Construction of optical needles and spot arrays using radiation pattern of collinear antenna array

An optimization-free approach is proposed for generating optical needle and bright spot arrays with controllable distributions. This is achieved by inversely focusing the radiation pattern from a collinear antenna array whose elements are placed along an optical axis and whose currents in each element also flow along the direction of that axis, situated at the focus of a 4pi configuration. The shape and central location of the nth focusing field only depend on the length l
_n
and the coordinate z
_n
of the nth element in the collinear array, respectively. The number of focal fields is determined by the number of elements in the array. The focal field is dominantly polarized along the longitudinal direction and its full width at half-maximum along the radial direction remains unchanged within the entire depth of focus. Three types of engineered focal field arrays, namely pure needle arrays, pure spot arrays, and hybrid arrays of needles and spots, are presented as examples to demonstrate the validity and flexibility of the proposed method. The light needle and bright spot arrays generated by this approach may find potential applications in multi-particle acceleration and in multi-particle trapping and manipulation.