Near-field characterization of bound plasmonic modes in metal strip waveguides.

Vladimir A Zenin,Andrei V Lavrinenko,Radu Malureanu,Ilya P Radko,Sergey I Bozhevolnyi

doi:10.1364/oe.24.004582

Abstract

Propagation of bound plasmon-polariton modes along 30-nm-thin gold strips on a silica substrate at the free-space wavelength of 1500 nm is investigated both theoretically and experimentally when decreasing the strip width from 1500 nm down to the aspect-ratio limited width of 30 nm, which ensures deep subwavelength mode confinement. The main mode characteristics (effective mode index, propagation length, and mode profile) are determined from the experimental amplitude- and phase-resolved near-field images for various strip widths (from 30 to 1500 nm), and compared to numerical simulations. The mode supported by the narrowest strip is found to be laterally confined within ~ 100 nm at the air side, indicating that the realistic limit for radiation nanofocusing in air using tapered metal strips is ~ λ/15.

Highlights

Surface plasmon polaritons (SPPs) are electromagnetic excitations, bound to and propagating along metal-dielectric interfaces [1]
The mode supported by the narrowest strip is found to be laterally confined within ∼ 100 nm at the air side, indicating that the realistic limit for radiation nanofocusing in air using tapered metal strips is ∼ λ /15
The results of this work are especially valuable for applications utilizing the ultimate field enhancement achieved by nanofocusing, since the knowledge of evolution of mode properties for progressively narrow strip widths is crucial for optimum design of nanofocusing configurations

Summary

Introduction

Surface plasmon polaritons (SPPs) are electromagnetic excitations, bound to and propagating along metal-dielectric interfaces [1]. Similar properties are present for a gap SPP mode, confined inside a dielectric between two strips, which allowed effective squeezing of the mode field into a subwavelength 14 × 80 nm area at λ = 830 nm [14]. In this case the field is mainly distributed in the dielectric between the metal strips, which might be a disadvantage compared to the nanofocusing with the strip waveguide, where the enhanced field in the air around the metal strip is readily accessible [10, 11].

Numerical investigation of dispersion properties

Experimental arrangement

Near-field maps

Fitting of near-field maps

Experimental results

Conclusion