Abstract
Much interest has recently been directed to infrared-to-visible upconversion in rare- earth doped materials sensitized with trivalent ytterbium. The approach permits the realization of upconversion based optical devices excited by commercially available high power solid state sources around 1.0 μm. For such pump wavelengths the excitation of the Yb3+- sensitizer is a multiphonon-assisted anti-Stokes absorption process [1] and as a result the energy transfer upconversion pumping of the Pr3+ emitting levels is strongly dependent upon the phonon population in the host material. In this work, thermally induced enhancement of infrared-to-visible frequency upconversion in Pr3+/Yb3+-codoped chalcogenide glasses excited at 1.064 μm is demonstrated. The Pr3+/Yb3+-codoped 70%Ga2O3:30%La2S3 glass samples were excited at 1.064 μm, and heated in the temperature range of 20°C to 200°C. For a fixed excitation power and temperature, the upconversion emission spectrum obtained from the sample is depicted in Fig. 1. It presents three bands centered around 500, 546, and 650 nm corresponding to the 3P1 - 3H4, 3P1 - 3H5, and 3P0 - 3F2 transitions, respectively. The temperature dependence of the fluorescence intensity was studied and the results revealed an upconversion efficiency enhancement of x3.5 in the temperature range of 20 °C - 130 °C, as indicated in Fig. 2. The behavior of the system is described by conventional rate equations [2] for the pertinent Pr3+-Yb3+ energy-levels, including an effective temperature dependent absorption cross-section σd(T) = σd0.F(T) for the Yb3+- sensitizer, where σd0 is the cross section at resonance and F(T) = [exp(hνphonon/kBT) - 1]-p. The exponent p is the number of phonons involved in the sensitizer excitation. The population of the Pr3+ excited-state emitting levels was accomplished through anti-Stokes multiphonon-assisted excitation of the Yb3+ ions and subsequent energy-transfer to and excited-state absorption from the Pr3+ ions.
© 2000 IEEE
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