Single‐Spot Focusing with Plasmonic Phase Manipulation

Abstract

The ring structure has an inherent symmetry that can be used to focus light to a single in‐plane spot for near‐field plasmonic applications. However, when excited with a linearly polarized light, it yields split focal spot at the center due to destructive interference of surface plasmon polaritons (SPPs). Here, twin configurations of a slit‐based plasmonic lens for in‐plane focusing of a linearly polarized light are proposed. Lenses are designed based on the ring geometry in order to exploit symmetric nature of the ring structure. Either a metallic or a dielectric semicircular protrusion is introduced in the symmetric ring structure for compensating the phase mismatch between SPPs counter‐propagating from two sides of the ring. The focusing properties of the lenses are analyzed using finite‐difference time‐domain simulations. A single focal spot with full width at half maxima of 0.27λ0 and 0.28λ0 is obtained for plasmonic lenses with metallic and dielectric protrusions, respectively. Thus the diffraction limit is beaten by focusing the light to a subwavelength region of approximately quarter its wavelength. The lenses have potential applications in nanosensing, nanoimaging, and nanolithography, wherein closely populated focused spots are desired using linearly polarized excitation.

Asymmetry‐Based Plasmonic Phase Manipulation for a Compact Far‐Field Optical Lens

“…Fabrication complexity of such 3D structure that needs a step height can be met with a standard e‐beam lithography procedure followed by lift‐off and subsequent milling of slits with a direct fabrication process such as focused‐ion beam (FIB). [ 21 ] Similarly, PPMZPs in Figure 1c can be fabricated by lithographic techniques aided by direct site‐specific milling with FIB.…”

“…[12][13][14][15][16][17][18] SPPs are excited at the metal dielectric interface when p-polarized light is incident on the interface and they get reradiated at the edges. Local phase manipulation of SPPs with the aid of geometrical modifications has enabled design of various optical components for different functionalities such as near-field focusing, [19][20][21] generation of orbital angular momentum (OAM) in near-field [22] as well as far-field. [23] Concept of merging the geometric phase with plasmon retardation phase is illustrated for arbitrary generation of OAM.…”

“…However, the component of electric field perpendicular to the structure yields a split focal spot at the desired focal region. [21,26] The reason for such split in the focal region is attributed to the inherent nature of linearly polarized light.…”

“…Previously, we have designed a near‐field phase manipulative plasmonic lens which yields a single subwavelength focal spot in the near‐field and improves the resolution. [ 21 ] In this work, we extend the concept of plasmonic phase manipulation to get a far‐field focusing plasmonic ZPs with improved resolution. By directly applying the concept, a 3D phase manipulative ZPs (PMZPs) is proposed and with modification its planar version planar phase manipulative ZPs (PPMZPs) is proposed.…”