Sensitized Ho3+ up-conversion luminescence in Nd3+–Yb3+–Ho3+ co-doped ZrF4-based glass

As previously reported, a selectively strong green emission due to the ${\mathrm{Ho}}^{\text{3+}}$ : ${(}^5{F}_4{\mathrm{,\hspace{0.17em}}}^5{S}_2{\mathrm{)\to }}^5{I}_8$ transitions is observed in ${\mathrm{Nd}}^{\text{3+}}$ –${\mathrm{Ho}}^{\text{3+}}$ co-doped ${\mathrm{ZrF}}_4$ -based fluoride glasses under 800 nm excitation. As an attempt to more enhance ${\mathrm{Ho}}^{\text{3+}}$ up-conversion luminescences in the ${\mathrm{Nd}}^{\text{3+}}–{\mathrm{Ho}}^{\text{3+}}$ co-doped ${\mathrm{ZrF}}_4$ -based glasses, ${\mathrm{Yb}}^{\text{3+}}$ ions were added to the glasses. As a result it was found that, in 800 nm excitation of 60${\mathrm{ZrF}}_4\text{⋅30}{\mathrm{BaF}}_2\text{⋅(8−x)}{\mathrm{LaF}}_3\text{⋅1}{\mathrm{NdF}}_3\mathrm{\cdot x}{\mathrm{YbF}}_3\text{⋅1}{\mathrm{HoF}}_3$ glasses $\text{(x=0}$ to 7), sensitized up-conversion luminescences are observed at around 490 nm (blue), 545 nm (green), and 650 nm (red), which correspond to the ${\mathrm{Ho}}^{\text{3+}}:$ ${}^5{F}_3{\to }^5{I}_8,$ ${(}^5{F}_4{\mathrm{,\hspace{0.5em}}}^5{S}_2{\mathrm{)\to }}^5{I}_8,$ and ${}^5{F}_5{\to }^5{I}_8$ transitions, respectively. The intensities of the green and red emissions in a 3 mol % ${\mathrm{YbF}}_3$ -containing glass were about 50 times stronger than those in no ${\mathrm{YbF}}_3$ -containing glass. This is based on sensitization due to ${\mathrm{Yb}}^{\text{3+}}$ ions. In particular, the green emission is extremely strong so that the ${\mathrm{Nd}}^{\text{3+}}$ –${\mathrm{Yb}}^{\text{3+}}$ –${\mathrm{Ho}}^{\text{3+}}$ co-doped ${\mathrm{ZrF}}_4$ -based glasses have a high possibility of realizing a green up-conversion laser glass. Up-conversion processes for the blue, green, and red emissions were two-photon processes assisted by ${\mathrm{Nd}}^{\text{3+}}\to {\mathrm{Yb}}^{\text{3+}}\to {\mathrm{Ho}}^{\text{3+}}$ energy transfer. The up-conversion mechanism in the glasses is discussed.