  The recent advance in the assembly of metallic nanoparticles (NPs) has
enabled sophisticated engineering of unprecedented light-matter interaction at
the optical domain. In this work, I expand the design flexibility of NP optical
metamaterial to push the upper limit of accessible refractive index to the
unnaturally high regime. The precise control over the geometrical parameters of
NP superlattice monolayer conferred the dramatic increase in electric resonance
and related effective permittivity far beyond the naturally accessible regime.
Simultaneously, effective permeability, another key factor to achieving high
refractive index, was effectively suppressed by reducing the thickness of NPs.
By establishing this design rule, I have achieved unnaturally high refractive
index (15.7 at the electric resonance and 7.3 at the quasi-static limit) at
broadband optical frequencies (100 THz ~ 300 THz). I also combined this NP
metamaterial with graphene to electrically control the high refractive index
over the broad optical frequencies.
