two-photon absorption

Nondegenerate two-photon absorption is a nonlinear absorption process whereby two photons of slightly different energies are absorbed simultaneously by an atom, ion or molecule and an electron is promoted from a lower energy level to a higher energy level. The total energy of the transition is equal to the sum of the two photon energies.

In the case of well-known degenerate two-photon absorption (TPA), two photons of the same energy are absorbed (almost) simultaneously: the first photon is absorbed to excite the molecule to its resonant virtual state followed by absorption of the second photon to reach the first excited state (see the right diagram on the picture below).

For the case of non-degenerate two-photon absorption, the energies of the absorbed photons are near the virtual state resonances (see the left diagram on the picture below).

radiative relaxation

Extremely non-degenerate two-photon absorption has been recently observed and measured yielding significantly higher values of absorption coefficients for semiconductors. Intermediate virtual states in this case are located close to both the ground state or the excited state.

radiative relaxation

See also: Single-photon absorption, Two-photon absorption, Three-photon absorption, Relaxation.

SimphoSOFT logo SimphoSOFT® supports modeling degenerate two-photon (as well as single- and three-photon) absorption. Its mathematical model includes a variant of PROP equation, written in the terms of electromagnetic field, coupled with time-resolved radially-dependent RATE equations. App Notes SimphoSOFT logo

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