  Negative index metamaterials (NIMs) give rise to unusual and intriguing
properties and phenomena, which may lead to important applications such as
superlens, subwavelength cavity and slow light devices. However, the negative
refractive index in metamaterials normally requires a stringent condition of
simultaneously negative permittivity and negative permeability. A new class of
negative index metamaterials - chiral NIMs, have been recently proposed. In
contrast to the conventional NIMs, chiral NIMs do not require the above
condition, thus presenting a very robust route toward negative refraction. Here
we present the first experimental demonstration of a chiral metamaterial
exhibiting negative refractive index down to n=-5 at terahertz frequencies,
with only a single chiral resonance. The strong chirality present in the
structure lifts the degeneracy for the two circularly polarized waves and
relieves the double negativity requirement. Chiral NIM are predicted to possess
intriguing electromagnetic properties that go beyond the traditional NIMs, such
as opposite signs of refractive indices for the two circular polarizations and
negative reflection. The realization of terahertz chiral NIMs offers new
opportunities for investigations of their novel electromagnetic properties, as
well as important terahertz device applications.
