Abstract
We present a scheme for the construction of coaxially equidistant multiple focal spots with identical intensity profiles for each individual focus and a predetermined number and spacing. To achieve this, the radiation field from an antenna is reversed and then gathered by high numerical aperture objective lenses. Radiation patterns from three types of line sources, i.e., the electric current, magnetic current and electromagnetic current distributions, with cosine-squared taper are respectively employed to generate predominately longitudinally polarized bright spots, azimuthally polarized doughnuts, and focal spots with a perfect spherically symmetric intensity distribution. The required illuminations at the pupil plane of a 4Pi focusing configuration for the creation of these identical multiple focal spots can be easily derived by solving the inverse problem of the antenna radiation field. These unique focal field distributions may find potential applications in laser direct writing and optical microscopy, as well as multiple-particle trapping, alignment, and acceleration along the optical axis.
Highlights
We present a scheme for the construction of coaxially equidistant multiple focal spots with identical intensity profiles for each individual focus and a predetermined number and spacing
The study of cylindrical vector (CV) beams, including the radially polarized beam, azimuthally polarized beam, and generalized CV beam, especially when focused under a high numerical aperture (NA) lens, has drawn much attention[1,2,3,4] owing to their fascinating properties and intriguing applications, including optical microlithography[5], microscopy[6], spectroscopy[7], and so on
We introduce a tapered line source antenna, which is often used in the antenna engineering field
Summary
From Eq (5), the required input field at the pupil plane of the high NA lens for creating these focal patterns can be calculated as a radially polarized field with spatial amplitude modulation, as shown in Fig. 3(d) with N = 6 (8)–(10), the identical spherical spots along the optical axis are readily attainable by the use of the radiation fields from the electromagnetic current tapered line sources with parameters N = 5, 6, 11, as illustrated, respectively. Note that when N ≥ (L/λ ), the distance between two adjacent spots will be compressed, and as a result, these spots will be combined into a quasi-optical needle or quasi-optical tunnel
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