Experimental studies of non-radiative relaxation rates in solid state laser materials

Abstract

Techniques for the measurement of non-radiative decay rates between the optically excited states in solid-state laser materials are described. Two crystals have been investigated, ruby (Cr³⁺:Al₂O₃) and Sm²⁺:CaF₂. A review of the spectroscopy of these materials is presented with particular emphasis on aspects relevant to non-radiative decay.

An experiment is described using a second harmonic Nd³⁺ Nd:glass Q-switched laser to pump ruby crystals. Limiting non-radiative relaxation times were derived from the rise of fluorescence. Rubies varying in concentration between 0.05% and 0.4% were studied over a range of temperatures, 77 -> 7000 K. ⁴T₂ -> ²E(E + 2A), ⁴T₂ -> ²T₁ (Eb) and ⁴T₂ -> ²T₁ (2A + Ea) non-radiative relaxations were all found to be <4ns in good agreement with the experimental results of other workers.

Two experiments employing a ruby laser to pump Sm²⁺ :CaF₂ are described. The first used a Q-switched laser and, apart from some improvements was similar to the ruby experiment. An upper limit of 4f⁵ 5d pump -> metastable non-radiative decay of 2ns was found for Sm²⁺:CaF₂, The fluorescence decay constant at 295 K was also measured to be 63ns. The second experiment employed a mode-locked laser and the Sm²⁺:CaF₂ non-radiative decay time was <0.5ns.

A new effect - provisionally called 'surface emission' - discovered during the ruby measurements, has been examined in detail and the results of this investigation are presented.

A phase-shift technique for the measurement of non-radiative decay times is examined in detail. A pilot experiment using a self-mode-locked He-Ne laser at 633 nm to pump Sm²⁺:CaF₂ is described. The non-radiative relaxation time found by this method was <33ps.

Suggestions for improved experiments, both pulsed and phase-shifts, are made and the application of the techniques to other materials is discussed.

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