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Nanotechnol."],"published-print":{"date-parts":[[2026,11]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                  <jats:p>\n                    Nonlinear frequency conversion underpins important technologies such as telecommunications and quantum computation; however, weak nonlinearities and architectures that resist miniaturization currently limit devices\u2019 efficiency and widespread adoption. Here we combine a band-structure-engineered GaAs\/AlGaAs multi-quantum-well heterostructure with a high-quality-factor dielectric metasurface and symmetry-broken guided-mode field profiles to enhance the material nonlinear susceptibility. By engineering a resonant interband transition in the heterostructure, we realize a second-order nonlinear tensor element of 1.6\u2009nm\u2009V\n                    <jats:sup>\u22121<\/jats:sup>\n                    at 1.57\u2009\u03bcm wavelength. We then make it free-space accessible and boost the effective nonlinearity to ~14 nm\u2009V\n                    <jats:sup>\u22121<\/jats:sup>\n                    using a metasurface patterned on the material. Our proof-of-concept experiment establishes that combining interband-transition engineering and metasurfaces enables giant effective nonlinearities in the near-infrared to visible spectrum. This addresses material and device-level constraints in nonlinear photonics, providing a scalable route to compact, efficient devices.\n                  <\/jats:p>","DOI":"10.1038\/s41565-026-02268-0","type":"journal-article","created":{"date-parts":[[2026,9,2]],"date-time":"2026-09-02T09:03:01Z","timestamp":1788339781000},"page":"1558-1564","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Quantum-well metasurface for free-space-accessible enhanced nonlinear polarization"],"prefix":"10.1038","volume":"21","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1021-5059","authenticated-orcid":false,"given":"Pernille Undrum","family":"Fathi","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Irene","family":"Occhiodori","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0006-6746-7555","authenticated-orcid":false,"given":"Patrick","family":"Devaney","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Amberly","family":"Ricks","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7026-9580","authenticated-orcid":false,"given":"Rithvik","family":"Ramesh","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yiwei","family":"Ju","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6297-5194","authenticated-orcid":false,"given":"Moaz","family":"Waqar","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0007-0757-2377","authenticated-orcid":false,"given":"Theodore P.","family":"Letsou","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Christina M.","family":"Sp\u00e4gele","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3925-3238","authenticated-orcid":false,"given":"Hyunseung","family":"Jung","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2139-5182","authenticated-orcid":false,"given":"Igal","family":"Brener","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0965-8568","authenticated-orcid":false,"given":"Xiaoqing","family":"Pan","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5925-8768","authenticated-orcid":false,"given":"Marcus","family":"Ossiander","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Seth R.","family":"Bank","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4534-8249","authenticated-orcid":false,"given":"Federico","family":"Capasso","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,9,2]]},"reference":[{"key":"2268_CR1","doi-asserted-by":"publisher","DOI":"10.1038\/s41467-018-04944-9","volume":"9","author":"S Liu","year":"2018","unstructured":"Liu, S. et al. 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