Interband polaritonics with topological insulators

Subwavelength nanostructures in metallic platforms provide an efficient avenue to engineer light below the diffraction limit due to resonances that arise from the excitation of free charge carriers. A similar response may be elicited from exotic materials such as topological insulators owing to the excitation of interband polaritonic transitions above the band gap. The resulting negative values for the real part of the dielectric function is typically confined to the visible part of the spectrum, thus restricting their usability. By fabricating nanostructures into topological insulator crystals belonging to the Bi2xSb2(1 x)Te3ySe3(1 y) (BSTS) family, we show that this is not the case, with nanostructured resonances persisting well beyond the region of negative material permittivities. Counterintuitively, characteristics such as nature of the resonant mode and surface wave propagation length remain unaffected across regions of negative and positive permittivities