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. (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. The ability to control and focus light is at the heart of the interdisciplinary field of optics and for many applications an exact knowledge of the structure of the focal field is required. Many areas in optical sciences make use of a tightly focussed light beam such as confocal microscopy [1] and optical data storage [2]. A highly concentrated and well matched field is also a necessary requirement for coupling to small quantum systems [3] and applying light forces to microscopic particles [4]. In the regime of strong focussing the widely used scalar theories are inadequate to describe the focal field. A vectorial focussing theory is required instead. In the concrete example of a beam which is linearly polarized along the y–axis, the rays propagating in the xz-and in the yz-plane, respectively, contribute differently to the focal field (see figures 1(a),(b)). This has two closely related consequences. The effective numerical aperture in the yz-plane is reduced and the focal spot is elongated in the direction of polarization of the input beam because the rays that propagate in this plane do not add up perfectly at the focus. Associated with this elongation, field components in directions orthogonal to the direction of polarization of the input beam arise. These effects are neglected in scalar descriptions [5], which therefore do not predict the correct spot shape and size. All aspects of the field distribution of a tightly focussed linearly polarized light beam have been studied in theory taking into account the vector properties of the field [6, 7, 8, 9]. The theoretical calculations predict pronounced deviations from the results of the scalar theory. Nevertheless, a major part of the intensity 1 is still predicted to be contained in a field 1 In the following, intensity always refers to electric energy density, which is the part of the field energy that couples to standard photodetectors and photosensitive materials. 1 Figure …