Microcavity enhanced directional transmission through a subwavelength plasmonic slit.

We show numerically that a compact structure, consisting of multiple optical microcavities at both the entrance and exit sides of a subwavelength plasmonic slit, can lead to greatly enhanced directional transmission through the slit. The microcavities can increase the reflectivity at both sides of the slit, and therefore the resonant transmission enhancement. In addition, the microcavities can greatly improve the impedance matching, and therefore the coupling between free-space waves and the slit mode. An optimized structure with two microcavities on both the entrance and exit sides of the slit leads to ~16 times larger transmission cross section per unit angle in the normal direction compared to the optimized reference slit without microcavities. We also show numerically that the operation frequency range for high emission in the normal direction is broad.