Manipulation of radial-variant polarization for creating tunable bifocusing spots

Creation of identical multiple focal spots with three-dimensional arbitrary shifting

We propose a simple and flexible method to create identical multiple focal spots with three-dimensional arbitrary shifting without moving lenses or laser beams. The incident cylindrical vector (CV) beam superposed with predesigned phase and amplitude modulations is tightly focused by a single lens. The multiple focal spots with predetermined number and positions are generated and the identical intensity distribution as well as the polarization distribution for each individual focal spot is demonstrated. We also present a three-dimensional dynamic shifting with four identical focal spots along Pyramid-like trajectory by continuously regulating the phase and amplitude modulations on the incident CV beam. Furthermore, multiple focal spots with unique intensity profile can also be achieved when proper diffractive optical element (DOE) is associated in the focusing system. These engineered focal fields may find potential applications in 3D laser printing, moving multiple particles trapping and manipulations.

Creation of multiple on-axis foci and ultra-long focal depth for SPPs

We present the design of a plasmonic lens (PL) which is composed of pixelated nano-grooves on a gold film for the coupling and focusing of surface plasmon polaritons (SPPs) into multiple focal spots on the optical axis. The pixelated grooves are arranged along the y-axis and the x-position of each groove is optimized by the simulated annealing algorithm. PLs that implement two and three on-axis foci are presented and the designed structures have been validated with FDTD simulations. We also successfully constructed a long-focal-depth PL with a longitudinal FWHM of the focus that reached 25 plasmonic wavelengths, while its transverse field profile is maintained over 15 µm distance. The presented design method constitutes a new basis for plasmonic beam engineering, and the proposed particular SPP focal fields have potential applications in multiple imaging, particle manipulating, and plasmonic on-chip signal transmission.

Creation of identical multiple focal spots with prescribed axial distribution

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.