  It is found that the electric field can be enhanced strongly inside a
permittivity-near-zero object in free space, when the transverse cross section
of the object is small and the length along the propagation direction of the
incident wave is large enough as compared with the wavelength. The physical
mechanism is explained in details. The incident electromagnetic energy can only
flow almost normally through the outer surface into or out of the
permittivity-near-zero object, which leads to large energy stream density and
then strong electric field inside the object. Meanwhile, the magnetic field
inside the permittivity-near-zero object may be smaller than that of the
incident wave, which is also helpful for enhancing the electric field. Two
permittivity-near-zero objects of simple shapes, namely, a thin cylindrical
shell and a long thin rectangular bar, are chosen for numerical illustration.
The enhancement of the electric field becomes stronger when the
permittivity-near-zero object becomes thinner. The physical mechanism of the
field enhancement is completely different from the plasmonic resonance
enhancement at a metal surface.
