Revisiting the Photon-Drag Effect in Thin Metal Films

Current flow in metal films can be induced by the photon momentum carried by an obliquely-incident electromagnetic wave, a phenomenon known as photon drag. The prevailing intuition for the sign of this current is based on the assumption that the absorbed light transfers momentum to the free electrons of the metal, generating electron flow in the direction of the in-plane incident photon momentum. However, the direction of this flow has been reported to puzzlingly vary with polarization state, surface morphology, and excitation of surface plasmons. Here we demonstrate that for a smooth gold film, s-polarized light unequivocally produces a photon drag current proportional to the Minkowski photon momentum and unrelated to plasmonic effects. But the sign of the observed current is opposite to that implied by the intuitive model above, requiring a reimagination of the microscopic processes of light-metal interaction to include momentum transfer to all the electrons of the metal, free and bound. To this end, we propose a new model which also accounts for an observed marked change in amplitude and sign of the photon drag current upon polarization rotation.