Photodarkening

Photodarkening is optical effect observed in interaction of laser radiation with amorphous media (glasses) in optical fibers. Until now, such creation of color centers was reported only in glass fibers . Photodarkening limits the density of excitations in fiber lasers.

Definition
From ethimology, one could expect that photodarkening may refer to any process when any object becomes non-transparent (dark) due to illumination with light. Formally, the darkening of the photo-emulsion also could be considered as photodarkening. However, recent papers use this term meaning reversible creation of absorbing color centers in optical fibers. One may expect that the effect is not specific for fibers; therefore, the definition should cover wide class of phenomena, excluding, perhaps, non-reversible darkening of photographic emulsions.reference wanted! According to Encyclopedia of Laser Physics and Technology , Photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths.

Based on recent publication, we may also define photodarkening as reversible creation of absorption centers in optical media at the illumination with light.

Photodarkening rate
The inverse of the timescale at which photodarkening occurs can be interpreted as photodarkening rate

Color centers
Usually, photodarkening is attributed to creation of color centers due to resonant interation of electromagnetic field with an active medium

Possible mechanisms of photodarkening
The phenomenon, similar to photodarkening in fibers, was recently observed in chunks of Yb-doped ceramics and crystals. At the high concentration of excitations, the absorption jumps up, causing the avalanche of the broadband liminescence . The jump of absorption can be caused by formation of color centers by electrons in the conduction band, created by several neighbor excited ions. (The energy of one or two excitations is not sufficient to pop an electron into the conduction band). This explains, why the rate of darkening is strong function of the intensity of the exciting beam (as in the case with optical fibers discussed above). In the experiments , the thermal effects are important; therefore only the initial stage of the avalanche can be interpreted as photodarkening, and such interpretation is not yet confirmed.