Creation of a 50,000λ long needle-like field with 036λ width

Abstract

It is a great challenge to create a needle-like field with properties of long beam length, narrow lateral width, uniformity, and high optical efficiency. Here we show a method that can realize these properties all at once. The key element is a 90° apex-angle concave conical mirror. By using this condenser along with a radially polarized incident beam of a specific field distribution, we numerically created a super slim, uniform, pure needle-like axially polarized field. This axially polarized field has a length of 50,000λ along the optical axis, and its lateral width still maintains a minimum 0.36λ size.

Creation of a 50,000λ long needle-like field with 036λ width: comment

“…At optical wavelengths, that length is an order of magnitude larger than the needle proposed in Ref. [1].…”

“…[1] the authors claim that as the result of the evaluation of the integral of the theorem of Helmholtz and Kirchhoff, they obtain a continuous transformation from the incident radially polarized plane wave onto the axially polarized cylindrical wave. However, from a propagation perspective since a CW beam is considered and assuming a metallic conical mirror (the field must be zero at the surface of the cone), the incident beam after being reflected transforms into a cylindrical wave traveling perpendicular to the incident beam.…”

“…(2) in Ref. [1] it is not a good approximation at wavelength scales since there is a π∕4 phase shift not taken into account that is needed to get the standing wave described by the Bessel function J 0 and the π∕2 phase shift that occurs when passing from incoming wave to outgoing wave. This phase shift is part of the incoming Hankel wave for which its asymptotic expression is to first order [8][9][10],…”

“…In a recent remarkable article, a simple method to produce a needle of 0.36λ and with a length of 50; 000λ was proposed by Zhu et al in which they use a conical mirror [1]. The point that makes it relevant is that the theory of the axisymmetric needle is intimately related to the physics of Bessel beams, one of the most investigated and intriguing optical beams [2].…”

“…The larger the mirror the larger the needle created with a field that requires a suitable apodization to reduce edge diffraction effects, as discussed in Ref. [1].…”