Luminescence spectra of Tm<SUP>3+</SUP> ion have been investigated for YVO<SUB>4</SUB> and LiNbO<SUB>3</SUB> crystals by excitation with 798 nm laser diode of high powers in a range from 0.6 W up to 10 W. In addition to the one-photon excited infrared emission bands, emission bands at 700 and 1208 nm are observed in YVO<SUB>4</SUB> by high power excitation and the similar emission bands are observed at 702 and 1216 nm in LiNbO<SUB>3</SUB>. It is observed that each of the 700 and 1208 nm emission intensities in YVO<SUB>4</SUB> has quadratic pump-power dependence in a pump power range of 0.6 - 4 W and cubic dependence in 4 - 10 W range. Same result is observed for the 702 and 1216 nm emission in LiNbO<SUB>3</SUB> but the change from the quadratic to cubic dependence appears at 5 W. Discussion is made on the luminescence process for these up-conversion.
Up-conversion by Yb<SUP>3+</SUP> sensitization in Er3+ doped YVO4 crystals has been studied under excitation with 976 nm laser diode. Up-converted (available in paper) emission bands are observed at 490, 547, 554, 660 and 670 nm at room temperature, together with down-converted Yb<SUP>3+</SUP> emission at 1010 nm and Er<SUP>3+</SUP> emission at 1470 - 1630 nm. Excited state absorption from the <SUP>4</SUP>I<SUB>11/2</SUB> state of Er<SUP>3+</SUP> to the <SUP>4</SUP>F<SUB>7/2</SUB> state is observed in the Yb<SUP>3+</SUP> emission spectrum. It is confirmed, from quadratic dependence on pump intensity, the up-conversion is induced by two-photon excitation process.
Up-conversion of red light with wavelength of 660 nm in Tm<SUP>3+</SUP>-doped BaY<SUB>2</SUB>F<SUB>8</SUB> powder results in the two violet luminescence bands with peaks at 417 and 430 nm and two blue luminescence bands with peaks at 455 and 470 nm. The two violet bands are observed to be stronger than the blue bands. The blue luminescence is also observed by pumping with 993 nm light. The up-conversion is explained by a multiple excited state absorption process.
In this paper, the absorption and fluorescence spectra of Tm<SUP>3+</SUP>:LiNbO<SUB>3</SUB> crystals at room temperature were reported for the first time. The energy levels and experimental absorption oscillator strengths were obtained from the absorption spectrum. According to Judd-Ofelt theory, the intensity parameters (Omega) <SUB>(lambda</SUB> ) ((Omega) <SUB>2</SUB> equals 3.30 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>, (Omega) <SUB>4</SUB> equals 1.37 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>, (Omega) <SUB>6</SUB> equals 0.84 multiplied by 10<SUP>-20</SUP> cm<SUP>2</SUP>) were worked out by a least-square-fitting procedure. With these values, electric dipole oscillator strength, radiative transition probability, radiative lifetime, branching ratio and integrated emission cross-section were calculated. The potential use of Tm<SUP>3+</SUP>:LiNbO<SUB>3</SUB> crystal as a laser material was also discussed.