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

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.