Extraordinary optical diffraction from hole pair based on near-field optical thin-microcavity theory

To realize optical coupling between various nanostructured elements and nanoscale field confinement, a fundamental model of the near-field diffraction from a subwavelength hole pair within a conducting thin film is derived theoretically. Coupling effects between these two holes are investigated in detail using the near-field optical thin microcavity theory and the coupled-mode method, and the results are then verified via finite element simulations. Some extraordinary diffraction phenomena are found and are attributed to the following two coupling mechanisms: (1) the edge effect from the thin microcavity theory that occurs at each hole edge when an incident electric field illuminates the upper surface of the thin film; and (2) the enhanced in-plane electromagnetic waves that propagate along the two film surfaces and interact with each other in the vicinity of the holes. Analysis of the effects of the structural parameters on the transmittance shows that the coupling effect can be enhanced or attenuated by the interference conditions, depending on the incident wavelength and the phase difference determined by the hole-hole distance. The extraordinary diffraction effects presented here have potential for applications in near-field optical devices.