Fig. 4

Upconversion luminescence spectra (a) and the relationship between log(I _em) and log(P) (b) for the (La_0.97Ho_0.01Yb_0.02)₂O₂S phosphor, where I _em and P are the emission intensity and excitation power (in watt), respectively. The inset in a is a photograph showing the appearance of strong UC emission under 50 mW of 978-nm laser excitation