Abstract

Recently many ideas have been proposed for the use of a longitudinal field for particle acceleration, fluorescent imaging, second-harmonic generation and Raman spectroscopy. A few methods to enhance the longitudinal field component have been suggested, but all have insufficient optical efficiency and non-uniform axial field strength. Here we report a new method that permits the combination of very unusual properties of light in the focal region, permitting the creation of a |[lsquo]|pure|[rsquo]| longitudinal light beam with subdiffraction beam size (0.43|[lambda]|). This beam is non-diffracting; that is, it propagates without divergence over a long distance (of about 4|[lambda]|) in free space. This is achieved by focusing a radially polarized Bessel–Gaussian beam with a combination of a binary-phase optical element and a high-numerical-aperture lens. This binary optics works as a special polarization filter enhancing the longitudinal component.