Ballistic Metamaterials.

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.