  By tuning the radiative coupling of localized surface plasmons to diffracted
orders, we demonstrate how stop-gaps in plasmonic crystals of nanorods may be
opened and tuned. The stop-gap arises from the mutual coupling of surface
lattice resonances (SLRs), which are collective resonances associated with
counter-propagating surface polaritons. We present experimental results for
three different nanorod arrays, where we show how the dispersion of SLRs can be
controlled by modifying the size of the rods. Combining experiments with
numerical simulations, we show how the properties of the stop-gap can be
tailored by tuning a single structural factor. We find that the central
frequency of the stop-gap falls quadratically, the frequency width of the
stop-gap rises linearly, and the in-plane momentum width of the standing waves
rises quadratically, as the width of the nanorods increases. These
relationships hold for a broad range of nanorod widths, including duty cycles
of the array between 20% to 80%. We discuss the physics in terms of a coupled
oscillator analog, which relates the tunability of the stop-gaps to the
coupling strength of plasmonic modes.
