The optical response of nanostructured surfaces and the composite diffracted evanescent wave model

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Abstract

Investigations of the optical response of subwavelength-structure arrays milled into thin metal films have revealed surprising phenomena, including reports of unexpectedly high transmission of light. Many studies have interpreted the optical coupling to the surface in terms of the resonant excitation of surface plasmon polaritons (SPPs), but other approaches involving composite diffraction of surface evanescent waves (CDEW) have also been proposed. Here we present a series of measurements on very simple one-dimensional subwavelength structures to test the key properties of the surface waves, and compare them to the CDEW and SPP models. We find that the optical response of the silver metal surface proceeds in two steps: a diffractive perturbation in the immediate vicinity (2–3 μ m) of the structure, followed by excitation of a persistent surface wave that propagates over tens of micrometres. The measured wavelength and phase of this persistent wave are significantly shifted from those expected for resonance excitation of a conventional SPP on a pure silver surface.

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Figure 1: Essential elements of the CDEW model.
Figure 2: The incoming plane wave Ei impinges on the subwavelength slit (or hole) and a groove milled on the input side.
Figure 3: Goniometer setup for measuring far-field light intensity and angular distributions.
Figure 4: Scanning electron micrograph of one of the series of single-slit, single-groove structures FIB-milled into a 400-nm-thick silver layer deposited on 1-mm-thick flat quartz microscope slides.
Figure 5: Scanning electron micrograph of one of the series of single-groove, single-hole structures fabricated similarly to the single-groove, single-slit structures in Fig. 4.
Figure 6: Normalized far-field intensity I/I0 as a function of slit–groove distance xsg for the series of single-slit, single-groove structures mounted facing the input side with respect to plane-wave excitation.
Figure 7: Normalized far-field intensity I/I0 as a function of hole–groove distance xhg for the series of single-hole, single-groove structures mounted facing the input side with respect to plane-wave excitation.

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Acknowledgements

Support from the Ministère délégué à l’Enseignement supérieur et à la Recherche under the programme ACI-‘Nanosciences-Nanotechnologies’, the Région Midi-Pyrénées [SFC/CR 02/22], and FASTNet [HPRN-CT-2002-00304] EU Research Training Network, is gratefully acknowledged, as is support from the Caltech Kavli Nanoscience Institute and from the AFOSR under Plasmon MURI FA9550-04-1-0434. Discussions and technical assistance from P. Lalanne, R. Mathevet, F. Kalkum, G. Derose, A. Scherer, D. Pacifici, J. Dionne, R. Walters and H. Atwater are also gratefully acknowledged.

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Correspondence to J. Weiner.

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