Increased cut-off wavelength for a subwavelength hole in a real metal.

A waveguide mode of a subwavelength rectangular hole in a real metal is analyzed. Due to coupling between surface plasmons on the long edges of the hole, the cut-off wavelength increases as the hole-width is reduced. The cut-off wavelength is found to be much larger than Rayleigh's criterion for perfect metals - 2.3 times as large for a 15 nm wide hole. The analytical results are verified by finite-difference calculations. The finite difference calculations also show the influence of including material loss.