In various situations, absorption processes are nonlinear; this means that the absorption coefficient depends on the optical intensity (radiant flux) – either only on the instantaneous intensity (for an instantaneous nonlinear response) or to the intensities in the near past.
In some cases, there is only nonlinear absorption and no linear absorption: the absorption coefficient vanishes for vanishing intensities. In other cases, there is absorption at low intensities, but the absorption coefficient rises or is reduced for higher intensities. The latter case (with reduced absorption coefficients for rising intensities) is called saturable absorption. It is often exploited for laser pulse generation by passive mode locking or passive Q switching.
A typical nonlinear absorption process is two-photon absorption (TPA). Here, the energies of two photons are combined to excite a single electron into a higher state – e.g. above the band gap of a semiconductor. Thus, there can be absorption (at high enough intensities) even when the energy of a single photon is insufficient for getting across the band gap. The effective absorption coefficient is proportional to the optical intensity.
Similarly, there are multiphoton absorption processes involving more than two photons per process. These are relevant, for example, for laser material processing in transparent media such as glasses. The deposition of laser energy is possible only with high peak powers as can be reached in short or ultrashort pulses of light.
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