  Recently, invisible cloaks have attracted much attention due to their
exciting property of invisibility, which are based on a solid theory of
transformation optics and quasi-conformal mapping. Two kinds of cloaks have
been proposed: free-space cloaks, which can render objects in free space
invisible to incident radiation, and carpet cloaks (or ground cloaks), which
can hide objects under the conducting ground. The first free-space and carpet
cloaks were realized in the microwave frequencies using metamaterials. The
free-space cloak was composed of resonant metamaterials, and hence had
restriction of narrow bandwidth and high loss; the carpet cloak was made of
non-resonant metamaterials, which have broad bandwidth and low loss. However,
the carpet cloak has a severe restriction of large size compared to the cloaked
object. The above restrictions become the bottlenecks to the real applications
of free-space and carpet cloaks. Here we report the first experimental
demonstration of broadband and low-loss directive free-space cloak and
compact-sized carpet cloak based on a recent theoretical study. Both cloaks are
realized using non-resonant metamaterials in the microwave frequency, and good
invisibility properties have been observed in experiments. This approach
represents a major step towards the real applications of invisibility cloaks.
