A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation

The emerging field of nanophotonics1 addresses the critical challenge of manipulating light on scales much smaller than the wavelength. However, very few feasible practical approaches exist at present. Surface plasmon polaritons are among the most promising candidates for subwavelength optical confinement. However, studies of long-range surface plasmon polaritons have only demonstrated optical confinement comparable to that of conventional dielectric waveguides, because of practical issues including optical losses and stringent fabrication demands3,11–13. Here, we propose a new approach that integrates dielectric waveguiding with plasmonics. The hybrid optical waveguide consists of a dielectric nanowire separated from a metal surface by a nanoscale dielectric gap. The coupling between the plasmonic and waveguide modes across the gap enables ‘capacitor-like’ energy storage that allows effective subwavelength transmission in non-metallic regions. In this way, surface plasmon polaritons can travel over large distances (40 –150 mm) with strong mode confinement (ranging from l/400 to l/40). This approach is fully compatible with semiconductor fabrication techniques and could lead to truly nanoscale semiconductor-based plasmonics and photonics. The need for fast, compact and efficient light sources and detectors with high spatial and temporal resolution has motivated research into optical structures capable of guiding light with deep subwavelength confinement. Photonic crystals have been used to guide light, although fundamentally, the confinement is limited to the order of a wavelength in each direction. Subwavelength confinement along one dimension has been shown in all-dielectric coupled silicon waveguides. This geometry is of fundamental importance in nanophotonics, although the relatively large portion of energy propagating in the surrounding regions may restrict the compactness of the waveguide. On the other hand, plasmonic waveguides can provide subwavelength confinement by storing optical energy in electron oscillations within dissipative metallic regions. This, however, leads to high optical loss, which is further exacerbated when high-permittivity dielectric materials such as semiconductors are involved18. Consequently, semiconductorbased plasmonics faces a fundamental challenge at telecommunications and visible frequencies. In this letter, we report a hybrid plasmonic waveguide capable of subwavelength confinement in two dimensions with low propagation loss. The hybrid mode can be strongly confined to sizes more than 100 times smaller than the area of a diffractionlimited spot, while maintaining propagation distances exceeding those of surface plasmon polaritons (SPPs) of the equivalent high-permittivity dielectric–metal interface, which are only confined in one dimension. Moreover, by tuning the geometrical properties of this structure, we can increase the propagation distance up to the millimetre range while still maintaining moderate confinement. This approach naturally extends the capabilities of both plasmonics and semiconductor photonics and can be applied to subwavelength laser devices, such as visible nanolasers and terahertz lasers as well as optically integrated circuits. The hybrid waveguide geometry shown in Fig. 1 consists of a high-permittivity semiconductor nanowire (cylinder waveguide) embedded in a low-permittivity dielectric near a metal surface (SPP waveguide). In the following study, we vary the cylinder diameter, d, and the dielectric gap width, h, between the cylinder and the metal plane to control the propagation distance, Lm, h d