Two-photon Absorption - TPA Coefficients

TPA Coefficients

The two photon absorption coefficient is defined by the relation

so that

Where is the two-photon absorption coefficient, is the absorption coefficient is the transition rate for TPA, is the irradiance, is the photon frequency and the thickness of the slice is .

The SI units of the coefficient are:

Where N is the number density of molecules per cm3, E is the photon energy (J), σ(2) is the two-photon absorption cross section (cm4s/molecule).

If β (m/W) is multiplied by 10−9 it can be converted to the CGS system (cal/cm s/erg).

Due to different laser pulses the TPA coefficients reported has differed as much as a factor 3. With the transition towards shorter laser pulses, from picosecond to subpicosecond durations, noticeably reduced TPA coefficient have been obtained.