Plasmonics uses collective electron motion in metals to squeeze optical fields into volumes far smaller than the wavelength of light. This ability has made it central to enhanced light-matter interactions, super-resolution imaging, compact optical circuits, and low-threshold nanolasers. Yet the same metals that confine light also absorb it. The resulting ohmic loss turns optical energy into heat, limiting device performance and creating a long-standing tradeoff between propagation length and field confinement.
Silver and gold remain the most widely used plasmonic metals, while sodium and emerging transparent conductors have reduced losses in selected spectral ranges. However, none has yet provided both extremely low optical damping and strong optical confinement in the visible to near-infrared range. Potassium has long been considered attractive because of its simple, nearly free-electron electronic structure, but its high chemical reactivity and low melting point make it difficult to obtain clean, flat, crystalline films. Surface oxidation, roughness, and sample instability have therefore obscured its intrinsic optical properties for decades.
In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Lin Zhou from Nanjing University and cooperated with Zhejiang Gongshang University as well as Johns Hopkins University, demonstrates that potassium can act as an ultralow-loss plasmonic metal. The study combines a momentum-gap-based theory of electron scattering with a new fabrication method called slipping-assisted oxide-free crystallization, or SOC. The theoretical model shows that potassium can suppress both normal electron-phonon scattering and a high-momentum pathway called umklapp scattering, placing it close to the low-loss limit for optical plasmonics.
The SOC method was designed to solve the key materials problem. Inside an inert-gas glovebox, the researchers first removed the oxide and impurity shell from potassium. A fresh liquid potassium droplet then fell onto a rapidly moving quartz substrate. The shear force split and spread the droplet, exposing a fresh interface that crystallized before oxidation could occur. In this way, the team created centimeter-scale potassium films that are smooth, continuous, highly crystalline, and close to oxide-free.
Optical measurements confirmed the advantage of the material. Spectroscopic ellipsometry measured the dielectric function across 300 to 2500 nm. The potassium films showed an imaginary permittivity of about 0.1 across the visible to near-infrared region, from 400 to 2000 nm. The measured optical damping rate reached 2.27 meV, about one to two orders of magnitude lower than previously reported low-loss plasmonic candidates such as silver, sodium, indium tin oxide, and hypergap transparent conductors. Propagation-length measurements of potassium plasmonic waveguides independently confirmed the low loss.
The researchers then tested whether low loss could also enable strong light confinement. Using an ultrathin encapsulation strategy, they made potassium-silicon nitride-air plasmonic devices that could be measured under non-vacuum conditions. Scattering-type scanning near-field optical microscopy directly imaged surface plasmon polaritons from 700 to 900 nm. The measured effective index exceeded 10, showing deeply subwavelength optical confinement that is nearly one order of magnitude stronger than low-loss silver and other leading candidates in comparable devices.
These results show that potassium can overcome the conventional loss-confinement tradeoff. Instead of sacrificing light compression to reduce absorption, the potassium platform offers both ultralow optical damping and tight surface-wave confinement. As the authors note, "our findings establish a practical pathway toward approaching the intrinsic low-loss limit of plasmonic metals" and could enable new opportunities for deep-subwavelength photonics.
The technology could support future plasmonic waveguides, nanoscale light sources, metasurfaces, optical sensors, and platforms for extreme light-matter interaction. Because the SOC strategy also addresses oxidation and crystallization in a highly reactive metal, it may inspire broader materials approaches for alkali-metal photonics and for searching for new potassium-like low-loss plasmonic materials.
Light: Science & Applications
Approaching the low optical loss limit of plasmonics using potassium