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 glassfibers cite journal
url=http://bookstore.spie.org/index.cfm?fuseaction=detailpaper&cachedsearch=1&productid=712545&producttype=pdf&CFID=6551164&CFTOKEN=3433140
author= J.Koponen
coauthors=M.Söderlund, H.J.Hoffman, D.Kliner, J.Koplow,J.L.Archambault, L.Reekie, P.St.J.Russell, and D.N.Payne
title=Photodarkening measurements in large mode area fibers
journal=Proceedings of SPIE
volume=6553
pages=64531E
year=2007
doi=10.1117/12.712545] cite journal
url=http://www.opticsinfobase.org/abstract.cfm?URI=ao-34-18-3436
author= L. Dong
coauthors= J. L. Archambault, L. Reekie, P. St. J. Russell, and D. N. Payne
title=Photoinduced absorption change in germanosilicate preforms: evidence for the color-center model of photosensitivity
journal=Applied Optics
volume=34
pages=3436
year=1995] . 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 centersin 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 [http://www.rp-photonics.com/photodarkening.html Encyclopedia of Laser Physics and Technology - photodarkening, photochromic damage, photo-induced loss, ultraviolet, gray tracking, color centers ] ] ,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 withlight.

Photodarkening rate

The inverse of the timescale at which photodarkening occurs can be interpreted as"photodarkening rate"cite journal]

Color centers

Usually, photodarkening is attributed to creation of color centers due to resonant interation of electromagnetic field with an active medium cite journal
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVF-4MBTTXJ-1&_user=119453&_coverDate=02%2F15%2F2007&_alid=557798969&_rdoc=1&_fmt=summary&_orig=search&_cdi=5533&_sort=d&_docanchor=&view=c&_ct=1&_acct=C000009578&_version=1&_urlVersion=0&_userid=119453&md5=7011dd0240684071fba56253ae081f98
author= L.C.Courrol
coauthors= I.M.Ranieri, W.B.Izilda, S.L.Baldochi, R.E.Ricardo, A.Z.de Freitas,L. Gomes,N.D.J.Vieira.
title=Study of color centers produced in thulium doped YLF crystals irradiated by electron beam and femtosecond laser pulses
journal=Optics Communications
volume=270
pages=340–342
year=2007
doi=10.1016/j.optcom.2006.09.071]

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 liminescencecite journal
url=http://there.is.no.such.link!
author= J.-F.Bisson
coauthors=D.Kouznetsov, K.Ueda, T.Fredrich-Thornton, K.Petermann, G.Huber
title=Switching of emissivity and photoconductivity in highly doped Yb3+:Y2O3 and Lu2O3 ceramics
journal=Applied Physics Letters
doi=10.1063/1.2739318
volume=90
pages=066101
year=2007] .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 notsufficient 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 cite journal
url=http://there.is.no.such.link!
author= J.-F.Bisson
coauthors=D.Kouznetsov, K.Ueda, T.Fredrich-Thornton, K.Petermann, G.Huber
title=Switching of emissivity and photoconductivity in highly doped Yb3+:Y2O3 and Lu2O3 ceramics
journal=Applied Physics Letters
doi=10.1063/1.2739318
volume=90
pages=066101
year=2007] , the
thermal effects are important; therefore only the initial stage of the avalanchecan be interpreted as photodarkening, and such interpretation is not yet confirmed.

References