Measurement of ac-Stark shift by a two-photon dressing process via four-wave mixing Huaibin Zheng, Utsab Khadka, Jianping Song, Yanpeng Zhang and Min Xiao EPL, 93 2 (2011) 23002 Published online: 03 February 2011 DOI: 10.1209/0295-5075/93/23002