  In this letter, we show that the bandwidth of optical band-stop filters made
of subwavelength metal structures can be significantly increased by the strong
plasmonic near-field coupling through the corners of the periodic metal
squares. The effect of small gap coupling on the spectral bandwidth is
investigated by varying the gap size between the metal squares. An equivalent
transmission line model is used to fit the transmission and reflection spectra
of the metal filters. The transmission line model can characterize well the
metal structures with the gap size larger than the near-field decay length.
However, it fails to model the transmission and reflection spectra when the gap
size reaches the decay range of the near-field in the small gaps.

Optics