Surface plasmon polariton modes in a single-crystal Au nanoresonator fabricated using focused-ion-beam milling

Abstract

We use focused-ion-beam milling of a single-crystal Au surface to fabricate a 590-nm-long linear ridge that acts as a surface plasmon nanoresonator. Cathodoluminescence imaging spectroscopy is then used to excite and image surface plasmons on the ridge. Principal component analysis reveals distinct plasmonic modes, which proves confinement of surface plasmon oscillations to the ridge. Boundary-element-method calculations confirm that a linear ridge is able to support highly localized surface plasmon modes ͑mode diameter Ͻ100 nm͒. The results demonstrate that focused-ion-beam milling can be used in rapid prototyping of nanoscale single-crystal plasmonic components. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2885344͔ Surface plasmon polaritons ͑SPPs͒ are electromagnetic waves confined to a metal-dielectric interface. As shown in recent experiments, SPPs can be manipulated using waveguides 1,2 and resonators. 3,4 Surface plasmon polaritons hold promise for application in sensing, photovoltaics, telecommunications, and optoelectronic circuit integration due to their ability to concentrate and guide electromagnetic energy at the nanoscale.Fabrication of components that guide and confine SPPs involves structuring of metals, typically using methods such as electron beam lithography, nanoimprint lithography, selfassembly, or templating techniques. These methods offer high spatial resolution, but require complex multistep processing. Moreover, the metal films, most often obtained by thermal evaporation, typically have a polycrystalline structure. Grain boundaries and surface roughness in polycrystalline films are known to cause undesired scattering of SPPs.A single-step method to structure metals for plasmonic applications, that is gaining widespread acceptance, is focused-ion-beam ͑FIB͒ milling. 5,6 In a typical FIB system, Ga + ions are extracted from a liquid-metal ion source, accelerated to 30 keV, and focused by an electrostatic lens system to a spot size with diameter as small as 5 -10 nm.In this letter, we show how FIB milling of a singlecrystal Au substrate can be used for highly reproducible, maskless fabrication of a smooth plasmonic resonator, with minimum lateral dimension of 50 nm, and surface roughness on the scale of only a few nanometers. We use cathodoluminescence ͑CL͒ imaging spectroscopy to generate SPPs ͑Refs. 7 and 8͒ and image resonant modes within the metal nanostructures. The data demonstrate that FIB is an ideal tool for fabrication of nanoscale plasmonic components, in particular, when single-crystal metal substrates are employed. Our conclusions are supported by boundary-element-method ͑BEM͒ calculations of the local fields at the metal nanostructures. 9,10 Nanoplasmonic device fabrication consisted of focusedion-beam milling a polished ͗111͘ single-crystal Au substrate, grown using the Czochralski process. A 10 pA, 30 keV Ga beam from a FEI Nova 600 dual-beam workstation was rastered in 40 passes over a 4 ϫ 5 m 2 area, using a 1000ϫ 1250 pixel grid using a variable dwell time per pi...

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