Short-wavelength luminescence in Ho3+-doped KGd(WO4)2 crystals

Emissions from the high-lying excited states, energy transfer and upconversion processes are investigated in Ho3+-activated KGd(WO4)2 crystal. The spectral assignment based on time-resolved emission spectra allowed to identify various near ultra-violet (UV), blue and green emissions starting from the excited 3H5, 5G4, 5G5, 5F3 and 5S2 levels. The temporal behavior of these transitions after pulsed excitation was analyzed as a function of temperature and holmium ions concentration. The shortening and nonexponentiality of the decays, observed with increasing activator concentrations, indicated cross-relaxation (CR) among the Ho3+ ions. Cross-relaxation rates were experimentally determined as a function of activator concentration and used to evaluate the values of the nearest-neighbor trapping rates X01 and to model the decays. It was observed that KGW, despite higher than in YAG maximum phonon energy of about 900 cm−1, is more efficient short-wavelength emitter than YAG. Examples of the excited-state absorption (ESA) and energy transfer (ET) mechanisms responsible for the upconverted, short-wavelength emissions were identified by analyzing fluorescence dynamics and possible energy resonances.