Vacuum-mediated multiphoton transitions - art. no. 051402

We have measured the emission spectra of two-level-like atoms driven by a s patially inhomogeneous bichromatic field comprised of one resonant and one off-resonant component. Under these excitation conditions, observed spectra consist of several narrow peaks appearing on top of a broad continuumlike structure. This experimental approach provides for selective observation of those spectral features having a Rabi-frequency-independent emission frequ ency. Such emission processes involve multiphoton transitions: with one leg being spontaneous, between dressed-state levels differing only in their ph otonic quantum numbers. i.e., between equivalent dressed-state-doublet subl evels. Related vacuum-mediated transitions have recently been invoked as an underlying mechanism of inversionless gain.