Chongfan Technology
News
27
2025
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02
Disordered Topological Optics
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In optics, bound states in the continuum (BIC) are a special type of optical singularity where energy is localized and cannot propagate outward, forming a non-radiating, infinitely Q-value polarization singularity in momentum space, with a non-trivial topological charge in the polarization distribution around this singularity. Therefore, BIC has broad prospects in optical applications such as vortex light generation, field enhancement, and high Q-value. Traditionally, BICs realized using metasurfaces and photonic crystals rely on strict periodic structures, and disorder in the structure can destroy periodicity, leading to the degradation of BIC into quasi-BIC (QBIC), with its topological properties disappearing as well. Thus, past research on BIC has often sought to mitigate the effects of disorder. However, from another perspective, disorder provides additional degrees of freedom for structural control, which is crucial in wavefront control applications. Therefore, how to introduce effective disorder information into BIC without destroying its topological characteristics has become an important challenge in the field of topological optics.
Research has found that there exists a special BIC resonance mode in photonic crystals, which also contains a topological singularity in its field distribution, and the phase around this singularity has a non-trivial topological charge. This topological resonance mode is immune to structural perturbations, meaning that when the structure undergoes slight changes, the resonance mode will not be affected due to the topological protection of the singularity (Figure 2), significantly enhancing the stability of the BIC. In addition, the BIC topological singularity in momentum space of this topological photonic crystal coexists with the Pancharatnam-Berry (PB) phase distribution in real space, which can be used to introduce disordered states, thereby achieving additional wavefront control information encoding.
As a proof of concept, the research team introduced PB phase by rotating the photonic crystal structure in real space, successfully achieving real-momentum dual vortices with nested patterns and high-dimensional topological charges using dual topological charges in the two spaces.The upper left corner of Figure 3shows the experimental results of the momentum space vortex under Gaussian beam illumination.The upper right corner of Figure 3shows the experimental results of real space vortices at blue, green, and red wavelengths under plane wave incidence.The lower part of Figure 3shows the experimental results of real-momentum space dual vortices, characterized by smaller vortices nested within larger ones. Near the green light wavelength, the internal momentum space vortex with a topological charge of 2 is well confined, with a zero intensity singularity at the center, while the real space vortex with a topological charge of 6 exhibits a larger radius and diffusion pattern. The cross-sectional view further clarifies the two singularities and the annular shape. Unlike real space vortices, momentum space BIC vortices do not rely on a specific spatial center, thus exhibiting high robustness against beam position shifts on the photonic crystal. This can be demonstrated by offsetting the beam from the center of the sample; after the position shift, the real space vortex undergoes significant deformation, especially on the right side of the vortex light. In contrast, the internal momentum space vortex remains unchanged. However, the momentum space BIC vortex is more sensitive to dispersion; when the wavelength changes, the internal vortex transforms into a Gaussian spot without a topological singularity, while the real space vortex still exists.
Furthermore, this study further verifies the feasibility of real-momentum photonic crystals in high information capacity encoding applications. Through real space PB phase, holographic phase information can be encoded to project superstructured holograms while simultaneously generating momentum vortices. Using the Gerchberg-Saxton algorithm, any image can be converted into the target phase distribution, which can then be encoded by the corresponding rotation of the photonic crystal structure unit.Figure 4shows the encoding of the image of the letter "A" into the photonic crystal structure, and the corresponding hologram can be obtained through cross-polarization transmission measurements at blue, green, and red light. Meanwhile, at the resonant wavelength of green light, a ring-shaped momentum vortex with a topological charge of 2 was observed, indicating that this photonic crystal still maintains its momentum topological properties under perturbation.
This study proposes a novel topological photonic crystal that successfully achieves wavefront control based on disorder while designing topological resonance modes immune to structural perturbations, and simultaneously maintains its momentum space BIC topological characteristics, providing a new approach for applications in the field of topological optics. This innovative research is expected to promote the development of micro-nano optical devices such as photonic chips and be applied in high stability, high capacity optical communication, complex structured light generation, high-dimensional quantum entanglement technology, fine optical manipulation of biological particles, and AR/VR displays. This research is anticipated to have a significant impact on fields such as optics, electromagnetics, acoustics, topology, and quantum technology.
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