  The collective response of closely spaced metal particles in non-periodic
arrangements has the potential to provide a beneficial angular and frequency
dependence in sensing applications. In this paper, we investigate the optical
response of a Sierpinski fractal arrangement of gold nanoparticles and show
that it supports a collective resonance similar to the surface lattice
resonances that exist in periodic arrangements of plasmonic resonators. Using
back focal plane microscopy, we observe the leakage of radiation out of a
surface lattice resonance that is efficiently excited when the wavenumber of
the incident light matches a strong Fourier component of the fractal structure.
The efficient coupling between localized surface plasmons leads to a collective
resonance and a Fano-like feature in the scattering spectrum. Our experimental
observations are supported by numerical simulations based on the coupled-dipole
approximation and finite-difference time-domain methods. This work presents a
first step towards the application of fractal arrangements for plasmonic
applications
