Complex Mechanism of Enhanced Optical Transmission Through a Composite Coaxial/Circular Aperture

Making use of the FDTD simulation, we study light transmission properties of a composite coaxial/circular aperture milled in a thin metallic film. Representing the aperture as consisting of segments of coaxial and hollow waveguides, connected in series, we show that there are three characteristic frequencies (the cutoff frequencies of the coaxial and hollow waveguides and the frequency of a longitudinal standing wave in the coaxial waveguide segment) and four regimes of operation (bounded by these frequencies, as well as by low- and high-frequency limits) which determine the behavior of the transmission efficiency. For two regimes of operation (for frequencies between the cutoff frequency of the coaxial waveguide and the resonant frequency of the longitudinal standing wave), both segments can contribute to the overall transmission. For other two regimes, either no enhancement occurs or only one segment contributes to the transmission efficiency. A way is proposed to optimize the transmission through the composite aperture. In particular, as we show, the transmission efficiency of the aperture can be enhanced by decreasing the exit hole size (radius of the circular aperture). In the considered case, an increase of the transmission efficiency exceeds 50%. The effect of the enhanced transmission is shown to result from both vertical and in-plane surface plasmon resonances occurring in the aperture.