Beam propagation near the dispersionless wavelength at 790 nm in rubidium
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2016 American Physical Society. Between any two resonance lines for an atomic medium there exists a dispersionless wavelength where the ac Stark shift from the two resonances cancel and the real part of the resonant susceptibility is 0. We experimentally demonstrate the effects of this wavelength in a pump-probe experiment in high-density atomic vapor. A strong pump field excites the medium, in which we counterpropagate a broadband probe field. In a long cell, only at the dispersionless wavelength will scattering and self-focusing or defocusing not cause attenuation, allowing a spectrometer to read off the dispersionless wavelength. We perform a propagation experiment that shows the passband centered around the 790-nm dispersionless wavelength between the D1 and the D2 lines of rubidium.