Infrared-laser-induced upconversion from Nd3+:LaF3 heteroepitaxial layers on CaF2(111) substrates by molecular beam epitaxy

This paper reports a systematic analysis on the upconversion fluorescence from a Nd3+-doped LaF3 planar waveguide grown on (111) oriented CaF2 substrates by molecular beam epitaxy. A spectroscopic study of upconversion emission has been carried out at low temperature. Upon infrared excitation into the 2H9/2, 4F5/2, or 4F3/2 multiplets of Nd3+, strong uv upconversion emissions originating from 4D3/2 to 4IJ (J=(9/2), (11/2), and (13/2)) transitions by a three-photon process have been observed. In addition, other upconversion emissions in the green and orange regions due to two-photon processes were also obtained and attributed to the 4G7/2→4I9/2, 2G7/2+4G5/2→4I9/2, and 4G7/2→4I11/2 transitions, respectively. In accordance with our experimental results, two kinds of energy-transfer processes are proposed as upconversion mechanisms responsible for the different emissions, and which are supported by a rate-equation analysis. The green and orange upconversions originate from an energy-transfer process involving two Nd3+ ions excited in the 4F3/2 state. For the three-photon upconversion, two successive energy cross-relaxation are suggested to populate the 4D3/2 level. The concentration dependence study has shown that the optimum concentration for a Nd3+ dopant is about 1 at. % for all the upconversion emissions. Owing to a guided configuration, upconversion emissions that are hardly detectable in a nonguided configuration at temperature higher than 100 K have now been recorded at room temperature