Transport of energy through the focal region of a high-numerical-aperture system with efficient annular focusing of light beam and its optimum thin-film linear-to-radial polarization conversion

Abstract

In a previous paper [Opt. Commun. 407 (2018) 217–226], there was proposed a light-efficient, high-numerical-aperture focusing system which is based on a monolithic assembly of an axicon and an inclined parabolic annular reflector (AXIPAR) optimally matched to thin-film linear-to radial polarization conversion of the illuminating beam. Also, there were derived integral expressions for the Poynting vector components of the respective, tightly focused beams having a near longitudinal polarization along the optical axis. In the present paper, being a direct continuation of the previous one, these expressions are used to analyze numerically a very specific behavior of the Poynting vector magnitude and the energy flow trajectories of deep UV laser beams in the focal regions localized in an immersion liquid. In particular, it is demonstrated graphically that, in the case of an incomplete suppression of the azimuthal component of the beams, their energy flow trajectories will have not only a small waviness near the focus, but also a moderate twisting around the optical axis, carrying an orbital angular momentum. The spatial frequency filtering of the beams is found to increase the axial size of the focal region where a more pronounced twisting is observed.