Tunable Quasi‐Resonances in Polypyrrole Nanodisks

Redox‐tunable conducting polymers have emerged as promising materials for dynamic plasmonics and metasurfaces. However, only a few conducting polymers with particularly high conductivities (>1000 S/cm) have been reported to provide spectral regions with plasmonic behavior. Here, we use polypyrrole (PPy) as a model system to address whether optically metallic properties can also be generated in conducting polymers with moderate conductivity (around 110 S/cm). We found that the material possesses negative permittivity but only in a limited spectral region and with magnitude limited to below 2. Furthermore, the negative permittivity region originates primarily from a polaronic absorption band rather than purely from mobile carriers as in conventional plasmonic materials. Nanostructures made from the material exhibit size‐dependent optical extinction peaks despite not fulfilling the traditional quasistatic resonance condition. These peaks, denoted as quasi‐resonances, can be reversibly tuned by electrochemical modulation, analogous to those presented in conducting polymer plasmonic nanostructures.