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 electromagnetically induced transparency coherence resonance on Rb vapor when the two incident Laguerre-Gaussian laser beams have opposite topological charges. The observations closely agree with theoretical predictions.

Figure 1

Scheme used for the observation of rotational Doppler effect on Hanle/EIT resonances illustrated on a $F=1\to F^{\prime }=0$ transition. The two spatially overlapped fields have identical frequency, opposite circular polarizations, and topological charges $l_1$ and $l_2$.

Figure 2

Experimental setup. ECDL is the extended cavity diode laser; PBS, polarizing beam splitter; PD, photodiode. M1 and M2 are the Laguerre-Gaussian mode-generating masks.

Figure 3

Observed Hanle/EIT resonances as a function of the total Zeeman shift $\delta =2g{\mu }_{B}B$ for different pairs of LG mode-generating masks. Dashed (solid) lines denote the same (opposite) topological charge on each field.

Figure 4

Line width dependence on topological charge. Circles experimental results are reflected by solid (hollow) symbols to denote equal (opposite) charge on each field. Triangles show theoretical prediction.