Subwavelength structures hybridized with slits and grooves in metallic films exhibit extraordinary optical properties for manipulation of light. A Green method is developed to investigate the wave coupling physics in the hybrid subwavelength structures, while source-image arrangement in bounded spaces is employed to obtain the Green's functions for the hybrid units. We find that secondary imaging is induced at a horizontal boundary to provide an inward wave propagation mechanism to couple the wave scattered by groove back into slit. Sophisticated self-consistent framework derived from our method that involves the wave couplings is then developed to study the physics origin of the extraordinary optical properties. The case of a slit in a metal film with patterned grooves on the output side for beaming and that of a two-layer metal film with indented double slit for beyond-limit focusing are analyzed. Our Green method, which automatically includes the Fourier high-order modes, is concise in formulation and thus yields accurate solutions. Explicit analytical models are rigorously developed to address wave coupling physics problems in subwavelength structure.
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