  We report the first experimental realization of a three-dimensional thermal
cloak shielding an air bubble in a bulk metal without disturbing external
thermal flux. The cloak is made of a thin layer of homogeneous and isotropic
material with specially designed three-dimensional manufacturing. The thickness
of cloak is 100 um while the cloaked air bubble has a diameter of 1 cm,
achieving the ratio between dimensions of the cloak and the cloaked object two
orders smaller than previous thermal cloaks which were mainly realized in a
two-dimensional geometry. This work can find applications in novel thermal
devices in the three-dimensional physical space.
