Tunable terahertz topological edge and corner states in designer surface plasmon crystals

Mode-order conversion in a Mach-Zehnder interferometer based on Chern insulators

Mode-order conversion devices can provide a flexible platform to achieve mode coupling and optimizing in mode division multiplex (MDM) that can eliminate the restrain of capacity and density in photonic integration and communication. However, mode-order converters based on traditional photonic crystal (PC) waveguides are susceptible to defects, which always render device incapacitation in mode-order conversion. Herein, a mode converter designed by the Mach-Zehnder interferometer (MZI) structure is proposed to manipulate the conversion of topological edge states (TESs) based on Chern insulators consisting of gyromagnetic PCs. The back-and-forth conversion between fundamental and high-order modes is numerically demonstrated based on phase modulation in our proposed device, in which each mode can be immune to defects. This unique approach for converting the mode order of TES exploits a new perspective in MDM to design a high-performance multimode device, leading to potential applications in photonic integrated circuits (PIC), on-chip processors, and optical fiber communication.

Terahertz topological photonic crystals with dual edge states for efficient routing

Topological photonic crystals with robust pseudo-spin and valley edge states have shown promising and wide applications in topological waveguides, lasers, and antennas. However, the limited bandwidth and intrinsic coupling properties of a single pseudo-spin or valley edge state have imposed restrictions on their multifunctional applications in integrated photonic circuits. Here, we propose a topological photonic crystal that can support pseudo-spin and valley edge states simultaneously in a single waveguiding channel, which effectively broadens the bandwidth and enables a multipath routing solution for terahertz information processing and broadcasting. We show that distorted Kekulé lattices can open two types of bandgaps with different topological properties simultaneously by molding the inter- and intra-unit cell coupling of the tight-binding model. The distinct topological origins of the edge states provide versatile signal routing paths toward free space radiation or on-chip self-localized edge modes by virtue of their intrinsic coupling properties. Such a powerful platform could function as an integrated photonic chip with capabilities of broadband on-chip signal processing and distributions that will especially benefit terahertz wireless communications.

Tunable topological edge and corner states in an all-dielectric photonic crystal

Topological photonics has become a new and fascinating area in recent years, which enables electromagnetic waves to propagate with negligible backscattering and excellent robustness even when encountering sharp corners or defects. But the flexible tunability of edge and corner states is challenging once the topological photonic crystals (PhCs) have been fabricated. In this paper, we propose a new all-dielectric PhC with C₃ symmetry constructed by hexagonal array of petal-like aperture embedded in silicon background. The proposed configuration has much wider energy gap than its triangular counterpart, and hence is suitable for wideband and high-capacity applications. When the apertures are filled with liquid crystals (LCs), the topologically-protected edge and corner states can be regulated through changing the refractive index of the LCs under different bias voltages. Moreover, the robustness of topological protection of edge and corner states is further demonstrated. This is the first demonstration of LC based tunable valley higher-order photonic topological insulator. The tunability of the proposed topological PhCs may be beneficial for development of tunable optical waveguides, reconfigurable topological microcavities, and other intelligent topological optical/terahertz devices.

A pixelated frequency-agile metasurface for broadband terahertz molecular fingerprint sensing

Terahertz (THz) plasmonic resonance based on an arbitrarily designed resonance metasurface is the key technique of choice for enhancing fingerprint absorption spectroscopy identification of biomolecules. Here, we report a broadband THz micro-photonics sensor based on a pixelated frequency-agile metasurface and illustrate its application ability to enhance and differentiate the detection of broadband absorption fingerprint spectra. The design uses symmetrical metal C-shape resonators with the functional graphene micro-ribbons selectively patterned into the gaps. A strong electric resonance with a high quality factor was formed, consisting of an electric dipole mode associated with the excitation of a dark toroidal dipole (TD) mode through the coupling from the electric dipole moment of the individual frequency-agile meta-unit. The resonance positions are nearly linearly modulated with the varying Fermi level of graphene. The configuration arranges a certain metapixel of the metasurface to multiple response spectra assembling a one-to-many mapping between spatial and spectral information which is instrumental in greatly shrinking the actual size of the sensor. By the synchronous regulation of graphene and C-shape rings, we have obtained highly surface-sensitive resonances over a wide spectral range (∼1.5 THz) with a spectral resolution less than 20 GHz. The target multiple enhanced absorption spectrum of glucose molecules is read out in a broadband region with high sensitivity. More importantly, the design can be extended to cover a larger spectral region by altering the range of geometrical parameters. Our microphotonic technique can resolve absorption fingerprints without the need for spectrometry and frequency scanning, thereby providing an approach for highly sensitive and versatile miniaturized THz spectroscopy devices.