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Discovering energy-transfer paths in laser crystals.

We demonstrate a new technique for discovering the main mechanisms of energy transfer inside the active crystal of a solid-state laser. It consists of measuring the harmonic content of fluorescence when the pump has a sinusoidal modulation. In particular we discuss excited-state absorption and upconversion and present the equations for the fundamental and the second harmonic for both of these processes. We measured the fluorescence from the (4)S(3/2) multiplet of Er(3+) in three hosts, YLF, YAG, and CaSGG, with different doping. The results of the new method show that we are able to identify which multiplets transfer their energy to the observed one and through which process.