UV light induced photoreduction in phosphate and fluoridephosphate glasses doped with Ni2+, Ta5+, Pb2+, and Ag+ compounds

Abstract

The photoreduction of polyvalent ions was studied in high purity fluoride-phosphate and metaphosphate glasses doped with Ni2+ (3d8), Ta5+ (5d0), Pb2+ (6d2), and Ag+ (3d10). Compared to the undoped base glasses all doped samples display different electronic transitions in the UV at the irradiation wavelength. Glass samples containing 50 to 5000 ppm dopants were irradiated with excimer lasers at 193 and 248 nm, respectively. The subsequent defect centers, formed at ppm levels, were characterized by EPR and optical UV-VIS spectroscopy. The observed laser induced transmission losses in the UV and visible range increased in the order Ni, Ta, Pb to Ag. Extrinsic electron centers are formed by photoreduction of the dopants. (Ni2+)- is characterized by an optical transition with a maximum at 355 nm and an EPR signal around g ≈ 2.07. The maxima of the optical transitions of the (Pb2+)- -EC are positioned at 395 and 500 nm, of the (Ta5+)- -EC at 465 nm. The photoionization products of silver depend strongly on the silver concentration. At a silver content of 50 ppm only the (Ag+)- -EC is formed, visible in the optical spectra with a maximum around 450 nm. Α second silver species, (Ag+)2 -, which absorbs at 305 nm, is additionally observed in the sample doped with a silver concentration of 500 ppm. In the sample doped with 5000 ppm silver a third defect, the photooxidized (Ag+)+ -HC, with an optical band maximum at 405 nm and an EPR signal around g ≈ 2.3 is observed as well. The formation of extrinsic electron centers causes in all glasses an increase in the formation of intrinsic hole centers and often a decrease in the formation of intrinsic electron centers. Defect generation curves show that a very rapid darkening in the glasses is initiated by the addition of any of these dopants. The recovery rates of the defeets formed depend strongly on the dopant, not on the glass matrix.