A method is presented for generation of a subwavelength (0.43λ) longitudinally polarized beam, which propagates without divergence over lengths of about 4X in free space. This is achieved by controlling the amplitude, phase and polarization property of the Bessel-Gaussian field on the aperture of a high numerical aperture focusing lens.
A plane electromagnetic wave is purely transversal. Thus, it was for many years assumed impossible to create longitudinally polarized light in free space. However, later it was shown that any beam of finite diameter has a longitudinal field component, even in a free space [1,2]. A strong longitudinal component appears at the focal region of a tightly focused laser beam [2-5]. It also arises with focusing of radially polarized light [6-12]. Besides academic interest, this longitudinal field has many attractive applications, e.g. in particle acceleration [2, 6, 7, 13], fluorescent imaging , second harmonic generation [15-17] and Raman spectroscopy . It can permit the achievement of higher resolution in z-polarized confocal fluorescence microscopy  and scattering scanning near-field optical microscopy .
The longitudinal field can be suppressed or enhanced by amplitude, polarization and phase modulation of the incident beam . For example, a longitudinal field can be completely suppressed in an azimuthally polarized beam [10, 22]. Several methods to enhance the longitudinal field component have been suggested [6, 11, 21, 23, 24], however all of them have insufficient optical efficiency (on the level of a few percents) and non-uniform axial field strength. Conversion efficiency is an critical characteristic for real applications, such as the use of radially and azimuthally polarized laser radiation for material processing , put into practice soon after the development of effective methods for radial and azimuthal beam polarization.