The role of surface plasmon polaritons in the optical response of a hole pair

The optical emittance of a hole pair perforated in an opaque metal film is studied from first-principles using the coupled-mode method. The geometrical simplicity of this system helps to understand the fundamental role played by surface plasmon polaritons (SPPs) in its optical response. A SPP interference model without fitting parameters is developed from the rigorous solution of Maxwell's equations. The calculations show that the interference pattern of the hole pair is determined by two scattering mechanisms: (i) re-illumination of the holes by the in-plane SPP radiation and (ii) an effective impedance depending on the single-hole response. The conditions for constructive and destructive interference only depend on the phase difference provided by each of the two scattering mechanisms.