  In the nanophotonics community, there is an active discussion regarding the
origin of the selective absorption/scattering of light by the resonances with
nanorod arrays. Here we report a study of the resonances in ordered
indium-tin-oxide (ITO) nanorod arrays resulted from the waveguide modes. We
discover that with only 2.4% geometrical coverage, the micron-length nanorod
arrays strongly interact with light across an extra-wide band from visible to
mid-infrared resulting in less than 10% transmission. Simulations show
excellent agreement with our experimental observation. Near-field profile
obtained from simulation reveals the electric field is mainly localized on the
surfaces of the nanorods at all the resonances. Theoretical analysis is then
applied to explain the resonances and it was found that the resonances in the
visible are different from those in the infrared. When the light arrives at the
array, part of the light wave propagates through the free space in between the
nanorods and part of the wave is guided inside the nanorods and the phase
difference at the ends of the rod interactions forms the basis of the
resonances in the visible region; while the resonances in the infrared are
Fabry-Perot resonances of the surface guided waves between the two ends of the
nanorods. The simple analytical formulae developed predict the spectral
positions of these resonances well. This information can be used to design
devices like wavelength-selective photodetector, modulators, and nanorod-based
solar cells.
