Fano resonances in metasurfaces made of linear trimers of plasmonic nanoparticles.

We investigate Fano resonances in planar two-dimensional periodic arrays of linear trimers of plasmonic nanoparticles that appear under plane wave incidence. The observed Fano resonances are associated to resonances belonging to the trimer (metamolecule) itself, where some are found to be strongly affected by the array periodicity. We observe that array-dependent resonances appearing for oblique incidence are resistant to losses, whereas narrow dipolar-like Fano resonances associated mainly to the metamolecule, which appear also under normal incidence, disappear when losses are too high. In particular, we prove the latter by theoretical (dipolar approximation) and full-wave simulations, in good agreement. We propose that the use of very low-loss plasmonic materials or the use of gain materials to mitigate plasmonic losses may lead to (high-quality factor) dipolar-like Fano resonances under normal incidence, exhibiting a certain degree of fabrication defect tolerance, which might be employed to improve sensors, lasing, switching, and nonlinear devices, for example.