Laser optical double resonance and efficient infrared quantum counter upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+

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Abstract
In this paper, we report the first laser‐excited optical double‐resonance experiments in rare‐earth‐doped crystals. Infrared upconversion in LaCl3 : Pr3+ and LaF3 : Pr3+ has been achieved by means of a tunable cw dye laser pump. The quantum efficiency of the upconversion process at 2.03 μ in LaCl3 : Pr3+ is 1.2×10−3, which is an improvement of 4×105 over the highest efficiency found previously with incoherent sources. It is shown that an efficiency of 1 can be easily obtained with higher fluxes and improved crystal quality. Direct excitation of the output fluorescence from the 3P0 level is observed in both LaCl3 : Pr3+ and LaF3 : Pr3+ when the dye laser frequency is resonant with the states of the lower‐lying 1D2 manifold in the absence of the infrared input. Detailed assignments of the observed double‐resonance transitions have been made from wavelength and polarization measurements.