Two-photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the energies of the two photons. Two-photon absorption is a third-order process several orders of magnitude weaker than linear absorption. It differs from linear absorption in that the strength of absorption depends on the square of the light intensity, thus it is a nonlinear optical process.
Read more about Two-photon Absorption: Background, Two-photon Emission, Development of The Field and Potential Applications, TPA Compounds, TPA Coefficients, See Also
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“These philosophers dwell on the inevitability and unchangeableness of laws, on the power of temperament and constitution, the three goon, or qualities, and the circumstances, or birth and affinity. The end is an immense consolation; eternal absorption in Brahma.”
—Henry David Thoreau (1817–1862)