Visible upconversion luminescence mediated by energy transfer in fluorogermanate glass doped with Tm3+ and Ho3+

Multicolor blue (480 nm), green (540 nm) and red (655 nm) upconversion light generation in Tm3+/Ho3+-codoped fluorogermanate glass excited by high-order stimulated Raman scattering Stokes emissions, generated in an optical fiber pumped by a laser operating at 1.064 µm, is demonstrated. The excitation route for the 480 nm, 650, and 800 nm Tm3+ emissions is accomplished through stepwise multiphonon-assisted ground-state, and excited-state absorption of 1.06, 1.12, 1.18, and 1.24 µm pump photons. The 540 nm emission of Ho3+ ions is effectuated through an initial ground-state absorption (5I8-5I6) in conjunction with energy-transfer Tm3+ to Ho3+ connecting the 5I6 - 5F2, 3K8 followed by rapid multiphonon relaxation to the 5F4, 5S2 green emitting levels. Energy-transfer Tm3+ (3H5-3H6) to Ho3+(5I8-5I6), and Tm3+(1G4) to Ho3+(5F2,3K8), also enhance the holmium ions upconversion excitation process. Pump power dependence of the emitted light, lifetime measurements, as well as the CIE-1931 color coordinates for fixed Tm3+ concentration and varying H3+ content, were also examined and discussed