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Hence, it is prudent to explore low-loss alternatives to the current mainstay of plasmonics\u2014the 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\u2009meV, with a measured imaginary permittivity of ~0.1 across the entire visible to near-infrared range (400\u20132000\u2009nm). 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\u2013matter interactions in a variety of plasmonic systems.<\/jats:p>","DOI":"10.1038\/s41377-026-02400-8","type":"journal-article","created":{"date-parts":[[2026,7,21]],"date-time":"2026-07-21T08:00:04Z","timestamp":1784620804000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Approaching the low optical loss limit of plasmonics using potassium"],"prefix":"10.1038","volume":"15","author":[{"given":"Yi","family":"Zhang","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yuhan","family":"Yang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jie","family":"Liang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Shizhuang","family":"Wang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yuhong","family":"Shi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Pengcheng","family":"Yao","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hanyu","family":"Fu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0725-8736","authenticated-orcid":false,"given":"Jacob B.","family":"Khurgin","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8268-7373","authenticated-orcid":false,"given":"Fengrui","family":"Hu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2871-4369","authenticated-orcid":false,"given":"Jia","family":"Zhu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3472-6497","authenticated-orcid":false,"given":"Shining","family":"Zhu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8405-5310","authenticated-orcid":false,"given":"Lin","family":"Zhou","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,7,21]]},"reference":[{"key":"2400_CR1","doi-asserted-by":"publisher","first-page":"24","DOI":"10.1021\/acsphotonics.7b00674","volume":"5","author":"DG Baranov","year":"2018","unstructured":"Baranov, D. 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