Spectroscopic studies of Tm-doped CaF2 single-crystal

Thulium-doped CaF₂ single crystal has been investigated in detail for its optical absorption, Raman spectra, and fluorescence spectra. The cubic structure of Tm: CaF₂ has been confirmed using a single-crystal XRD investigation. Seven absorption transitions spanning 300–2000 nm corresponding to 4f–4f transitions of Tm³⁺ ions have been identified in the absorption spectra. Three Judd–Ofelt (JO) parameters are estimated by fitting the measured and predicted transition strengths using least-square fitting. The Calculated optimal values of JO parameters are Ω₂ = 0.34, Ω₄ = 0.54, Ω₆ = 1.37 in 10⁻²⁰ cm² units. A lower value of Ω₂, which is sensitive to the local environment, suggests a stronger ionic bond in fluorite structures doped with rare earth elements, as a result of charge compensation with interstitial F − anions. Using JO parameters, we have found the transition probabilities, which is then used to determine the radiative lifetimes of the primary emitting levels and the branching ratios of respective transitions. There are four distinct bands in the Tm³⁺-CaF₂ emission profile namely: ¹D₂ → ³F₄ (451 nm), ¹G₄ → ³H₆ (477 nm) excited by a 356 nm wavelength, another ³F_2,3 → ³H₆ (690 nm) excited by a 461 nm wavelength, and ³H₄ → ³H₆ (822 nm) excited by a 675 nm wavelength.