We proposed a method to design a ternary optical element in order to achieve a needle of super-resolution longitudinally polarized beam with ultra-long depth of focus, and obtained a beam with a size of 0.3995λ and depth of focus of 12.83λ after focusing a ternary optical element modulated, radially polarized Bessel-Gaussian beam with an aplanatic lens of numerical aperture 0.95. The algorithm we used to design the ternary optical element is based on axial uniformity in the focal region, which allows rapid searching speeds and excellent performance. The ratio of pupil radius to the beam waist was set as 0.57, making the peak intensity of the incident beam occur at the rim of the lens aperture, which maximized the possible resolution of the focused beam.

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A terahertz absorber that can be dynamically tuned via an external magnetic field is proposed. The absorber is composed of periodic gold-disks and an underlying graphene sheet on a dielectric/gold reflector substrate. Simulated results reveal that a new absorption peak appears under the perpendicularly applied external magnetic field. The new absorption peak under a 10 T magnetic field red-shifts from 14.22 THz to 4.47 THz as the Fermi level of graphene increases from 0.1 eV to 0.3 eV. At a fixed Fermi level, the new absorption peak blue shifts as the magnetic field increases. The new absorption peak can be enhanced by using multilayer graphene. The absorber is polarization independent. These results may promote the development and applications of flexibly tunable terahertz absorbers.