Generation of subdiffraction longitudinal bifoci by shaping a radially polarized wave.

Lenses with two or more foci along the longitudinal direction exhibit immense potential in several optical applications. In this study, we propose an approach for generating subdiffraction longitudinal bifoci by binary-phase bifocal super-oscillatory lenses (SOLs), which are realized by simple AND operation between two single-foci SOLs with different focal lengths. Three bifocal SOLs with =70 are designed at an operating wavelength of =118.8µ. Simulation results demonstrate that the minimum full width at half maximum (FWHM) is 0.397, and the maximum FWHM is 0.449, which is still smaller than the Abbe diffraction limit of 0.510, while all the sidelobe ratios are small (<15.1). By properly choosing the focal length of the single-foci SOLs in the design process, the distance between the two foci can be easily controlled. Significantly, the generated bifoci with relatively uniform intensity contain a strong longitudinal electric field, which indicates their excellent prospects in optical imaging, particle acceleration, and other optical applications. In addition, the proposed bifoci-SOLs are based on the binary phase modulation, which facilitates easy fabrication compared with other approaches. These outstanding properties indicate the wide application prospects of bifocal SOLs.