Saturable absorbers based on impurity and defect centers in crystals

Saturation of near-infrared absorption and transmission dynamics are investigated in tetravalent-chromium-doped Gd/sub 3/Sc/sub 2/Ga/sub 3/O/sub 12/, Gd/sub 3/Sc/sub 2/Al/sub 3/O/sub 12/, and Mg/sub 2/SiO/sub 4/ crystals, as well as in reduced SrTiO/sub 3/ using 20 ps 1.08 /spl mu/m laser pulses. An absorption cross section of (5/spl plusmn/0.5)/spl times/10/sup -18/ cm/sup 2/ in garnets and (2.3/spl plusmn/0.3)/spl times/10/sup -18/ cm/sup 2/ in forsterite is estimated for the /sup 3/A/sub 2/-/sup 3/T/sub 2/ transition of tetrahedral Cr/sup 4+/. Q-switched and ultra-short pulses are realized in neodymium lasers using chromium-doped crystals as the saturable absorbers. Saturation of free-carrier absorption with ultra-short relaxation time is observed in SrTiO/sub 3/ at 10/sup 8/-10/sup 10/ W/cm/sup 2/ pump intensities, while at 10/sup 10/-10/sup 11/ W/cm/sup 2/ three-photon interband transitions predominate. The free-carrier absorption cross section is estimated to be (2.7/spl plusmn/0.3)/spl times/10/sup -18/ cm/sup 2/.<
>