Two-photon-excited luminescence from a Eu~(3+)-doped lithium niobate crystal pumped by a near-infrared femtosecond laser

摘要

Red upconversion luminescence (UCL) is observed using an 800 nm femtosecond laser to irradiate a Eu~(3+)-doped LiNbO_3 single crystal. Power-dependent and polarization-dependent experiments demonstrate that the red UCL originates from Eu~(3+) two-photon simultaneous absorption rather than second harmonic generation of nonlinear optical crystal followed by reabsorption of Eu~(3+). Two different kinds of Eu~(3+)-centers, called Eu~(3+)-pairs, are revealed via emission spectra. Besides using femtosecond laser excitation to simulate diode laser excitation with broad line, the efficient two-photon simultaneous absorption can populate more Eu~(3+) to upper levels, these are beneficial to the study of cross relaxation with submicrosecond resolution between Eu~(3+)-pairs occurred in practical application. Transient UCL spectra and decay curves suggest that the luminescent state ~5D_0 is fed by way of both multiphonon relaxation through state ~5D_1 and cross relaxation of ~5D_2+~7F_0 →~5D_0+~7F_5.