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Generation of visible and NIR frequency upconversion emission light within the first biological window spectral region using excitation at 1319 nm in the second biological window in Tm3+/Er3+-codoped TeO2-based glass, is demonstrated. Efficient energy upconversion of cw radiation at 1319 nm into near infrared 800 nm emission in Tm3+ single-doped 60TeO2-10GeO2-10K2O-10Li2O-10Nb2O5 glass is observed. The 800 nm signal is the sole light emission observed in the entire UV-VIS-NIR spectral region. Luminescence emission around 1480 nm, and 1800 nm was also observed. The proposed excitation mechanism for the 800 nm thulium emitting level is assigned to a multiphonon-assisted excitation from the ground-state 3H6 to the 3H5 excited-state level, a rapid relaxation to the 3H4 level and followed by an excited state absorption of the pump photons mediated by multiphonons connecting the 3H4 level to the 3F4 emitting level. For Tm3+/Er3+-codoped tellurite glass samples, radiations at 550 nm, 660 nm and 980 nm owing to erbium ions in addition to the main 800 nm signal originating form thulium ions is readily observed. Here, energy-transfer from Tm3+ (3H6 - 3F4) to Er3+(4I9/2) producing the NIR emission, followed by excited-state absorption of 1319 nm photons populating 2H11/2, 4S3/2, and 4F9/2 visible emitting levels. Erbium single-doped samples did not exhibited visible detectable emissions for excitation powers as high as 1.8 W of cw radiation at 1319 nm. This spectroscopic characteristics is highly requested for applications in biomedicine such as fluorescence based nano-thermometers and fluorescence-based bio-imaging, once the penetration depth is a major issue to be considered.
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