Spectroscopic observation of the rotational Doppler effect
  We report on the first spectroscopic observation of the rotational Doppler shift associated with light beams carrying orbital angular momentum. The effect is evidenced as the broadening of a Hanle/EIT coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions. 
S. Barreiro, J.W.R. Tabosa, H. Failache and A. Lezama