  The interaction of free electrons with electromagnetic excitation is the
fundamental mechanism responsible for ultra-strong confinement of light that,
in turn, enables biosensing, near-field microscopy, optical cloaking,
sub-wavelength focusing, and super-resolution imaging. These unique phenomena
and functionalities critically rely on the negative permittivity of optical
elements resulting from the free electrons. As result, progress in
nanophotonics and nano-optics is often related to the development of new
negative permittivity (plasmonic) media at the optical frequency of interest.
Here we show that the essential mobility of free charge carriers in such
conducting media dramatically alters the well-known optical response of free
electron gases. We demonstrate that a ballistic resonance associated with the
interplay of the time-periodic motion of the free electrons in the confines of
a sub-wavelength scale nanostructure and the time periodic electromagnetic
field leads to a dramatic enhancement of the electric polarization of the
medium - to the point where a plasmonic response can be achieved in a composite
material using only positive bulk permittivity components. This ballistic
resonance opens the fields of plasmonics, nanophotonics, and metamaterials to
many new constituent materials that until now were considered unsuitable for
such applications, and extends the operational frequency range of existing
materials to substantially shorter wavelengths. As a proof of concept, we
experimentally demonstrate that ballistic resonance in all-semiconductor
metamaterials results in strongly anisotropic (hyperbolic) response well above
the plasma frequencies of the metamaterial components.
