Approaching the low optical loss limit of plasmonics using potassium

Plasmonics enables the miniaturization of photonic devices beyond the optical diffraction limit, yet its potential is hindered by inherently large ohmic losses. Hence, it is prudent to explore low-loss alternatives to the current mainstay of plasmonics-the noble metals. In this work, we demonstrate the potential of potassium as a plasmonic material with intrinsically low losses in the optical region. The ultra-flat, high-quality potassium film, fabricated via a rapid slipping-assisted oxide-free crystallization process, achieves measured optical damping rate down to 2.27 meV, with a measured imaginary permittivity of ~0.1 across the entire visible to near-infrared range (400-2000 nm). Near-field optical spectroscopic measurements further confirmed the reduced losses by revealing deeply subwavelength confinement of optical modes. This result overcomes the loss-confinement tradeoff existing in state-of-the-art plasmonic materials and devices, establishing a new platform for exploring extreme light-matter interactions in a variety of plasmonic systems.