ZnO as a tunable metal: new types of surface plasmon polaritons

The use of the free-electron gas in a heavily doped semiconductor (ZnO:Ga) enables the realization of almost arbitrarily shaped surface-plasmon-polariton dispersion curves in planar geometries. In particular, by preparing metal-metal-type interfaces, we demonstrate surface-plasmon polaritons exhibiting finite frequencies in the long-wavelength limit. Moreover, coupling of surface plasmon polaritons at adjacent interfaces allows for the controlled formation of frequency gaps or, alternatively, the opening of otherwise forbidden regions by an appropriate layer design. Our findings reveal a considerable plasmonic potential of this semiconductor-based approach, e.g., for achieving propagation control or phase matching for nonlinear optical processes as well as novel many-particle phenomena.