Co-propagating photonic topological interface states with hybridized pseudo-spins

Nature Physics (2026)Cite this article Topological interface states in quantum spin Hall systems, which are characterized by spin–momentum locking, enable robust unidirectional propagation for each spin component. Conventionally, such interfaces support only a single topological state in each propagation direction. This limitation impedes applications, such as those requiring multichannel signal switching. Here we demonstrate co-propagating topological interface states in a photonic topological insulator system. This is enabled by a hybridized pseudo-spin-flipping coupling mechanism that occurs across the interface between two topologically identical domains. The coupling mechanism facilitates power transfer and mode switching, which inherit the topological protection of the underlying states in each domain. The incorporation of optical gain further activates flexible switching, even in the presence of geometric defects. 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ECCS-2023780, ECCS-2425529 and DMR-2326699 to L.F. and DMR-2326698 to L.G.).These authors contributed equally: Xilin Feng, Tianwei Wu.Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, PA, USAXilin Feng, Tianwei Wu & Liang FengDepartment of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USATianwei Wu & Liang FengDepartment of Physics and Astronomy, College of Staten Island, The City University of New York, Staten Island, NY, USALi GeThe Graduate Center, The City University of New York, New York, NY, USALi GeSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarT.W., X.F. and L.F. conceived the project. X.F. and T.W. performed the tight-binding calculations and numerical simulations, fabricated the samples, and performed the measurements. L.F. and L.G. guided the research. All authors contributed to the discussions and the preparation of the paper.Correspondence to Liang Feng.The authors declare no competing interests.Nature Physics thanks Kun Ding and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.a, Design schematic of the microring array with a pseudo-spin-flipping interface. Adjacent site rings (black) within each domain are coupled via a link ring (green), with gap \({d}_{1}\) controlling the coupling strength. To implement a synthetic gauge phase of \(\varphi =2\pi / 3\), the link rings that connects the site rings in y-direction are vertically shifted in an arithmetic sequence with a displacement of \(\Delta y={\lambda }_{0} / (6{n}_{{\rm{eff}}})\), where \({\lambda }_{0}\) is the free-space wavelength and neff is the effective index of the guided mode inside the ring resonators. The site rings along the interface are coupled via a pair of link rings (purple), with gap \({d}_{2}\) (between the link rings) and \({d}_{3}\) (between the link rings and the site rings) jointly controlling the pseudo-spin–flipping coupling strength. b, Fabrication flow of the topological microring array. HSQ: hydrogen silsesquioxane; ICP: inductively coupled plasma; PECVD: plasma-enhanced chemical vapor deposition.A 1064 nm pump laser is used to illuminate from both sides of the sample, controlling the pumping area on the topological lattice and exciting the microring laser input. Optical signal emitted at ~1550 nm is collected using a microscope objective from the front side of the sample for imaging light transport on the sample and spectral characterization. The pump beam and the optical signal collected from the sample are represented by blue and red lines, respectively. BS: beam splitter; ND: neutral-density; SLM: spatial light modulator; BPF: bandpass filter; CCD: charge-coupled device.Supplementary Figs. 1–6 and Discussion.Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.Reprints and permissionsFeng, X., Wu, T., Ge, L. et al. Co-propagating photonic topological interface states with hybridized pseudo-spins. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03172-zDownload citationReceived: 07 September 2025Accepted: 07 January 2026Published: 02 February 2026Version of record: 02 February 2026DOI: https://doi.org/10.1038/s41567-026-03172-zAnyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative