Enhanced optical transmission through a star ... - OSA Publishing

Enhanced optical transmission through a starshaped bull’s eye at dual resonant-bands in UV and the visible spectral range Tavakol Nazari, Reza Khazaeinezhad, Woohyun Jung, Boram Joo, Byung-Joo Kong, and Kyunghwan Oh* Photonic Device Physics Laboratory, Institute of Physics and Applied Physics, Yonsei University, Seoul 120–749, South Korea * [email protected]

Abstract: Dual resonant bands in UV and the visible range were simultaneously observed in the enhanced optical transmission (EOT) through star-shaped plasmonic structures. EOTs through four types of polygonal bull’s eyes with a star aperture surrounded by the concentric star grooves were analyzed and compared for 3, 4, 5, and 6 corners, using finite difference time domain (FDTD) method. In contrast to plasmonic resonances in the visible range, the UV-band resonance intensity was found to scale with the number of corners, which is related with higher order multipole interactions. Spectral positions and relative intensities of the dual resonances were analyzed parametrically to find optimal conditions to maximize EOT in UV-visible dual bands. ©2015 Optical Society of America OCIS codes: (240.6680) Surface Plasmons; (120.7000) Transmission; (050.6624) Subwavelength structures; (050.1220) Apertures; (240.5440) Polarization-selective devices.

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Received 7 Apr 2015; revised 26 Jun 2015; accepted 26 Jun 2015; published 9 Jul 2015 13 Jul 2015 | Vol. 23, No. 14 | DOI:10.1364/OE.23.018589 | OPTICS EXPRESS 18589

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Introduction Surface plasmon polaritons (SPPs) have shown a hyper-sensitive response to variations of structural parameters in both reflection and transmission, which has opened new opportunities not only in photonics but also in biological and material science [1–6]. In nano-aperture

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plasmonic arrays, extraordinary transmission has been observed by Ebbesen nearly a decade ago, which initiated intensive research on plasmonic structures with unique spectral control capability in the optical transmission [7]. However, those nano-aperture arrays in a metallic film have suffered from low transmission efficiency, and concentric grooves around the central aperture have been added form a bull’s eye structure to further maximize enhanced optical transmission (EOT) [8–11]. Bull’s eye structures have shown that the transmitted light can be strongly enhanced via resonant SPP excitation in the periodic corrugations. These enhanced optical transmissions are directly involved in a number of applications, such as refractive index biosensors, optical tweezers, and enhanced Raman scattering sensors, to name a few [12–14]. Recent plasmonic investigations have renewed unique importance of bull’s eye structures and various geometrical modifications from the original circular structure have been attempted. Elliptical aperture surrounded by circular grooves, elliptical aperture surrounded by elliptical grooves, and polygonal aperture surrounded by polygonal grooves have been reported in recent years [15–21]. In previous studies, main concerns have been focused on the single fundamental resonance of surface plasmon polariton, and dualresonance bands have been far less investigated [22]. It is only very recent that both a relatively weak UV-band resonance and fundamental SP resonance have been observed in EOT through a triangular silver nano-plate [23–25]. Systematic and parametric analyses on this unique dual-band transmission have not been rigorously attempted despite its high potentials in photonic applications. In this study, we proposed a new type of bull’s eye structure based on a polygonal starshaped aperture and concentric star-shaped grooves that can consistently provide UV-band response as well as the fundamental SPP resonance in the visible range, by utilizing the nonlinear quadrupole excitation though the narrow edges with an acute internal angle. The UV band was optimized to have a flexibly controlled intensity comparable to that of visible range, for the first time to the best knowledge of the authors. We found that these polygonal star bull’s eyes effectively excited not only the dipole SPP but also the quadrupole SPP, to provide a UV- EOT whose intensity is comparable to that of the visible SPP band. We numerically analyzed EOT characteristics through the star-shaped bull’s eye with 3, 4, 5, and 6-corners on the silver film by using finite-difference time-domain (FDTD) method [26] by varying geometrical parameters systematically. We also studied effects of the incident light polarization on surface plasmons excitation, which can be potentially applicable for lighting and display devices requiring a variable polarization extinction ratio. Schematic diagrams of the proposed bull’s eyes in this study are summarized in Fig. 1. In comparison to prior bull’s eyes, the proposed structure is based on a star-shaped aperture composed of identical isosceles triangular corners having an acute internal angle attached to an equilateral polygon at the center. The isosceles triangle corners are shown in red and the central equilateral polygons are shown in blue, on the last column of Fig. 1. These isosceles corners were found to efficiently induce a quadrupole to generate an EOT in the UV region and its mechanism is discussed in the latter part of this paper. Here we assumed a silver film with the thickness of 300 nm, which has been used in prior reports to take advantage of its large negative real part and a small imaginary part of dielectric constant in the UV-visible spectral range [27,28]. The structure in Fig. 1(a) is a 3-corner star aperture surrounded by 3-corner star grooves. The aperture includes an equilateral triangle with the side length of ‘b’ at the center and three isosceles triangles with the base of ‘b’ and height ‘h’. In a similar manner, we constructed a 4-corner star aperture surrounded by 4-corner star grooves in Fig. 1(b). The aperture includes a square with the side ‘b’ at the center and four identical isosceles triangles. Figure 1(c) shows a 5-corner star aperture surrounded by 5-corner star grooves. The aperture includes a regular pentagon with the side ‘b’ at the center and five identical isosceles triangles. 6-corner star aperture surrounded by 6-corner star grooves is shown in Fig. 1(d). The aperture includes a regular hexagon with the side ‘b’ and six identical isosceles triangles.

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Fig. 1. Perspective and top views of the proposed star-shaped bull’s eye structures. (a) 3-corner star aperture surrounded by 3-corner star grooves. (b) 4-corner star aperture surrounded by 4corner star grooves. (c) 5-corner star aperture surrounded by 5-corner star grooves. (d) 6corner star aperture surrounded by 6-corner star grooves. (e) comparison of the central polygonal aperture sizes. The silver film with thickness of ‘T’ is structured by ‘n’ grooves with width ‘L’, depth ‘H’, arranged in a periodic pitch of ‘P’, The aperture radius ‘R’ is the radius of a circumscribed circle, and ‘L1’ is the distance between aperture and first groove. Input and output surface structures are identical. (f) Tope and side view of FDTD simulation box.

Structural parameters of the metallic grooves are defined in Fig. 1: the distance from the aperture to the first groove (L1), the groove height (H), the width (L), the period (P), and the silver film thickness (T). The first groove is located at the distance of L1 from the aperture and the following grooves were periodically arranged with the spatial periodicity of P. For the systematic and consistent comparison of transmission through these bull’s eyes, we considered two factors. Firstly, we kept the general requirement of sub-wavelength plasmonic condition such that the diameter of the central aperture (2R) should be less than light wavelength (λ), 2R