  Metals are highly opaque, yet we show that densely packed arrays of metallic
nanoparticles can be more transparent to infrared radiation than dielectrics
such as germanium, even for arrays that are over 75% metal by volume. Despite
strong interactions between the metallic particles, these arrays form effective
dielectrics that are virtually dispersion-free, making possible the design of
optical components that are achromatic over ultra-broadband ranges of
wavelengths from a few microns up to millimetres or more. Furthermore, the
local refractive indices may be tuned by altering the size, shape, and spacing
of the nanoparticles, allowing the design of gradient-index lenses that guide
and focus light on the microscale. The electric field is also strongly
concentrated in the gaps between the metallic nanoparticles, and the
simultaneous focusing and squeezing of the electric field produces strong
`doubly-enhanced' hotspots which could boost measurements made using infrared
spectroscopy and other non-linear processes over a broad range of frequencies,
with minimal heat production.
