Saturable absorption

Saturable absorption is a property of materials where the absorption of light decreases with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities.

Saturable absorbers are useful in laser cavities. The key parameters for a saturable absorber are its wavelength range (where it absorbs), its dynamic response (how fast it recovers), and its saturation intensity and fluence (at what intensity or pulse energy it saturates). They are commonly used for passive Q-switching.

At high intensity of pump, the laser gain medium is so excited that the lower laser level is drained. In this case, the absorption of pump is saturated. However, in many optically-pumped lasers, the density of excitations is not high, and the absorption of pump is not saturated.