Blue luminescence in YVO4 and KGd(WO4)2 crystals excited by CW 1064-nm radiation

Upconversion luminescence near lambda = 475 nm is studied in KGd(WO4)(2) and YVO4 crystals excited by cw radiation at a wavelength of 1064 nm. The luminescence spectra for intracavity and extracavity excitation are compared. The upconversion emission of a YVO4 crystal excited outside the cavity is studied in detail. The spectrum recorded in the visible and near-IR regions exhibits luminescence bands around 475, 650, and 800 nm, which coincide with the (1) G (4) -> (3) H (6) and (1) G (4) -> (3) F (4) transitions, as well as with the (1) G (4) -> (3) H (5) and (3) H (4) -> (3) H (6) transitions of Tm3+ ions. Blue upconversion slope n was measured to be 1.3-1.6, which is considerably lower than that for the three-photon excitation process. The reasons for the anomalous value of n are discussed.