Engineering of variable length optical needle based on uniform line source antenna

A flexible method is presented to generate an arbitrary length optical needle with high purely longitudinal polarization and consistent beam size (~0.36λ) over flattop range. By reversing the field radiated from a uniform line source, for which the electric current is constant along its extent, situated at the focus of the 4Pi focusing system formed by two confocal high-NA objective lenses, the required illumination distribution at the pupil plane for creating optical needle field can be obtained. Numerical results show that the length of light needle field is determined by the length of uniform line source. The optical needle created by this method might found potential applications in particle acceleration, microscopy, and optical trapping.