Switching Between Rayleigh-like and Lorentzian Lineshapes of the Dispersion in Driven Two-level Atoms

We investigate the structure of the dispersion and absorption lineshapes for driven two-level systems against a weak probing laser field. It is shown that the shape of both curves depends critically on the detuning of the driving laser to the atomic resonance frequency and can be transformed continuously and oppositely between Rayleigh (or ‘dispersion-like’) and Lorentzian (or ‘absorption-like’) lineshapes. An analysis in the dressed state basis explains these features via the different influences of coherences and populations of the corresponding dressed states.