Dynamics and spectroscopy of infrared-to-visible upconversion in erbium-doped cesium cadmium bromide (CsCdBr3:Er3+)

Trivalent erbium ions in ${\mathrm{CsCdBr}}_3$ exist predominantly as dimers along one-dimensional lattice chains. Very efficient upconversion from near-infrared to various visible wavelengths was observed. ${}_{}{}^2{}_{}{}^{}$ ${\mathit{H}}_{9/2}$ ${\to }^4$ ${\mathit{I}}_{15/2}$ (414 nm) and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{7/2}$ ${\to }^4$ ${\mathit{I}}_{15/2}$ (493 nm) fluorescence dominated for ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{15/2}$ ${\to }^4$ ${\mathit{I}}_{9/2}$ (801 nm) and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{15/2}$ ${\to }^4$ ${\mathit{I}}_{11/2}$ (984 nm) excitation, respectively. Simultaneous excitation at 801 and 984 nm preferentially populated ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{5/2}$, giving ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{5/2}$ ${\to }^4$ ${\mathit{I}}_{15/2}$ (455 nm), ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{5/2}$ ${\to }^4$ ${\mathit{I}}_{13/2}$ (651 nm), and, by cross relaxation, ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{9/2}$ ${\to }^4$ ${\mathit{I}}_{15/2}$ (671 nm). The efficiency of multiple-color upconversion is attributed to low phonon energies and close coupling between ions. Upconversion mechanisms are proposed, and energy-transfer rates determined from the temporal transients.