Observation of OAM sidebands due to optical reflection

We investigate how the orbital angular momentum (OAM) of a paraxial light beam is affected upon reflection at a planar interface. Theoretically, the unavoidable angular spread of the (paraxial) beam leads to OAM sidebands which are found to be already significant for modest beam spread (0.05). In analogy to the polarization Fresnel coefficients we develop a theory based upon spatial Fresnel coefficients; this allows straightforward prediction of the strength of the sidebands. We confirm this by experiment. PACS numbers: A light beam, either classical or quantum, possesses spa-tially transverse degrees of freedom. A very popular ex-ample is the Orbital Angular Momentum (OAM) of light [1, 2]; this has important applications in quantum com-munication [3–5]. The key advantage of OAM for quan-tum communication is its high dimensionality [6]; this allows a single photon to carry much more [7, 8] than the two bits of information (qubit) enabled by the po