Stokes and anti-Stokes luminescence in Tm3+/Yb3+-doped Lu3Ga5O12 nano-garnets: a study of multipolar interactions and energy transfer dynamics

Abstract

Nanocrystalline Lu₃Ga₅O₁₂ garnets doped with Tm³⁺/Yb³⁺ ions have been synthesized by a low cost and environmentally benign sol–gel technique and characterized for their structural, Stokes and anti-Stokes luminescence properties. The diffuse reflectance spectra of doped Lu₃Ga₅O₁₂ nano-garnets have been measured to derive the partial energy level structure of Tm³⁺ and Yb³⁺ ions and possible energy transfer channels between them. Upon laser excitation at 473 nm, weak red and intense near-infrared Stokes emissions have been observed in the nano-garnets. The decay curves of ³H₄ and ¹G₄ levels of Tm³⁺ ions and the ²F_5/2 level of Yb³⁺ ions have been measured upon resonant laser excitation and are found to be non-exponential in nature due to multipolar interactions. In order to know the kind of multipolar interaction among optically active ions, the decay curves are analyzed through the generalized Yokota–Tanimoto model. Moreover, under 970 nm laser excitation, intense blue anti-Stokes emission is observed by the naked eye in Tm³⁺–Yb³⁺ co-doped Lu₃Ga₅O₁₂ nano-garnets. The results show that as-synthesized nano-garnets may be useful in the field of phosphors and photonics.