  An optical antenna can convert a propagative optical radiation into a
localized excitation, and reciprocally. Although optical antennas can be
readily created using resonant nanoparticles (metallic or dielectric) as
elementary building blocks, the realization of antennas sustaining multiple
resonances over a broad range of frequencies remains a challenging task. Here,
we use aluminum self-similar, fractal-like structures as broadband optical
antennas. Using electron energy loss spectroscopy, we experimentally evidence
that a single aluminum Cayley tree, a simple self-similar structure, sustains
multiple plasmonic resonances. The spectral position of these resonances is
scalable over a broad spectral range spanning two decades, from ultraviolet to
mid-infrared. Such multi-resonant structures are highly desirable for
applications ranging from non-linear optics to light harvesting and
photodetection, as well as surface-enhanced infrared absorption spectroscopy.
