Light has taken on a new form at the corners of a fractal structure. Researchers have created “corner vortex solitons” that combine two previously separate features of photonics: the topological protection of corner-confined light and the twisted phase structure of optical vortex beams. The advance could lead to photonic chips capable of carrying large volumes of information while remaining resistant to manufacturing defects, deformation, and other forms of structural damage.
The work addresses a long-standing limitation in higher-order topological photonic systems. Topological corner states are special optical modes that become trapped at the corners of carefully designed lattices. Because their existence is determined by the global topology of the structure rather than by individual design details, they can remain stable even when imperfections are introduced. This robustness has made them attractive for optical communications and signal processing. However, conventional corner states are usually zero-dimensional, tightly localized modes with relatively simple, nearly uniform phase profiles.
That compact structure creates a serious obstacle for generating optical vortices. A vortex beam contains a phase that winds continuously around a central singularity, where the phase is undefined. This spiral phase gives the beam orbital angular momentum, a property that can be used to encode additional information into light. Vortex beams are therefore considered promising for high-capacity communications, optical encryption, and advanced imaging. Yet the broad, multi-site phase pattern required for a vortex is difficult to fit inside the highly confined single-mode profile of a conventional topological corner state.
Previous attempts to produce corner solitons—self-sustaining localized light structures formed through the interaction of diffraction and optical nonlinearity—had not overcome this barrier. Although those solitons could remain confined at lattice corners, their phase distributions were essentially flat. They did not display the phase singularities or winding behavior associated with true optical vortices. The new study reports a system in which both effects emerge in the same state: topological corner confinement and orbital angular momentum.
The researchers designed their photonic lattice using the geometry of a Sierpiński gasket, a fractal pattern generated by repeatedly removing triangular regions from a larger triangle. Unlike ordinary periodic lattices, this structure contains self-similar features across multiple length scales. It also supports pairs of degenerate corner modes—distinct modes with the same energy. By carefully superimposing these paired modes, the researchers produced a linear corner state whose phase varied across the lattice in a spiral-like pattern.
The lattice was fabricated in fused silica using femtosecond laser direct writing, a technique that uses tightly focused ultrashort laser pulses to permanently modify the refractive index inside transparent materials. By writing an array of microscopic waveguides into the glass, the team created a three-dimensional photonic platform in which light could hop between neighboring lattice sites. The resulting structure provided the controlled coupling and fractal topology needed to support the unusual corner modes.
When optical nonlinearity was introduced, the linear vortex corner states evolved into localized corner vortex solitons. In such a soliton, the light intensity modifies the refractive index of the material through nonlinear response. That refractive-index change can counteract diffraction, which would otherwise cause the beam to spread as it propagates. The result is a self-trapped beam whose spatial profile and twisted phase remain coordinated over distance. In this case, the self-trapped state also retained its topological confinement at the lattice corner.
Experiments revealed several properties that distinguish these states from vortex solitons in ordinary, topologically trivial lattices. The corner vortex solitons remained stable across an exceptionally broad range of input powers and showed strong resilience to structural imperfections. Most strikingly, they formed without a minimum power threshold. Conventional vortex solitons generally require sufficient nonlinear interaction before self-trapping can occur, meaning that the input beam must exceed a critical power. The new states instead benefit from the underlying linear corner modes, allowing vortex localization to appear even when no threshold power is required.
The researchers also used phase-resolved interference measurements to verify that the observed states were genuine optical vortices rather than ordinary localized beams with an unusual intensity pattern. Interference reveals how the optical phase changes across the beam, and the measured phase distribution showed the characteristic winding and singularity expected from a vortex. The result demonstrates that orbital angular momentum can be integrated directly into a topologically protected corner mode, opening a path toward photonic devices that combine robustness, compactness, and high-dimensional information encoding.
The findings could influence the design of future optical chips for communications, encryption, and signal processing. A device based on corner vortex solitons might carry information through both the conventional properties of light and its orbital angular momentum, increasing the number of independent channels available within a single platform. Because the states are protected against imperfections and do not require a power threshold to form, they may also simplify the operation of nonlinear photonic circuits. The study, published in Science Bulletin under the title “Thresholdless corner vortex solitons in fractal Sierpiński topological insulators,” establishes a new connection between fractal topology, nonlinear optics, and vortex-based photonics.
Subject of Research: Topological photonics, optical vortex solitons, fractal photonic lattices, and orbital angular momentum of light
Article Title: Thresholdless corner vortex solitons in fractal Sierpiński topological insulators
Web References: https://doi.org/10.1016/j.scib.2026.06.052
References: Science Bulletin, “Thresholdless corner vortex solitons in fractal Sierpiński topological insulators”
Image Credits: © Science Bulletin
Keywords: corner vortex solitons, topological insulators, topological photonics, Sierpiński gasket, fractal lattices, optical vortices, orbital angular momentum, nonlinear optics, photonic chips, optical communications, optical encryption
