When a wave is reflected from a moving object, its frequency is Doppler shifted1. Similarly, when circularly polarized light is scattered from a rotating object, a rotational Doppler frequency shift may be observed2,3, with manifestations ranging from the quantum world (fluorescence spectroscopy, rotational Raman scattering and so on3,4) to satellite-based global positioning systems5. Here, we observe for the first time the Doppler frequency shift phenomenon for a circularly polarized light wave propagating through a gas of synchronously spinning molecules. An ensemble of such spinning molecules was produced by double-pulse laser excitation, with the first pulse aligning the molecules and the second (linearly polarized at a 45° angle) causing a concerted unidirectional rotation of the ‘molecular propellers’6,7. We observed the resulting rotating birefringence of the gas by detecting a Doppler-shifted wave that is circularly polarized in a sense opposite to that of the incident probe.
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The authors thank E. Gershnabel and Y. Khodorkovsky for useful discussions and E. Grinvald for assistance with the notch experiments. Financial support for this research was provided by the Israel Science Foundation (grant no. 601/10) and the Deutsche Forschungsgemeinschaft (grant no. LE 2138/2-1). I.A. is the incumbent of the Patricia Elman Bildner Professorial Chair. Y.P. is the incumbent of the Sherman Professorial Chair. This research is made possible in part by the historic generosity of the Harold Perlman Family. R.J.G. thanks the Weston Foundation for support during a sabbatical visit.
The authors declare no competing financial interests.
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Korech, O., Steinitz, U., Gordon, R. et al. Observing molecular spinning via the rotational Doppler effect. Nature Photon 7, 711–714 (2013) doi:10.1038/nphoton.2013.189
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