Transmission, control and coherent propagation of surface plasmons in metal nanostructures

Abstract

Extraordinary transmission of light through sub-wavelength hole arrays in thick metals is imaged both at "allowed" and "forbidden" surface plasmon resonances. Detouring around the holes, coherent propagation along and beyond the hole patterns are revealed.Surface plasmon (SP) resonances are responsible for the extraordinary optical transmission through sub-wavelength periodic hole arrays in thick metal films [l]. So far, however, a microscopic understanding of this extraordinary phenomenon is still lacking. Near-field imaging of the spatial emission pattern of such arrays is likely to provide detailed insight. Preliminary work with off-resonance excitation seems to indicate that the light is emitted from the holes [2]. This is in contrast to the intriguing mystery that for excitation of the sapphire-metal interface, extraordinary transmission occurs also at the air-metal SP resonance, indicating that SP play an important role in forming the near-field emission pattern.In this contribution, we study, for the first time the spatial emission pattern of nanometric hole arrays with sub-100 nm spatial resolution for excitation of SP resonances. We demonstrate that, in striking contrast to earlier expectations, the near-field emission pattern of these nanostructures is mainly governed by SP propagating along the metal-air interface. By spatially imaging the coherent propagation of those SP, we show that such nanostructures may serve as novel emission sources for coherent SP waves that may find important applications in future nano-optic devices.We have performed near-field imaging of the spatial transmission patterns by collecting the light emitted from periodic hole arrays with metal-coated fiber probes with sub-100 nm apertures. The excitation wavelength was tuned to various plasmon resonances. A representative emission pattern of our samples is shown in Fig. 1. For resonant excitation of the air-metal (1,O) SP with linearly polarized light, we find that only a small fraction of the transmitted light is collected at the center of the nanometer-sized holes. Most of the transmitted signal is found between the holes. This shows directly that the interference of SP at the metal-air interface governs the emission pattern of the array (Fig. lb). Comparing near-field and far field emission (Fig. IC, Id), it becomes clear that light emission is predominantly governed by propagating air-metal SP even in the far field.We found that the spatial structure of the emission pattern depends strongly on the illuminating light polarization, suggesting that we can control the propagation direction of the emitted surface waves by manipulating incident polarization and wavelength. This is demonstrated in Fig. 2, showing the emission pattern for excitation of the sapphire-silver (1, 1) SP. The emission pattern is now directed along the array diagonal, showing that these nanostructures may serve as a source for spatially directed surface waves.Such plasmon modes may coherently propagate along the surface on time scales shorter than their de...