Sub-Wavelength Slit-Assisted Binary Metallic Lens Design for Effective Multifocusing via Phase Superposition Method

This paper proposes a multifocal lens design method named phase superposition method (PSM) and a methodology to increase the efficiency of metallic binary zone plates through a sub-wavelength slit array. The phase superposition method calculates the intensity at each focus, assuming a perfectly phase-matched modulation based on input light, and the size of the lens. Subsequently, based on the calculated focus intensity, the phases linearly related to the target intensity at each focus are combined. The sub-wavelength slit-assisted metallic binary plate assumes the sub-wavelength slit array as a dielectric in the light polarized perpendicular to the slit. It then controls the condition of the slit to invert the phase difference between the opened region and the slit region. By applying these methods, the trifocal lens was designed to verify our idea. The light was focused on three target positions. After applying the sub-wavelength slit structure, the amplitude was increased up to 90.11%, and the efficiency increase was 303.99% compared to that of a binary plate without the sub-wavelength slit.