Multiphoton excitation of visible emission in Y2O2S: Er,Yb phosphors by 810 nm excitation

During investigations of anti-Stokes effects in erbium—ytterbium doped yttrium oxysulphide phosphors we have found an excitation of the erbium green emission due to 810 nm radiation (⁴I$\text{15}\text{2}$ → ⁴I$\text{9}\text{2}$ transition), the effect being of a two-photon type. There is also a two-photon excited blue emission (²H$\text{9}\text{2}$ → ⁴I$\text{15}\text{2}$ transition) due to the same radiation. Introduction of ytterbium impurity has a deleterious effect on the above 810 nm excitation, in contrast to its sensitization of the 1000 nm anti-Stokes excitation. To distinguish between various excitation schemes a two-beam excitation technique has been used. Results show that ytterbium inclusions lead to loss of energy from the 810 nm excitation by increasing the probability of downward transitions from the ⁴F$\text{9}\text{2}$ level to ⁴I$\text{11}\text{2}$ and ⁴I$\text{13}\text{2}$ states, reached after onephoton absorption. This effect is indicated by an increase in 1500 nm emission (⁴I_{$\text{13}\text{2}$} → ⁴I_{$\text{15}\text{2}$} with ytterbium concentration.