Three-photon-excited upconversion luminescence of Ce(3+): YAP crystal by femtosecond laser irradiation.

Infrared to ultraviolet and visible upconversion luminescence was demonstrated in trivalent cerium doped YAlO(3) crystal (Ce(3+): YAP) under focused infrared femtosecond laser irradiation. The fluorescence spectra show that the upconverted luminescence comes from the 5d-4f transitions of trivalent cerium ions. The dependence of luminescence intensity of trivalent cerium on infrared pumping power reveals that the conversion of infrared radiation is dominated by three-photon excitation process. It is suggested that the simultaneous absorption of three infrared photons pumps the Ce(3+) ion into upper 5d level, which quickly nonradiatively relax to lowest 5d level. Thereafter, the ions radiatively return to the ground states, leading to the characteristic emission of Ce(3+).