However, for co-doped Tm3+-Nd3+:KPb2Cl5, the presence of the Tm3+ is known to increase the absorption of the pump and enhance the IR emission from the Nd3+ ions [44]. An additional example is co-doped Tm3+-Pr3+:CsCdBr3, in which pumping the 3H4 level of Tm3+ results in energy transfer and up-conversion to emitting

states in the visible [45]. Energy transfer from the 3H4 state of Tm3+ to the IR-emitting states of Pr3+ in a low phonon energy host crystal is also an interesting phenomenon. Like Ho3+, the Pr3+ ion also lacks absorption at 800 nm. However, transitions out of the first three excited states of Tm3+ that populate through cross-relaxation are resonant with absorption transitions out of the Pr3+ ground state to excited states of Pr3+ that radiate

in the mid-IR. Figure 6 selleck kinase inhibitor compares the lower energy levels of Tm3+ to the lower levels of Pr3+ and illustrates three possible pathways for resonant energy transfer that involve excited-state Tm3+-sensitizing ions interacting with ground-state Pr3+ acceptor ions. Figure 6 Energy INCB024360 molecular weight transfer processes for co-doped Tm 3+ -Pr 3+ :KPb 2 Cl 5 . The first three excited states of Tm3+-sensitizing ions are all resonant with ground-state transitions of Pr3+ acceptor ions. In contrast to Pr3+:YAG or Pr3+:YLF, Pr3+ ions in a chloride host crystal will radiate at mid-IR wavelengths because the lower energy levels are no longer quenched by multi-phonon relaxation. This effect was exploited to make 5.2- and 7.2-μm lasers using Pr3+:LCl3[11, 12]. For Pr3+ doped into KPb2Cl5, the lower energy

levels will also radiate in the mid-IR. The mid-IR fluorescence can be observed in singly doped Pr3+:KPb2Cl5 when the 3F4 level is pumped directly with a 1.5-W, 1,483-nm laser diode. For Pr3+:KPb2Cl5 under this pump, the room temperature fluorescence that results from 1,600 to 2,800 nm is shown in Figure 7 and from 3,000 to 5,500 nm is shown in Figure 8[32]. Each feature in the spectra is labelled with the associated Pr3+ energy level transition. Figure 7 Fluorescence from 1,600 to 2,800 nm resulting from 1,483-nm pumping of Pr 3+ :KPb 2 Cl 5 . The sample has a Pr3+ concentration of 1.5 × 1020 ions/cm3. Figure 8 Fluorescence Florfenicol from 3,000 to 5,500 nm resulting from 1,483-nm pumping of Pr 3+ :KPb 2 Cl 5 . The sample has a Pr3+ concentration of 1.5 × 1020 ions/cm3. The Tm3+ sensitization of Pr3+:KPb2Cl5 allows for more convenient 800-nm diode pumping. For a co-doped Tm3+-Pr3+:KPb2Cl5 crystal using a 1.5-W, 805-nm laser diode as a pump source, the same broadband mid-IR emission between 4,000 and 5,500 nm from the Pr3+ ions is observed. The room temperature fluorescence that results from 805-nm pumping of the co-doped crystal overlapped with the fluorescence that results from the 1,483-nm pumping of the same co-doped crystal from 1,600 to 2,800 nm is shown in Figure 9 and from 3,000 to 5,500 nm is shown in Figure 10[32].