Investigation of sensitized anti-stokes phosphors using sinusoidal modulation of the pump intensity

We describe a simple and relatively inexpensive method of investigating the processes by which incident IR radiation is converted to visible emission in anti-Stokes phosphors by multiphoton excitation. The procedure involves sinusoidal modulation of the intensity of the IR exciting radiation and analysis of the harmonic content of the visible output from the phosphor. A two-photon process, for example, results in some second harmonic modulation of the visible emission. The harmonic content depends on the relaxation times involved and on the nature of the conversion process. The experiments thus enable one to establish the order of the conversion process, to distinguish between possible mechanisms and to determine certain relaxation times. Representative results for the ytterbium-sensitized anti-Stokes phosphors Y₂O₂S-Ho-Yb and Y₂O₂S-Er-Yb are described.