Efficient optical coupling into metal-insulator-metal plasmon modes with subwavelength diffraction gratings

We demonstrate efficient optical coupling into metal-insulator-metal (MIM) plasmon modes. Subwavelength grating couplers are used to optically excite the MIM plasmon mode, which is observed with reflection spectroscopy. Coupling efficiencies of up to 28% are measured for insulator thicknesses of 12 nm. It is found that the MIM resonance has a significant shift in energy as a function of grating depth. This shift is much larger than that seen from traditional surface plasmon modes. MIM plasmons are promising tools for probing molecular junctions due to strong field confinement and high field intensities within the insulator. (C) 2008 American Institute of Physics.