EXCITED-STATE ABSORPTION MECHANISMS OF RED TO UV AND BLUE CONVERSION LUMINESCENCE IN TM3+ DOPED FLUOROPHOSPHATE GLASS

Characteristics and ion-ion interaction processes important in the optical dynamics of UV and blue upconversion luminescence in Tm3+ doped fluorophosphate glass have been investigated by exciting Tm3+ ions into the F-3(2) level with a DCM dye laser tuned at 657 nm. Two emission bands centered at 363, 451 nm from the 1D2 level and one emission band centered at 478 nm from the 1G4 level were observed. The 451 nm emission was stronger than the 478 nm emission. The excitation power dependence of all the upconverted emissions were found to be quadratic, conforming the two photon nature of these transitions. The mechanism leading to these emissions was attributed to the excited state absorption (ESA) from the F-3(4) and H-3(4) levels for the emissions the 1D2 and 1G4 levels, respectively. The loss mechanism due to ion-ion interaction in the F-3(4) level therefore was studied as function of temperature by measuring the spectral overlap between emission and absorption spectra of this level. From this data relevant microscopic interaction parameters that give a measure of Tm-Tm coupling have been calculated. Optical properties of the intermediate and the final levels involved in the upconversion processes were studied using the Judd-Ofelt theory. This theory was also used to determine radiative transition rates and the fluorescence quantum efficiencies of the excited levels, and the excited state absorption coefficient for the F-3(4) --> 1D2 and H-3(4) --> 1G4 transitions when the excitation was fixed at 657 nm. Lower excited state absorption coefficient of the former transition explains why the 478 nm emission intensity is weaker than 451 nm emission intensity in this glass. According to the rate equation model temperature dependence of the upconverted emission intensities from the 1D2 level was controlled by the temperature dependence of the excited state absorption coefficient corresponding to the F-3(4) --> 1D2 transition.