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Sharp-bending waveguide is a key element for ultra-compact and densely integrated photonic devices, which is promising to enlarge the capability of modern information processing in a single chip. Topological photonics manifest the nature of robust propagation against sharp bending and such robustness has been experimentally demonstrated in topological integrated interfaces. It is important to quantify the bending loss of topological interface but has remained exclusive. In this work, we report on the characterization of sharp-bending robustness in the integrated topological interface of valley photonic crystals (VPCs) by experimentally quantifying the ultralow bending loss. The VPCs are designed on a standard silicon-on-insulator platform with the inversion-symmetry broken in honeycomb lattice, and four types of topological interfaces can be constructed by two topologically-distinct VPCs. As one of the representative cases, zigzag-AA interface is applied to demonstrate the robust propagation along sharp bending. In experiment, we fabricate a series of VPC interfaces with different turn number and the same transmission distance, which perform the ultralow bending loss less than 0.02 dB per 120-deg turning. Furthermore, we experimentally characterize the propagation loss in the integrated interfaces. Our approach not only shows the ability of VPC topological interfaces to suppress backscattering stemming from sharp bending, but also paves the way for topological nanophotonic dense integration.

Topological boundary modes, which are localized at the edge of topological materials, have received significant attention for their various applications in robust waveguides, optical cavities, and topological lasers. To envision their further applications in tunable devices, we propose and demonstrate a scheme to dynamically manipulate topological boundary modes by exploiting the two translation parameters of photonic crystals. We find that the translation not only transports the Wannier state similar to conventional Thouless pumping but also induces a nonzero Chern number in the two-dimensional synthetic space while preserving the time-reversal symmetry in the real space. Through changing the translation, gapless and tunable topological boundary modes are demonstrated. As a specific application, we show a dynamic bandpass filter with real-time tuning over 100% bandgap, a capability that cannot be achieved with only one translation parameter. Our design opens a venue for the development of tunable topological devices based on synthetic parameter dimension and can be generalized to other bosonic systems. [ABSTRACT FROM AUTHOR]

Disorder, which is ubiquitous in nature, has been extensively explored in photonics for understanding the fundamental principles of light diffusion and localization, as well as for applications in functional resonators and random lasers. Recently, the investigation of disorder in topological photonics has led to the realization of topological Anderson insulators characterized by an unexpected disorder-induced phase transition. However, the observed photonic topological Anderson insulators so far are limited to the time-reversal symmetry breaking systems. Here, we propose and realize a photonic quantum spin Hall topological Anderson insulator without breaking time-reversal symmetry. The disorder-induced topological phase transition is comprehensively confirmed through the theoretical effective Dirac Hamiltonian, numerical analysis of bulk transmission, and experimental examination of bulk and edge transmissions. We present convincing evidence for the unidirectional propagation and robust transport of helical edge modes, which are the key features of nontrivial time-reversal invariant topological Anderson insulators. Furthermore, we demonstrate disorder-induced beam steering, highlighting the potential of disorder as a new degree of freedom to manipulate light propagation in magnetic-free systems. Our work not only paves the way for observing unique topological photonic phases but also suggests potential device applications through the utilization of disorder.

3-dB couplers, which are commonly used in photonic integrated circuits for on-chip information processing, precision measurement, and quantum computing, face challenges in achieving robust performance due to their limited 3-dB bandwidths and sensitivity to fabrication errors. To address this, we introduce topological physics to nanophotonics, developing a framework for topological 3-dB couplers. These couplers exhibit broad working wavelength range and robustness against fabrication dimensional errors. By leveraging valley-Hall topology and mirror symmetry, the photonic-crystal-slab couplers achieve ideal 3-dB splitting characterized by a wavelength-insensitive scattering matrix. Tolerance analysis confirms the superiority on broad bandwidth of 48 nm and robust splitting against dimensional errors of 20 nm. We further propose a topological interferometer for on-chip distance measurement, which also exhibits robustness against dimensional errors. This extension of topological principles to the fields of interferometers, may open up new possibilities for constructing robust wavelength division multiplexing, temperature-drift-insensitive sensing, and optical coherence tomography applications., (© 2024. The Author(s).)

Manipulating radiation asymmetry of photonic structures is of particular interest in many photonic applications such as directional optical antenna, high efficiency on-chip lasers, and coherent light control. Here, we proposed a term of pseudopolarization to reveal the topological nature of radiation asymmetry in bilayer metagratings. Robust pseudopolarization vortex with an integer topological charge exists in P-symmetry metagrating, allowing for tunable directionality ranging from -1 to 1 in synthetic parameter space. When P-symmetry breaking, such vortex becomes pairs of C points due to the conservation law of charge, leading to the phase difference of radiation asymmetry from π/2 to 3π/2. Furthermore, topologically enabled coherent perfect absorption is robust with customized phase difference at will between two counterpropagating external light sources. This Letter can not only enrich the understanding of two particular topological photonic behaviors, i.e., bound state in the continuum and unidirectional guided resonance, but also provide a topological view on radiation asymmetry, opening an unexplored avenue for asymmetric light manipulation in on-chip laser, light-light switch, and quantum emitters.

Directional emission of electromagnetic waves plays an essential role in laser radar and free-space communication. For most directional antennas, bandwidth and miniaturization are a pair of contradictions due to their underlying interference mechanism. Connection-type metamaterials exhibit exotic electromagnetic response near zero-frequency, which relies on the global topology of mesh connectivity rather than resonance and thus has a broad working bandwidth. In this Letter, we investigate the broadband orientation-dependent coupling effect of a 3D double mesh metamaterial. Based on this effect, we achieve a broadband directional emission (relative bandwidth of 37.72%) using a compact structure (compared to twice working wavelength). Our work provides a novel, to the best of our knowledge, scheme to manipulate a long-wavelength wave and may pave the way to a miniaturized directional antenna.