Beyond the Bethe Limit: Tunable Enhanced Light Transmission Through a Single Sub-Wavelength Aperture

Abstract

Since the advent of diffraction gratings in the last century, it has long been apparent that new physics and novel applications can arise from the ability to control structures on the scale of the wavelength of light. [1±7] Present nanofabrication technology allows the construction of artificial structures in a controlled way, leading to novel, and sometimes surprising, material properties. Recently we showed that a two-dimensional crystal of holes in a metallic film overcomes the fundamental constraint of low transmittance of apertures smaller than the wavelength of light. [8,9] Through the lattice of holes, the photons couple resonantly to surface plasmons (SPs)Ðcoherent collective excitations of electrons at a metal surface [10] Ðleading to an optical transmittance at well-defined wavelengths which exceeds by orders of magnitude what would be expected from extrapolating for a single hole from Bethe's theory. [11] The transmittance enhancement has recently been confirmed by numerical simulations. [12] In those lattices, there were literally millions of holes. [8,9] Ultimately, it would be much more practical if one could achieve the same control (wavelength selectivity and enhanced transmittance) in a single aperture. One possible approach would be to create a periodic structure on the metal surface to couple the light to the SPs, and then add a hole or set of holes, and determine whether these will transmit the SP-coupled light. For this scheme to work it is imperative that the periodic surface features do not themselves emit light. This is far from obvious since it is well known from SP studies [3,10] that surface roughness induces radiative decay of SPs. We demonstrate here that this is indeed possible: Even a single sub-wavelength hole, if surrounded by periodic surface corrugation, shows as large a transmittance enhancement as an array of holes. In addition, the transmission can be tuned to a given wavelength by the proper design of the 2D lattice. We also clarify other aspects of this rich phenomenon.To study the transmission properties of hole lattices on which are superimposed lattices of surface indentations, silver metal films (thickness t » 300 to 500 nm) were prepared on a quartz surface after which holes and indentations (radius r » 150 nm) were milled in the film using a focused ion beam (FIB) (Micrion System 9500, 50 keV Ga ions, 5 nm nominal spot diameter). The periodic surface modulation consists of an array of circular indentations, or dimples, that penetrate no more than halfway through the film. The surface plasmon skin depth being about 20 nm, [10] the two surfaces are well-isolated from each other, including in the indentations.To determine if it is possible to modify the transmission spectrum by introducing a periodic surface structure, we first fabricated the simplest configuration consisting of a square array of surface indentations with a period P i = 0.5 mm in which every other indentation is milled through the film to produce a hole. In other words, the holes have a period P h = 1.0...

Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures

“…Thus, it was postulated that plasmon-assisted evanescent tunneling through the subwave- length apertures allows light impinging on the opaque metal surface between the holes to be transmitted via the conversion into surface plasmons. 94 Intriguingly, the transmission enhancement is also present for a single aperture provided that the metallic surface on its input side shows a few periods of a regular corrugation, 95 and groove cavity modes and subsequent reemission as well as aperture waveguide modes have been postulated as mechanisms for this enhancement. 96 While corrugations on the input side lead to an enhanced transmission, the occurrence of highly directional beamsteering via corrugating the exit side of the screen around the aperture has been observed 97 and subsequently explained in this framework.…”

Metamaterial Grounded Planar Bilayers Supporting Leaky W aves: Principles and Applications

“…This setup may have several interesting potential applications such as spatially filtering electromagnetic radiation, photolithography, near-field microscopy, or microwave imaging. This phenomenon is related to the enhancement of wave transmission through small apertures exploiting the leaky-wave supported by proper corrugations over a metallic screen [14], as originally proposed by Oliner, Jackson and their coworkers [15], and to the concept applied to metamaterial single layers by our group in [16]. The extension of this concept to metamaterial bilayers, as reported in [7], may potentially reduce the overall thickness of the required covers to sub-wavelength dimensions, together with increasing the overall performance of the setup, thanks to the proper excitation of the anomalous interface resonance, characteristics of these bilayers.…”

Metamaterial Covers Over a Small Aperture

“…Independent of these developments in the area of metamaterials, in the past few years some researchers mostly in the physics community have experimentally shown that a significant enhancement of optical transmission through a periodic array of sub-wavelength holes [14] or a single sub-wavelength hole surrounded by a periodic corrugation [15] in a metallic opaque screen may be achieved by suitably choosing the corrugation periods. In both cases, they have originally attributed this phenomenon to the material surface plasmons (which are collective resonant excitations of the electron density on a surface), and in particular in the case of a single aperture they have shown how at that given period for which the transmission shows its peak, the corrugated surface supports a material polariton [15].…”

“…In both cases, they have originally attributed this phenomenon to the material surface plasmons (which are collective resonant excitations of the electron density on a surface), and in particular in the case of a single aperture they have shown how at that given period for which the transmission shows its peak, the corrugated surface supports a material polariton [15]. Oliner, Jackson and their co-workers [16], [17] have elegantly explained this effect in terms of the leaky-wave theory, showing also how the enhancement may be optimized by a proper choice of the corrugation periods.…”

“…As in the case of a hole with the corrugated surrounding, this huge enhancement effect in the wave transmission may have several similar interesting applications: it may be used to spatially filter the electromagnetic radiation, effectively going even beyond the diffraction limitations (see, e.g., [22]), for tunable optical filters [23], for photolithography, for near-field microscopy or to extract light from LED [15], [24]. Also the challenge of transporting optical and RF information below the diffraction limit has recently become of interest in the optics community; different solutions have been proposed [25], [26] and this effect may be another exciting way to overcome the diffraction limitation.…”