Spectroscopic and line-narrowing properties of resonant sum and difference frequency generation

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Abstract
Three wave mixing is investigated with respect to its spectroscopic applications under fully resonant conditions. A theoretical study of the appropriate second order susceptibility χ(2) reveals interesting features both for sum and difference frequency generation. For difference frequency generation two different mechanisms are possible one of which allows the observation of resonances between excited states and the study of dephasing induced coherent emission. The coherence decay rate between the two excited states may be measured and also its pure dephasing rate. The line‐narrowing characteristics of the fully resonant χ(2) are studied with respect to correlation between the inhomogeneous distribution for different levels. The line‐narrowing capabilities are found to be partly complementary for the sum—and the two difference frequency resonance configurations. This leads to the suggestion of new line‐narrowing techniques which could provide information about type and amount of correlation in inhomogeneously broadened systems.