Extreme Subwavelength Metal Oxide Direct and Complementary Metamaterials

Alternative plasmonic materials like conducting oxides are of great interest for extending the application range of plasmonic and metamaterials with functionalities complementary to traditional noble metals. Here, we investigate the feasibility and limitations of using metal oxides as magnetic resonators for metamaterials and demonstrate that split ring resonators fabricated out of conducting metal oxides show pronounced magnetic and electric modes. The low negative permittivity of the oxide material results in ultrasmall resonant metamolecules, which show only weak interparticle coupling and which consequently allow rational design of extremely dense metamaterials with up to 80 elements per square wavelength and without any significant cross-coupling between elements. Broadband transmission response of up to 50% is obtained over an octave spreading bandwidth using a single-layer design. We numerically explore the tuning of such metal oxide metamaterials through modification of the carrier concentration, offering a prospect which is not available in noble metals. Next to direct designs, we experimentally investigate indium–tin oxide complementary metamaterials obtained using focused ion beam milling. The frequency response of the inverse structures is shown to be complementary to the direct designs following Babinet’s principle, while penetration of fields results in an additional loss and lower performance of the complementary structures compared to direct designs. While ITO metamaterials cannot compete with traditional noble metals in their plasmonic performance, the extremely subwavelength scale of metal oxide resonators in combination with favorable properties such as tunability and visible-range transparency may find use in high-spatial frequency and nonlinear metasurfaces.