Field enhancement in a circular aperture surrounded by a single channel groove.

Numerical analysis of diffraction by a single aperture surrounded by a circular shallow channel in a metallic screen shows the possibility of a 50-fold increase of the electric field intensity inside the central aperture, when compared to the incident field. Detailed analysis of cavity modes and their coupling through surface plasmon wave determine the parameters leading to maximum field enhancement. This effect can be used in high-efficiency single-molecule fluorescence analysis in attoliter volumes.