Resonance mechanism for the electromagnetic enhancement by an isolated subwavelength metallic groove

Abstract

ABSTRACT We propose quantitative investigations of the Fabry-Perot re sonance condition for the field enhancement by an isolated subwavelength metallic groove. The resonance condition is obta ined from a simple Fabry-Perot model. Our calculations show that the electric field near the gr oove mouth that contacts the specimen is drastically enhanced under the resonance condition but is depressed out of the resonance condition. Giant electric-field enhancement can be obtained for narrow groove, which reveals nearly 10 8 magnification of Raman signals. A weak dependence of the electromagnetic enhancement on the metal type is presented and explained. Keywords: Surface enhanced Raman scattering, Electromagnetic enhancement, Fully-vectorial numerical method, Fabry-Perot model 1. INTRODUCTION Surface enhanced Raman scattering (SERS) [1] offsets the draw back of weak cross section of Raman scattering and can drastically enhance Raman signals. SERS offers high sensitiv ity up to single-molecule detection [2], which has a large potential for new developments in biochemical applications and attracts much more interests nowadays [3-5]. SERS is basically supported by the electric-field enhancement in metallic nanostructures. Suitable SERS substrates include, for instance, electrochemically roughened metallic electrodes, metal island films, metallic nano-particles [6] and other roughened surfaces. With the development of nanofabrication technologies, regularly arranged metallic structures can be fabricated successfully, which offer the possibility for a more systematic and reproducible investigation in contrast to irregular substrates. Metallic grooves down to nanometer dimensions are promising albeit simple structures that can provide a giant electromagnetic enhancemen t. Recently, enormous field enhancement by a single groove [7] or by almost flat silver gratings [8] is reported. Impacts of structure parameters and of the illumination condition on the enhanced electric field in a single meta llic groove have been investigated systematically [9]. In this report, we propose quantitative investigations of the Fabry-Perot resonance condition for the field enhancement by an isolated metallic nano-groove. The resonance condition is obtained from a simple Fabty-Perot model [9] that can precisely predict the data calculated by the fully vectorial nu merical method [10]. Our calcul ations show that the electric-field near the groove mouth that contacts the specimen is drastically enhanced under the resonance condition but is depressed out of the resonance condition. Giant electric-field enhancement can be obtained for narrow grooves, which reveals nearly 10