Effects of nano-scale surface geometry on surface plasmon are studied by the coordinate-transformation differential method to numerically calculate surface plasmon modes on a weakly disordered metallic surface. An air-silver surface profile with a subwavelength period and a nano-scale height at wavelength of 650 nm are chosen and it is found that the Bloch wave numbers and the surface plasmon modes are highly sensitive with distortions of only a few nanometers for periods much less than wavelength. On the contrary, distortions of long periods which are comparable to wavelength have little impact. Three typical surface profiles exhibit surface plasmon modes of wide variations.
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