  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.
