  The invisibility cloak has been a long-standing dream for many researchers
over the decades. By transforming space and light propagation, a
three-dimensional (3D) object can be perceived as having reduced number of
dimensions, in the form of points, lines, and thin sheets, making it
"undetectable" judging from scattered field. Although a variety of cloaking
devices have been reported at microwave and optical frequencies, the Terahertz
(THz) domain remains unexplored. Moreover, it should be noted that all the
previous experimental demonstrations are performed in a two-dimensional (2D)
waveguide configuration. Although those works represent a critical step in
validating the concept of the invisibility cloak, one would expect the cloaking
device to be realized in 3D with the ability to cloak an object of realistic
size. This requires the construction of an optically large cloaking device with
features much smaller than the wavelength. Fabricating 3D structures with
aspect ratio close to 100:1 is obviously a challenging task. Here, we report an
experimental demonstration of a 3D THz ground plane cloak. Reflection terahertz
time-domain spectroscopy (THz-TDS) was employed to characterize the cloaking
samples. Two distinct reflection peaks can be clearly observed across a broad
frequency range, which is caused by the reflection at the surface of the bump.
The measured peak positions are consistent with the numerical simulation peak
positions. By contrast, in the spectral map of the cloak sample, the wavefront
is relatively smooth with a single peak.

Optics