of the photon they would normally absorb. This phenomenon is called two-photon absorption (TPA). TPA is confined to a very small region at the focus of a laser since it occurs only at on high intensities of light. This means that as the intensity of laser decreases two-photon absorption rapidly wanes to nothing. This makes it highly confined to the focus of the laser where intensity is highest. The spatial confinement of two-photon fluorescence can be used for imaging of bilogical samples [1-2], for writing-reading data , for direct laser writing of 3D structures etc .
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 Kallepalli, D.L., Alshehri, A.M., Marquez, D.T., Andrzejewski, L., Scaiano, J.C. and Bhardwaj, R., 2016. Ultra-high density optical data storage in common transparent plastics. Scientific reports, 6.
 Lee, K.S., Kim, R.H., Yang, D.Y. and Park, S.H., 2008. Advances in 3D nano/microfabrication using two-photon initiated polymerization. Progress in Polymer Science, 33(6), pp.631-681.