We perform rigorous simulations of hybrid long-range modes guided by a central metal core and a twodimensional dielectric slab. We show that these modes are subject to fewer limitations than conventional long-range plasmon modes in terms of field confinement and guiding performance. These hybrid modes may offer substantial improvements for integrated plasmonic components, as illustrated here by the consideration of 90°bends. © 2007 Optical Society of America OCIS codes: 240.6680, 130.2790, 130.3120. Surface plasmon polaritons (SPPs) are electromagnetic-electronic waves propagating at the interface between a metal and a dielectric [1]. SPPs are associated with strongly enhanced local fields that can exhibit spatial variation on a scale much smaller than the photon wavelength. This particular aspect of SPPs renders them attractive for the development of photonic components whose size is not limited by the diffraction limit of light [2]. However, SPP modes with subdiffraction confinement propagate only over distances ranging from micrometers in the visible to hundreds of micrometers in the infrared, because a substantial part of the energy is contained in the metal-an extremely absorptive medium at those frequencies [1,3,4]. To reduce the losses by absorption, the field confinement must be considerably relaxed [4]: for example, thin metal films and metal strips support long-range SPPs that propagate as far as centimeters, but the lateral extension of these modes reaches several wavelengths [5][6][7][8][9][10][11][12][13][14][15].Recently, several studies have shown that the properties of long-range SPPs propagating along infinitely wide metal films can be tailored by constraining their field in a two-dimensional dielectric waveguide [12][13][14]. Here we generalize this approach by considering the case of arbitrary curved metal strips embedded in a thin dielectric slab. We show numerically that the long-range SPPs are hybridized by total internal reflection and that, although the general trade-off between loss and propagation distance still occurs for these modes, the electromagnetic fields can be localized to significantly smaller volumes for the same propagation distance when compared with long-range SPPs.Our numerical method is based on finding the eigenmodes of the strip geometry in either an infinite-or a finite-thickness dielectric region at the wavelength = 1550 nm. To simplify the calculations, we limit our investigations to straight and uniformly bent waveguides and investigate their modes in Cartesian and cylindrical coordinates, respectively (Fig. 1). The electric field of the mode can be formally written as E = E 0 ͑x , y͒exp͑i z z͒ for the straight waveguides and E = E 0 ͑r , y͒exp͑i ͒ for the bends, where E 0 is the field distribution along the waveguide cross section and  z (in inverse meters) and  (in inverse radians) are the complex propagation constants. With the field dependence on the propagation direction assumed, the strip waveguides can be fully characterized by performing a finite-e...
In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.
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