Excitation and Dispersion Characteristics of Transverse Magnetic Supermodes in Plasmonic Optical Fibers

We investigate the dispersion characteristics of the transverse magnetic (TM) guided supermodes (sTM<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf>, sTM<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</inf>, and a<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf>) in a tapered, metal-coated optical fiber core. The structure consists of a fiber core coated with a 25 nm gold film, tapered from an 8.2 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\mu} \mathbf{m}$</tex> diameter down to 0.4 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\mu} \mathbf{m}$</tex>. We use a cascaded eigenmode expansion algorithm to quantify the evolution of the supermodes and identify two primary loss mechanisms: i) catastrophic coupling loss when the fundamental sTM<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> mode cuts off, and ii) inherent propagation loss due to the plasmonic nature of the modes.