Conversion of infrared radiation into visible emission in NaGd(WO4)2:Yb3+, Ho3+ crystals

NaGd(WO4)(2):Yb3+, Ho3+ single crystals have been grown by the Czochralski technique along the (0 0 1) orientation. Conversion of the infrared (IR) radiation at 980 nm into the visible emission in NaGd(WO4)(2) crystals containing several different concentrations of Yb3+ and Ho3+ has been investigated. The NaGd(WO4)(2): 8 at. % Yb3+, 4 at. % Ho3+ system exhibits intense red upconverted emission originating from the F-5(5) level. The upconversion mechanism in a Ho3+-Yb3+ system under near infrared excitation is discussed. It is concluded that the green emission is excited by energy transfers from Yb3+ to Ho3+, whereas excited state absorption is involved in the excitation of red emission. The emission cross-section of the F-5(5) -> I-5(8) transition at about 660 nm was estimated by using the Fuchtbauer-Ladengurg formula.