In the diffraction shadow: Norton waves versus surface plasmon polaritons in the optical region

Surface electromagnetic modes supported by metal surfaces have a great potential for use in miniaturized detectors and optical circuits. For many applications, these modes are excited locally. In the optical regime, surface plasmon polaritons (SPPs) have been thought to dominate the fields at the surface, beyond a transition region comprising 3-4 wavelengths from the source. In this work, we demonstrate that at sufficiently long distances SPPs are not the main contribution to the field. Instead, for all metals, a different type of wave prevails, which we term Norton waves (NWs) for their resemblance to those found in the radio-wave regime at the surface of the Earth. Our results show that NWs are stronger at the surface than SPPs at distances larger than 6-9 SPP absorption lengths, the precise value depending on wavelength and metal. Moreover, NWs decay more slowly than SPPs in the direction normal to the surface.

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