The focus of light--linear polarization breaks the rotational symmetry of the focal spot

We experimentally demonstrate for the first time that a linearly polarized beam is focussed to an asymmetric spot when using a high-numerical aperture focussing system.This asymmetry was predicted by Richards and Wolf [Proc.R. Soc.London A 253, 358 (1959)] and can only be measured when a polarization insensitive sensor is placed in the focal region.We used a specially modified photodiode in a knife edge type set up to obtain highly resolved images of the total electric energy density distribution at the focus.The results are in good agreement with the predictions of a vectorial focussing theory.