Recently, a groundbreaking research titled "Disorder-assisted real-momentum topological photonic crystal" was published in Nature by Professor Song Qinghua's team from Tsinghua Shenzhen International Graduate School (Tsinghua SIGS). This research introduces a novel concept of real-momentum topological photonic crystals, which can incorporate disorder as an information carrier without compromising the topological properties of optical singularities, addressing a significant challenge in the field of topological photonics.
A Major Challenge in Topological Photonics
In topological photonics, Bound States in the Continuum (BICs) are specific optical singularities where energy is localized and cannot radiate outward. These states form non-radiative, high-Q polarization singularities in momentum space, with non-trivial topological charge in their surrounding polarization distributions. BIC hold great promise for applications in vortex beam generation, field enhancement, and high-Q optical systems.
Conventional BICs in metasurfaces and photonic crystals rely on strict periodic structures. Disorder in these structures can disrupt periodicity, causing BIC to degrade into quasi-BIC (QBIC) and lose their topological properties. Consequently, past research has mainly focused on minimizing the impact of disorder. However, the disorder also provides additional degrees of freedom for structural control, which is crucial for wavefront manipulation applications. Thus, one of the major challenges in topological optics is how to introduce effective disorder into BICs without compromising their topological characteristics.
The Unique Concept of Real-Momentum Topological Photonic Crystals
To address this challenge, the research team proposed the concept of real-momentum topological photonic crystals for the first time (Figure 1). They discovered a unique BIC resonance mode in photonic crystals, where the electric field distribution exhibits a topological singularity at the structural center. Encircling this singularity, a vortex phase profile with a non-trivial topological charge is formed. Remarkably, this topological resonance mode is immune to structural perturbations. When the structure undergoes minor changes, the resonance mode remains unaffected due to the topological protection of the singularity, thereby significantly enhancing the stability of BICs (Figure 2).
Fig. 1 Schematic illustration of the real-momentum topological photonic crystal.
Fig.2 Topological resonance mode with immunity to structural perturbations. The electric field distribution exhibits a singularity at the center of the structure, and the phase distribution has a non-trivial topological charge, which remains unaffected by structural perturbations.
Promising Future Applications
The BIC momentum-space topological singularity in these topological photonic crystals coexists with the real-space Pancharatnam-Berry (PB) phase distribution, which can be used to encode additional wavefront control information. As a proof of the concept, the research team introduced PB phase by rotating metasurface structures in real space, using dual topological charges in both spaces to successfully create nested patterns and high-dimensional topological vortices in real-momentum space. Additionally, the study encoded holographic images into the PB phase and experimentally verified the recovery of high-quality metasurface holograms and momentum vortex beams (Figure 3). The dispersion characteristics of the momentum singularity and the broadband operation of the PB phase allow for wavelength-controlled separation and recombination in both spaces, offering higher tunability and information capacity.
Fig.3 Wavefront manipulation at the BIC. The hologram formed by PB phase encoding exhibits broadband characteristics (top), while the vortex beam generated by the BIC topology has narrowband characteristics (bottom).
In the future, Professor Song's team will continue to address key challenges in this important field, and aim to apply the research findings to optical communication, holographic displays, and special-shaped lasers.
The corresponding authors are Song Qinghua, associate professor at Tsinghua SIGS, Cheng-Wei QIU, professor at the National University of Singapore (NUS), and Romain FLEURY, professor at ?cole Polytechnique Fédérale de Lausanne (EPFL). The first authors are Haoye QIN, research assistant at Tsinghua SIGS and doctoral candidate from EPFL, Zengping SU, doctoral student from Tsinghua SIGS, and Zhe ZHANG, postdoctoral fellow from EPFL. Research collaborators include academician Ji ZHOU from Tsinghua University, researcher Bo LI, doctoral candidate Wenjing LV, doctoral candidate Zijin YANG and postgraduate student Xinyue GAO from Tsinghua SIGS, postdoctoral fellow Weijin CHEN, doctoral candidate Heng WEI from NUS, and professor Yuzhi SHI from Tongji University. This work was supported by the National Natural Science Foundation of China and the Science, Technology and Innovation Commission of Shenzhen municipality.
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Source: Tsinghua SIGS
Editor: Guo Lili
