Optical fluorescence at the combinational frequency in coherently driven three-level systems
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We derive and compare the optical fluorescence spectra for all three transitions for three-level systems coherently coupled with two fields in a cascade or lambda configuration. It is shown that the total intensity of fluorescence at the combinational frequency (the sum or difference of the driving field frequencies in the ladder and lambda systems, respectively) may essentially exceed the fluorescence intensity at the driven transitions under the condition of two-photon resonance. The influence of additional homogeneous and inhomogeneous line broadening on the distribution of intensities between different fluorescence channels is studied. In particular, Doppler broadening is found to suppress fluorescence from the driven transitions but in some cases not from the combinational transition. 2011 Copyright Taylor and Francis Group, LLC.