  Two types of optical metamaterials operating at near-IR and mid-IR
frequencies, respectively, have been designed, fabricated by nanoimprint
lithography (NIL), and characterized by laser spectroscopic ellipsometry. The
structure for the near-IR range was a metal/dielectric/metal stack "fishnet"
structure that demonstrated negative permittivity and permeability in the same
frequency region and hence exhibited a negative refractive index at a
wavelength near 1.7 um. In the mid-IR range, the metamaterial was an ordered
array of four-fold symmetric L-shaped resonators (LSRs) that showed both a
dipole plasmon resonance resulting in negative permittivity and a magnetic
resonance with negative permeability near wavelengths of 3.7 um and 5.25 um,
respectively. The optical properties of both metamaterials are in agreement
with theoretical predictions. This work demonstrates the feasibility of
designing various optical negative-index metamaterials and fabricating them
using the nanoimprint lithography as a low-cost, high-throughput fabrication
approach.
