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 σ_d(T) = σ_d⁰.F(T) for the Yb³⁺- sensitizer, where σ_d⁰ is the cross section at resonance and F(T) = [exp(hν_phonon/k_BT) - 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