A novel plasmonic lens (PL) with simple nano-structure is proposed to realize the subwavelength focusing. The proposed PL is composed of the gold film with only five dielectric-filled nanoslits. The exit surface of the gold film is processed into quadric shape that can be parabolic, elliptical or hyperbolic cylinders. The film is fabricated to form five uniformly spaced nanoslits with different widths and depths. All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium (i.e., SiO2). Under the illumination of TM polarized beams, the surface plasmon polaritons are excited at the entrance surface of the PL, then pass through the SiO2-filled slits while acquiring specific phase retardations, and are finally coupled to the light waves in the free space. Each light wave originating from the slit can be regarded as an individual point source, and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL. We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface, filling medium and geometric parameters of the slits. A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape. Three kinds of quadratic cylindrical PLs, i.e., parabolic, elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum. The finite difference time domain method is employed to compute the light field and to investigate the focusing characteristics of the proposed PL. The performance measurements include the focal length, depth of focus and full-width half-maximum (FWHM). The simulation results confirm that the proposed PL with a 2-µm-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale. The attainable smallest FWHM of the focal spot is 0.405λ0 (λ0 denoting the wavelength of the incident light) which is well beyond the diffraction limit. It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL. For example, the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92% of that produced by the continuous elliptical cylindrical PL. The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices. Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices in the fields such as optical micro manipulation, super-resolution imaging, optical storage and biochemical sensing.
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