  Since the introduction of the artificially designed negative-index
metamaterial (NIM) introduced in 2001, its extraordinary electromagnetic
properties, which cannot be attained from naturally occurring materials, have
continuously attracted many researchers to study them. Various types of NIMs
with the resonant frequencies shifted from gigahertz all the way to higher
frequencies have been actualized over the past decade. For the actual
applications, the most fascinating and significant goal of research on NIMs is
to achieve negative refraction at visible wavelengths. According to the
effective medium theory, the interior structural unit of NIMs must be on a
scale much smaller than the operating wavelength.
