Enlarging the Bandwidth of Nano-scale Propagating Plasmonic Modes in Deep-subwavelength Cylindrical Holes
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1055-GG09-06
Enlarging the Bandwidth of Nano-scale Propagating Plasmonic Modes in Deepsubwavelength Cylindrical Holes Peter B. Catrysse, and Shanhui Fan Stanford University, Stanford, CA, 94305-4088 ABSTRACT Subwavelength cylindrical holes in optically thick metallic films always support a propagating HE11 mode near the surface plasmon frequency, regardless of how small the holes are. For holes filled with a uniform dielectric material, the bandwidth of the HE11 mode asymptotically approaches zero as the hole size is reduced to deep-subwavelength scales. We show that it is possible to create nano-scale propagating plasmonic modes with very large bandwidth in holes that are concentrically filled with two different dielectric materials, even when the hole radius goes to zero. INTRODUCTION The optical properties of nano-scale apertures in optically thick metallic films have been intensely researched in the past several years due to their fundamental importance in near-field optics and nano-photonics, as well as their practical significance for photonic devices and applications, including filters, near-field probes, optical data storage, and lithography [1, 2]. It is well-established that the transmission characteristics of apertures strongly vary depending on whether they allow or prohibit propagating modes [3, 4]. For example, enhanced transmission in metallic nano-slits is attributed to propagating transverse magnetic modes that are supported by slits independent of their width [4-9]. Cylindrical holes with a circular cross-section in a perfect metal, on the other hand, do not support propagating modes when the hole diameter is smaller than λ 2n 0 , where λ is the vacuum wavelength of incident light and n0 is the refractive index of the dielectric inside the hole [10, 11]. Following Ebbesen et al.’s experiments [12], extra-ordinary optical transmission has commonly been associated with the excitation of surface wave resonances on the front and back surfaces of the metallic film, and an evanescent tunneling process through the subwavelength holes [10, 12-16]. More recently, localized surface plasmon modes or shape resonances inside subwavelength cylindrical holes have been identified as an alternative pathway for extraordinary transmission [17-21]. In either case, the use of resonances results in transmission peaks of relatively narrow line width. In recent work of our own, we numerically demonstrated that subwavelength holes in a metal always support a propagating plasmonic HE11 mode near the surface plasmon frequency, regardless of how small the holes are, and that this mode can lead to near-complete optical transmission through a subwavelength hole array [22]. The importance of such propagating modes is that, under appropriate conditions, they provide a transmission window with a relatively broad bandwidth.
In this work, we analyze and design the bandwidth properties of the plasmonic HE11 mode. We show that its cut-off frequency (lower frequency limit) is co-determined by the hole radius and the (average) dielec
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