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ever-increasing demand for higher data transfer rates in wired and wireless communication links. The exponential growth in data rates has pushed the carrier frequencies toward the higher spectral region, the terahertz (THz) band. The availability of ultrahigh bandwidths in the THz region (0.1-10 THz) allows achievement of terabits per second connectivity, [1] making it ideal for sixth-generation (6G) communication. However, with the emergence of 6G networks, the development of efficient on-chip communication with low loss and active control is crucial to handle and process the massive volume of data transmitted using THz carrier frequencies. The existing solutions for high-speed on-chip interconnects, including copper-based electrical interconnects (EIs), [2] suffer from limited bandwidth, and optical interconnects (OIs) [3] possess integration complexity and electronic-to-optical (EO/OE) conversion losses. To circumvent the existing performance gaps (in terms of bandwidths, energy efficiency, and system simplicity) of on-chip interconnects, THz interconnects [4,5] offer a potential route by leveraging the advantages of both the electronic and photonic worlds. However, scaling carrier frequencies up to sub-THz and beyond requires further innovation toward on-chip photonic solutions to support higher radio-frequency (RF) electronics Rapid scaling of semiconductor devices has led to an increase in the number of processor cores and integrated functionalities onto a single chip to support the growing demands of high-speed and large-volume consumer electronics. To meet this burgeoning demand, an improved interconnect capacity in terms of bandwidth density and active tunability is required for enhanced throughput and energy efficiency. Low-loss terahertz silicon interconnects with larger bandwidth offer a solution for the existing inter-/intrachip bandwidth density and energy-efficiency bottleneck. Here, a low-loss terahertz topological interconnect-cavity system is presented that can actively route signals through sharp bends, by critically coupling to a topological cavity with an ultrahigh-quality (Q) factor of 0.2 × 10 6 . The topologically protected large Q factor cavity enables energy-efficient optical control showing 60 dB modulation. Dynamic control is further demonstrated of the critical coupling between the topological interconnect-cavity for on-chip active tailoring of the cavity resonance linewidth, frequency, and modulation through complete suppression of the back reflection. The silicon topological cavity is complementary metal-oxide-semiconductor (CMOS)-compatible and highly desirable for hybrid electronic-photonic technologies for sixth (6G) generation terahertz communication devices. Ultrahigh-Q cavity also paves the path for designing ultrasensitive topological sensors, terahertz topological integrated circuits, and nonlinear topological photonic devices.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202202370.

In periodic media, despite the close relationship between geometrical effects in the bulk and topological surface states, the two are typically probed separately. We show that when beams in a Weyl medium reflect off an interface with a gapped medium, the trajectory is influenced by both bulk geometrical effects and the Fermi arc surface states. The reflected beam experiences a displacement, analogous to the Goos-Hänchen or Imbert-Fedorov shifts, that forms a half-vortex in the two-dimensional surface momentum space. The half-vortex is centered where the Fermi arc of the reflecting surface touches the Weyl cone, with the magnitude of the shift scaling as an inverse square root away from the touching-point, and diverging at the touching-point. This striking feature provides a way to use bulk transport to probe the topological characteristics of a Weyl medium.

Valley polarized topological states of light allow for robust waveguiding which has been demonstrated for transverse-electric modes in THz and near-infrared parts of the spectrum. As the topological protection relies on guiding the light via a highly structured surface, direct imaging of the photonic modes at sub-unit cell resolution is of high interest but challenging in particular for transverse-magnetic modes. Here, we report mapping the transverse-magnetic modes in a valley photonic crystal waveguide using scattering-type scanning near-field optical microscopy at the optical telecom Cband wavelength. The waveguide based on a triangular air-hole motif with broken inversion symmetry is fabricated from suspended Germanium layer. We observed the launching and guiding of the transverse-magnetic edge mode along the boundary between topologically distinct domains with opposite valley Chern indices. These results are supported by theoretical simulations, and provide insight into the design and use of topological protected states for applications in densely integrated optical telecommunication devices.

We realize an unpaired Dirac cone at the center of the first Brillouin zone, using a gyromagnetic photonic crystal with broken square sub-lattice symmetry and broken time reversal symmetry. The behavior of the Dirac modes can be described by a gyromagnetic effective medium model with nearzero refractive index, and Voigt parameter near unity. When two domains are subjected to opposite magnetic biases, there exist unidirectional edge states along the domain wall. This establishes a link between topological edge states and the surface waves of homogenous magneto-optical media.

The supermoiré lattice, built by stacking two moiré patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoiré lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called “golden rule of three”, here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoiré lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moiré samples with an accuracy better than ~ 0.2°. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moiré materials.

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