Karl W. Krämer, Hans U. Güdel and Robert N. Schwartz
NIR to VIS upconversion in LaCl₃: 1% Er³⁺
One- and two-color excitations around 1000 and 800 nm
J. Alloys Comp. 275-277, 191-195 (1998)                 

Abstract: LaCl₃: 1% Er³⁺ shows visible luminescence upon near infrared excitation. In a first step the ⁴I_15/2 ⁴I_11/2 or ⁴I_15/2 ⁴I_9/2 transitions of Er³⁺ are excited by infrared laser radiation around 980 or 810 nm, respectively. In a second step the ⁴F_7/2 (490 nm), ⁴F_3/2 (445 nm) or ²H_9/2 (405 nm) levels are populated via excited state absorption from the ⁴I_11/2 or ⁴I_9/2 levels. The excitation wavelengths for the second step are again around 810 or 980 nm, respectively. The mechanisms of one- and two-color excitations were determined by polarized excitation spectroscopy. Efficient luminescence from those highly excited states is obtained in the blue, green and red spectral regions. The upconversion efficiency can be strongly enhanced by two-color excitation. Pure blue ⁴F_5/2 ⁴I_15/2 and red ⁴F_5/2 ⁴I_13/2 upconversion luminescence was detected for the first time upon NIR excitation of Er³⁺.