Infrared and visible photon upconversion in LaCl3:Pr3+ (Nd3+)

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Abstract
Investigation of the optical properties of single crystals of LaCl3 containing 1% Pr3+ and traces of N d3+ impurity has revealed several upconversion processes in which laser excitation results in the emission of photons of higher energy than that of the pump photons. The fluorescence upconversion flux dependences and decay times are analyzed in terms of two mechanisms: (1) a sequential two‐photon excitation process in which the initially excited ion decays to a lower excited level before the absorption of a second photon sends it to a higher excited level and (2) an energy transfer upconversion (ETU) process in which two nearby ions in excited states undergo an annihilation interaction resulting in one ion in a higher excited state and the other in a lower state. The time dependence of the upconverted fluorescence is derived from transient rate equations for the ETU process under a square wave excitation and the derivation is found to be in good agreement with the experimental data. Upconverted fluorescence resulting from infrared quantum counter action is also observed when the sample is in double resonance with a laser excitation source and a broad‐band (1.15–4 μ) infrared source.