Floating Thin Plasmonic Graded-Index Lens

Manipulation of optical information using surface plasmon polaritons (SPPs) has been rigorously studied by utilizing subwavelength nature of SPPs. Specifically, the intensity and launching direction of SPPs can be efficiently controlled when the geometry of metallic structure and the shape of dielectric material are designed [1]. There exist many types of in-plane plasmonic lenses based on principles of Fourier optics [2]. But thickness of the floating lens in previous study is about 5 µm and it is quite long for applying in plasmonic devices. Therefore, we propose a novel floating thin lens based on the graded-index method. Figures 1(a) and 1(b) show the top and front view of the lens structure. By changing the gap between floating silicon-dioxide and silver substrate, it is possible to make gradual variation of effective index of SPPs. Planar SPP waves, after passing through the thin flat lens region, are focused to the focal point on the central axis. Figure 1(c) shows simulation result of transverse H-field distribution at the boundary of silver substrate and air. The lens is designed to impose parabolic phase and numerically analyzed by full-wave simulation based on finite difference time domain method (FDTD) while the operating wavelength is designed to 633 nm. Lateral width and thickness of the lens is 1.1 µm and 100 nm, respectively. The floating gap thickness varies from 20 nm to 500 nm in 11 symmetrically separated lens sectors in order to form a discrete parabolic phase. We expect that the proposed method can be used to design various compact plasmonic devices that may help avoid loss problem by reduction of device size.

[1] S.-Y. Lee et al., "Role of magnetic induction currents in nanoslit excitation of surface plasmon polaritons," Physical Review Letters, 108, 213907 (2012).

[2] H. Kim, J. Hahn, and B. Lee, "Focusing properties of surface plasmon polariton floating dielectric lenses," Optics Express, 16, 3049-3057 (2008).

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