Focusing light with a deep parabolic mirror

TL;DR

This paper investigates how a deep parabolic mirror can focus light to a near-ideal point, comparing its performance with conventional methods and analyzing the effects of aberrations.

Contribution

It introduces a detailed calculation of electric energy density in the focus of a parabolic mirror and compares it with traditional high-NA illumination techniques.

Findings

Deep parabolic mirrors can achieve near-ideal focusing conditions.

Comparison shows advantages over conventional nearly 4pi illumination.

Aberrations due to misalignment significantly affect focus quality.

Abstract

The smallest possible focus is achieved when the focused wave front is the time reversed copy of the light wave packet emitted from a point in space (S. Quabis et al., Opt. Commun. 179 (2000) 1-7). The best physical implementation of such a pointlike sub-wavelength emitter is a single atom performing an electric dipole transition. In a former paper (N. Lindlein et al., Laser Phys. 17 (2007) 927-934) we showed how such a dipole-like radiant intensity distribution can be produced with the help of a deep parabolic mirror and appropriate shaping of the intensity of the radially polarized incident plane wave. Such a dipole wave only mimics the far field of a linear dipole and not the near field components. Therefore, in this paper, the electric energy density in the focus of a parabolic mirror is calculated using the Debye integral method. Additionally, a comparison with "conventional nearly 4pi" illumination using two high numerical aperture objectives is performed. The influence of aberrations due to a misalignment of the incident plane wave is discussed.