Experimental demonstration of low-loss optical waveguiding at deep sub-wavelength scales

Emerging communication applications call for a road map towards nanoscale photonic components and systems. Although metal-based nanostructures theoretically offer a solution to enable nanoscale photonics, the key demonstration of optical modes with deep sub-diffraction-limited confinement and significant propagation distances has not been experimentally achieved because of the trade-off between optical confinement and metallic losses. Here we report the first experimental demonstration of truly nanoscale guided waves in a metal-insulator-semiconductor device featuring low-loss and broadband operation. Near-field scanning optical microscopy reveals mode sizes down to 50x60 nm(2) at visible and nearinfrared wavelengths propagating more than 20 times the vacuum wavelength. Interference spectroscopy confirms that the optical mode hybridization between a surface plasmon and a dielectric mode concentrates the hybridized mode inside a nanometre thin gap. This nanoscale waveguide holds promise for next generation on-chip optical communication systems that integrate light sources, modulators or switches, nonlinear and quantum optics.