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Express"],"published-print":{"date-parts":[[2026,7,13]]},"abstract":"<jats:p>Spin\u2013orbit interaction (SOI) of light enables structural control of nonlinear harmonic generation by coupling spin and orbital angular momentum. However, in conventional transverse-field-driven processes, angular-momentum conversion is fundamentally constrained by symmetry. Here, we derive general selection rules showing that the longitudinal electric field of a tightly focused beam lifts this restriction without breaking rotational symmetry. By enhancing the longitudinal component at an epsilon-near-zero thin-film interface, we make nonlinear spin\u2013orbit conversion experimentally observable and generate phase-singular vortex harmonics from a non-singular Gaussian beam at both second and third order. These results reveal the longitudinal field as a key degree of freedom for angular-momentum control in harmonic generation.<\/jats:p>","DOI":"10.1364\/oe.596674","type":"journal-article","created":{"date-parts":[[2026,6,29]],"date-time":"2026-06-29T09:00:07Z","timestamp":1782723607000},"page":"26148","update-policy":"https:\/\/doi.org\/10.1364\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Nonlinear spin\u2013orbit conversion via enhanced longitudinal fields in an epsilon-near-zero thin film"],"prefix":"10.1364","volume":"34","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0129-6934","authenticated-orcid":true,"given":"Kohei","family":"Nagai","sequence":"first","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3647-3251","authenticated-orcid":true,"given":"Takuya","family":"Okamoto","sequence":"additional","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tomohiro","family":"Inaba","sequence":"additional","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3291-6980","authenticated-orcid":true,"given":"Yasushi","family":"Shinohara","sequence":"additional","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-8228-3689","authenticated-orcid":true,"given":"Haruki","family":"Sanada","sequence":"additional","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9448-5413","authenticated-orcid":true,"given":"Katsuya","family":"Oguri","sequence":"additional","affiliation":[{"name":"NTT, Inc."}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"285","published-online":{"date-parts":[[2026,7,8]]},"reference":[{"key":"oe-34-14-26148-R1","doi-asserted-by":"publisher","first-page":"253","DOI":"10.1038\/s41566-021-00780-4","type":"journal-article","volume":"15","author":"Forbes","year":"2021","journal-title":"Nat. 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