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Two-photon Absorption

Acronym: TPA

Definition: a nonlinear absorption process where two photons are simultaneously absorbed

More general term: nonlinear absorption

German: Zweiphotonenabsorption

Category: physical foundations


How to cite the article; suggest additional literature

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Two-photon absorption (TPA) is a process where two photons (usually of the same energy) are absorbed simultaneously, exciting e.g. an atom or ion to a higher-lying state, with the energy increase being equal to the sum of the photon energies. This is a nonlinear absorption process, occurring with significant rates only at high optical intensities because the absorption coefficient is proportional to the optical intensity:

$$\alpha = \beta \;I$$

with the TPA coefficient <$\beta$>.

The absorbed power is thus proportional to the square of the optical input power.

Two-photon absorption is the simplest variant of multiphoton absorption.

two-photon absorption in a semiconductor
Figure 1: Two-photon absorption in a semiconductor allows for absorption of light even if the photon energy is below the band gap energy.

In a dielectric material or a semiconductor, two-photon absorption can normally occur only if the photon energy is at least half the band gap energy. Therefore, there are e.g. no losses via two-photon absorption when ultrashort pulses at 800 nm wavelength propagate in a silica fiber. On the other hand, two-photon absorption at the same wavelength can occur in semiconductors such as GaAs, having a much smaller band gap.


The phenomenon of two-photon absorption finds applications in various technical areas. Some examples:

Under certain circumstances (illumination with ultrashort pulses), it is even possible that the human eye responds to infrared light due to two-photon absorption processes in the retina [7].

TPA can also modify the saturation characteristics of saturable absorbers such as SESAMs, leading to a roll-over of the saturation curve which can help e.g. to suppress Q-switching instabilities.

Problems with TPA

Detrimental TPA effects can occur for nonlinear frequency conversion of ultrashort pulses in nonlinear crystal materials, particularly for conversion of short wavelengths, e.g. in UV sources. The nonlinear absorption leads to additional power losses, to thermal effects and possibly also to degradation of the material (photodarkening).


[1]W. Kaiser and C. G. B. Garrett, “Two-photon excitation in CaF2:Eu2+”, Phys. Rev. Lett. 7 (6), 229 (1961), DOI:10.1103/PhysRevLett.7.229 (first experimental demonstration of two-photon absorption)
[2]E. W. Van Stryland et al., “Energy band-gap dependence of two-photon absorption”, Opt. Lett. 10 (10), 490 (1985), DOI:10.1364/OL.10.000490
[3]M. Sheik-Bahae et al., “Dispersion and band-gap scaling of the electronic Kerr effect in solids associated with two-photon absorption”, Phys. Rev. Lett. 65 (1), 96 (1990), DOI:10.1103/PhysRevLett.65.96
[4]E. R. Thoen et al., “Two-photon absorption in semiconductor saturable absorber mirrors”, Appl. Phys. Lett. 74, 3927 (1999), DOI:10.1063/1.124226
[5]F. R. Ahmad et al., “Energy limits imposed by two-photon absorption for pulse amplification in high-power semiconductor optical amplifiers”, Opt. Lett. 33 (10), 1041 (2008), DOI:10.1364/OL.33.001041
[6]M. Rumi and J. W. Perry, “Two-photon absorption: an overview of measurements and principles”, Advances in Optics and Photonics 2 (4), 451 (2010), DOI:10.1364/AOP.2.000451
[7]P. Artal et al., “Visual acuity in two-photon infrared vision”, Optica 4 (12), 1488 (2017), DOI:10.1364/OPTICA.4.001488

See also: nonlinear absorption, multiphoton absorption, nonlinearities

Questions and Comments from Users


How can I experimentally measure TPA for a photodiode?

The author's answer:

Yes – you can check how much signal you get when hitting the photodiode with long-wavelength light, having a photon energy well below the band gap energy. Usually, the light needs to consist of short or ultrashort pulses in order to reach sufficiently high peak intensities.


In silicon photonics, it is known that the TPA limits the amount of optical power that can be guided through standard single mode silicon waveguides on the SOI platform to about 100 mW. Is this the peak optical power or the average power?

The author's answer:

It is the peak power because TPA is mostly relevant for the power peaks. It just leads to increase propagation losses, although the light could in principle still be guided in such waveguides.

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