Upconversion processes in Pr³⁺-doped chalcohalide glasses

Infrared-to-visible upconversion obtained under continuous-wave infrared laser excitation has been investigated in Pr³⁺-doped chalcohalide glasses for different halide (Cl, Br, I) modifiers and Pr³⁺ concentrations. The excitation wavelength dependence suggests two different upconversion mechanisms. When the excitation wavelength is resonant with the ¹G₄ state an orange fluorescence from the ¹D₂ state is observed which is attributed to energy-transfer upconversion, whereas nonresonant excitation at higher energies than the ¹G₄ level originates emission from the ³P₀ state. This latter upconverted emission can be attributed to excited-state anti-Stokes absorption assisted by phonon emission. A model accounting for the photon-ion-phonon interaction is in good agreement with the observed temperature dependence of the excitation-upconverted fluorescence band. Orange-to-blue upconversion has also been observed and analysed for all the samples.