Upconversion in a divalent rare earth ion: optical absorption and luminescence spectroscopy of Tm2+ doped SrCl2

Crystals of 2% Tm2+ doped SrCl2 were studied by variable temperature optical absorption and luminescence spectroscopy. The absorption spectrum consists of the weak 4f13 intraconfigurational 2F7/2→2F5/2 transition around 8840 cm−1 and a series of intense parity allowed f→d transitions above 15,000 cm−1. Luminescence is observed from the 4f13 2F5/2 excited state as well as from the lowest-energetic 4f125d excited state at all temperatures between 10 and 300 K. Upconversion luminescence from the latter is observed after cw- (lamp) excitation at 8840 cm−1 into 2F5/2 at 15 K, as well as after pulsed (laser) excitation at 1064 nm at 300 K.