Abstract
This paper presents a method to generate a multi-segmented optical needle with a strong longitudinally polarized field, uniform intensity along the optical axis, and a transverse size (~0.36λ). The length of each segment in the optical needle and the spacing between adjacent segments are controllable by reversing and focusing the radiation pattern from a sectional-uniform line source antenna to the focal volume of a 4Pi focusing system. By solving the inverse problem, we can obtain the required incident field distribution at the pupil plane to create the multi-segmented optical needle. Numerical examples demonstrate that a multi-segmented optical needle with variable focal depth, adjustable interval, narrow lateral width, homogeneous intensity, and high longitudinal polarization purity can be formed using the proposed approach. The length of each needle segment is approximately equal to the length of the corresponding sectional uniform line source. The multi-segmented optical needle may be employed in applications such as multi-particle acceleration, multi-particle trapping and manipulation, laser machining, and laser material processing.
Highlights
In the past decade, the high-numerical-aperture (NA) focusing of a cylindrical vector beam has attracted considerable interest because of its novel focusing property[1,2,3,4,5]
The required illumination at the pupil plane to create such focal field can be analytically obtained from the radiation field F (θ) in Eq (1) and Richards-Wolf theory[22, 23]
The nth zFWHM is approximately equal to length Ln and only determined by Ln
Summary
The required illumination at the pupil plane to create such focal field can be analytically obtained from the radiation field F (θ) in Eq (1) and Richards-Wolf theory[22, 23]. The nth zFWHM is approximately equal to length Ln and only determined by Ln. The nth spacing between two adjacent segment needles is approximately equal to the blanking Sn. Figure 2b shows that the phase distribution of the Ez component exhibits a binary behavior between −90° and +90°; in particular, the value of −90° is maintained in the scope of the nth main lobe, whose axial length is equal to the length of Ln. The phase stability must be maintained to generate a high-quality light needle field[26]. This input field may be achievable at present using the latest technologies of spatial light modulation and metasurfaces[29,30,31]
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