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:
with the TPA coefficient β.
The absorbed power is thus proportional to the square of the optical input power.
Two-photon absorption is the simplest variant of multiphoton absorption.
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:
- It is used in simple autocorrelators for pulse characterization, where TPA in a photodiode, having a bandgap energy larger than the photon energy, is exploited to obtain a nonlinear response.
- Also, two-photon absorption is often used in fluorescence microscopy (two-photon microscopy) for exciting fluorescence with an infrared laser beam, which can easily penetrate the sample.
- In other cases, TPA is exploited for optical power limiting or for microfabrication.
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).
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