Optimal tapers for compensating losses in plasmonic waveguides

1 Introduction The study of the field enhancement and subwavelength focusing of light in optical waveguides is of great interest for many applications including sensing, lasing, and imaging. The light confinement in conventional optical waveguides is limited by diffraction and by that dielectric waveguides do not allow the effective field fo-cusing to dimensions less than a micron. In contrast, due to the light coupling to electrons in metals and the excitation of surface plasmon polaritons, the light localization near metal– dielectric surfaces can squeeze the field to the nanometer scale [1–3]. Using the plasmonic waveguides, it becomes possible to achieve extreme light focusing and the concentration of the optical energy in the volume of several nanometers [4]. Tapering of the plasmonic waveguides was suggested as an effective way for the nanofocusing of plasmons [5, 6]. Both tapered nanorod waveguides and plasmonic slot waveguides have been studied theoretically [7–9] and realized in experiment [10–13]. In particular , it was shown that the tapering leads to slowing down of plasmons propagating towards the taper tip, and consequent energy concentration [7]. A detailed analysis of two-dimensional plasmonic slot waveguides in the adiabatic approximation has been conducted in Refs. [8, 9] where it was shown that tapering can compete with losses leading to the field enhancement. The effect of the taper shape on nanofocusing has been discussed in Refs. [14, 15]. Although the plasmon nanofocusing in the adiabati-cally tapered waveguides has been already studied in the