  The invisibility cloak has been a long-standing dream for many researchers
over the decades. The introduction of transformational optics has revitalized
this field by providing a general method to design material distributions to
hide the subject from detection. By transforming space and light propagation, a
three-dimensional (3D) object is perceived as having a reduced number of
dimensions, in the form of points, lines, and thin sheets, making it
"undetectable" judging from the scattered field. Although a variety of cloaking
devices have been reported at microwave and optical frequencies, the
spectroscopically important Terahertz (THz) domain remains unexplored.
Moreover, due to the difficulties in fabricating cloaking devices that are
optically large in all three dimensions, hiding realistic 3D objects has yet to
be demonstrated. Here, we report the first experimental demonstration of a 3D
THz cloaking device fabricated using a scalable Projection
Microstereolithography process. The cloak operates at a broad frequency range
between 0.3 and 0.6 THz, and is placed over an {\alpha}- lactose monohydrate
absorber with rectangular shape. Characterized using angularresolved reflection
THz time-domain spectroscopy (THz-TDS), the results indicate that the THz
invisibility cloak has successfully concealed both the geometrical and
spectroscopic signatures of the absorber, making it undetectable to the
observer.
