A spectroscopic analysis of blue and ultraviolet upconverted emissions from Gd3Ga5O12:Tm3+, Yb3+ nanocrystals

The spectroscopic behavior of gadolinium gallium garnet (Gd3Ga5O12, GGG) nanocrystals codoped with 1% each of Tm3+ and Yb3+ prepared via a solution combustion synthesis procedure was investigated. Initial excitation of the codoped nanocrystals with 465.8 nm (into the (1)G(4) state) showed a dominant blue-green emission ascribed to the (1)G(4)-H-3(6) transition as well as red and NIR emissions from the (1)G(4)-F-3(4) and (1)G(4)-H-1(5)/H-1(4)-H-1(6) transitions, respectively. Excitation at this wavelength (465.8 nm) showed the existence of a Tm3+ -> Yb3+ energy transfer process evidenced by the presence of the F-1(5/2)-(F7/2Yb3+)-F-2 emission in the NIR emission spectrum. The decay time constants proved that the transfer of energy occurred via the H-3(4) state. Following excitation of the Yb3+ ion with 980 nm, intense upconverted emission was observed. Emissions in the UV (D-1(2)-H-3(6)) blue (D-1(2)-F-3(4)), blue-green ((1)G(4)-H-3(6)), red ((1)G(4)-F-3(4)), and NIR ((1)G(4)-H-3(5)/H-3(4)-H-3(6)) were observed and were the direct result of subsequent transfers of energy from the Yb3+ ion to the Tm3+ ion. Power dependence studies showed a deviation from expected values for the number of photons involved in the upconversion thus indicating a saturation of the upconversion process. An energy transfer efficiency of 0.576 was determined experimentally.