Upconversion luminescence from 2E state of Cr3+ in Al2O3 crystal by infrared femtosecond laser irradiation.

Visible upconversion luminescence was observed in Cr3+: Al2O3 crystal under focused femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from the 2E-4A2 transition of Cr3+. The dependence of the fluorescence intensity of Cr3+ on the pump power reveals that a two-photon absorption process dominates in the conversion of infrared radiation to the visible emission. It is suggested that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic visible emission from 2E state of Cr3+.