Intense green photoluminescence and upconversion-based intrinsic optical bistability in Ho3+/Yb3+ codoped CaSc2O4 and upconversion in Ho3+/Yb3+ codoped CaY2O4 phosphors

Abstract

The present study details the downshifting, frequency upconversion and intrinsic optical bistability in CaSc2O4:Ho3+/Yb3+ and upconversion in CaY2O4:Ho3+/Yb3+ phosphors synthesized by a complex-based precursor solution method. The X-ray powder diffraction confirms the formation of highly crystalline phosphors with pure orthorhombic phase. Fourier transform infrared studies of synthesized phosphors show vibrational bands due to the presence of Ca–O, Sc–O and Y–O groups. The diffuse reflectance spectra show a number of bands in the UV–vis–NIR regions due to presence of Ho3+ and Yb3+ ions. The values of optical band gap (Eg) are found to be 5.69 eV and 5.58 eV for the Ho3+/Yb3+ codoped CaSc2O4 and CaY2O4 phosphors, respectively. The Ho3+/Yb3+ codoped CaSc2O4 phosphors display intense green downshifting emission upon 454 nm excitations. The lifetime of green luminescence of CaSc2O4:Ho3+/Yb3+ phosphors with varied Yb3+ concentrations have also been measured. The decay curves show a non-exponential nature fitted to the Inokuti-Hirayama model and the energy transfer microscopic parameters have been also calculated. The Ho3+/Yb3+ co-doped CaSc2O4 and CaY2O4 phosphors display intense green alongwith weak blue, red, and near-infrared upconversion (UC) emissions upon 980 nm excitations. The spectral color purity (Sgr) of CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor is calculated to be 0.78. Importantly, the variation of pump power generates intrinsic optical bistability (IOB) in the CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor for the green emission, which is not observed in the CaY2O4:Ho3+(1%)/Yb3+(5%) phosphor. Therefore, the Ho3+/Yb3+ co-doped CaSc2O4 phosphor could potentially be used in UC-based devices, optical memory devices, optical gates, optical transistors and switching devices for optical communications.