Add drop multiplexers for terahertz communications using two-wire waveguide-based plasmonic circuits

Terahertz flexible multiplexing chip enabled by synthetic topological phase transitions

Flexible multiplexing chips that permit reconfigurable multidimensional channel utilization are indispensable for revolutionary 6G terahertz communications, but the insufficient manipulation capability of terahertz waves prevents their practical implementation. Herein, we propose the first experimental demonstration of a flexible multiplexing chip for terahertz communication by revealing the unique mechanism of topological phase (TP) transition and perseveration in a heterogeneously coupled bilayer valley Hall topological photonic system. The synthetic and individual TPs operated in the coupled and decoupled states enable controllable on-chip modular TP transitions and subchannel switching. Two time-frequency interleaved subchannels support 10- and 12-Gbit/s QAM-16 high-speed data streams along corresponding paths over carriers of 120 and 130 GHz with 2.5- and 3-GHz bandwidths, respectively. This work unlocks interlayer heterogeneous TPs for inspiring ingenious on-chip terahertz-wave regulation, allowing functionality-reconfigurable, compactly integrated and CMOS-compatible chips.

150 Gbps THz Chipscale Topological Photonic Diplexer

Photonic diplexers are being widely investigated for high data transfer rates in on-chip communication. However, dividing the available spectrum into nonoverlapping multicarrier frequency sub-bands has remained a challenge in designing frequency-selective time-invariant channels. Here, an on-chip topological diplexer is reported exhibiting terahertz frequency band filtering through Klein tunneling of topological edge modes. The silicon topological diplexer chip facilitates two high-speed channels with quadrature amplitude modulation (QAM) over a broad bandwidth of 12.5 GHz each. These channels operate at carrier frequencies of 305 and 321.6 GHz, achieving a combined diplexer capacity of 150 Gbit s-1. To ensure minimal interference between adjacent channels, a guard band is implemented. Topologically protected edge modes suppress the frequency selective fading of the broadband signals and hold promise for diverse integrated photonic applications spanning terahertz and telecommunication realms, including the design of lossless topological multiplexers, interconnects, antennas, and modulators for the sixth to X generation (6G to XG) wireless.

205-240 GHz free-space-to-fiber mode adapter with an 80% mode conversion efficiency

In this work, we propose an integrated terahertz mode adapter that couples broadband terahertz radiation from free-space to hollow-core fiber with a high mode conversion efficiency (Gaussian beam-to-T E 01) of up to 80%. The adapter consists of a pyramidal horn antenna, a broadband mode converter, and a conical horn. The simulation results indicate that the T E 01 mode in the hollow-core fiber can be efficiently excited by the terahertz mode adapter. The terahertz mode adapter successfully achieved a wide operating bandwidth of more than 15.7% ranging from 205 to 240 GHz in our simulation.

Resonant Gas Sensing in the Terahertz Spectral Range Using Two-Wire Phase-Shifted Waveguide Bragg Gratings

The development of low-cost sensing devices with high compactness, flexibility, and robustness is of significance for practical applications of optical gas sensing. In this work, we propose a waveguide-based resonant gas sensor operating in the terahertz frequency band. It features micro-encapsulated two-wire plasmonic waveguides and a phase-shifted waveguide Bragg grating (WBG). The modular semi-sealed structure ensures the controllable and efficient interaction between terahertz radiation and gaseous analytes of small quantities. WBG built by superimposing periodical features on one wire shows high reflection and a low transmission coefficient within the grating stopband. Phase-shifted grating is developed by inserting a Fabry-Perot cavity in the form of a straight waveguide section inside the uniform gratings. Its spectral response is optimized for sensing by tailoring the cavity length and the number of grating periods. Gas sensor operating around 140 GHz, featuring a sensitivity of 144 GHz/RIU to the variation in the gas refractive index, with resolution of 7 × 10-5 RIU, is developed. In proof-of-concept experiments, gas sensing was demonstrated by monitoring the real-time spectral response of the phase-shifted grating to glycerol vapor flowing through its sealed cavity. We believe that the phase-shifted grating-based terahertz resonant gas sensor can open new opportunities in the monitoring of gaseous analytes.

Transmission of sub-terahertz signals over a fiber-FSO-5 G NR hybrid system with an aggregate net bit rate of 227.912 Gb/s

Transmission of sub-terahertz (sub-THz) signals over a fiber-free-space optical (FSO)-fifth-generation (5 G) new radio (NR) hybrid system is successfully realized. It is a promising system that utilizes multiple media of optical fiber, optical wireless, and 5 G NR wireless to achieve a 227.912-Gb/s record-high aggregate net bit rate. The system concurrently transmits a 59.813-Gb/s net bit rate in the 150-GHz sub-THz frequency, 74.766-Gb/s in the 250-GHz sub-THz frequency, and 93.333-Gb/s in the 325-GHz sub-THz frequency through the fiber-FSO-wireless convergence, including 25-km single-mode fiber, 100-m FSO, and 30-m/25-m/20-m sub-THz-wave transmissions. This system achieves sufficiently low bit error rates (< hard-decision forward error correction (FEC) threshold of 3.8 × 10-3 at 16 and 20 Gbaud symbol rates; < soft-decision FEC threshold of 2 × 10-2 at 28 Gbaud symbol rate) and clear and distinct constellation diagrams, meeting the demands of 5 G NR communications in the sub-THz band. The development of fiber-FSO-5 G NR hybrid system represents a substantial development in the field of advanced communications. It has the ability to enhance the way we communicate in the future.

Realization of an optical nanostructure 4×1 multiplexer based on metal-insulator-metal plasmonic waveguides

The optical multiplexer was created at a nanoscale plasmonic structure utilizing the finite element method (FEM) with COMSOL version 5.5 software to enable maximum light confinement, high-speed optical systems, and a tiny structure. The metal-insulator-metal technology at a nanoscale dimension is used for creating the 4×1 multiplexer. In this design, the transmission threshold (T t h r e s h o l d ) is selected to be 100% for separating between logic "1" and logic "0" at a 1310 nm operating wavelength. The modulation depth (MD), contrast ratio (CR), and insertion loss (IL) characteristics were explained to evaluate the performance of the multiplexer. The CR has 3.48 dB, the MD offers an ideal performance with 95.28 %, and the IL has 3.31 dB.

Wideband circular polarizer for a photoconductive antenna

We report a thin-film circular polarizer consisting of three metal-grid layers to be used with a photoconductive antenna (PCA) to generate terahertz (THz) circularly polarized (CP) radiation. The polarizer has a high transmission with a measured 3 dB axial-ratio bandwidth of 54.7% from 0.57 to 1 THz. We further developed a generalized scattering matrix approach to provide insight into the underlying physical mechanism of the polarizer. We revealed that the Fabry-Pérot-like multi-reflection among gratings enables the high-efficiency polarization conversion. The successful realization of the CP PCA can find widespread application, such as THz circular dichroism spectroscopy, THz Mueller imaging, and ultrahigh-speed THz wireless communications.

Super-resolution THz endoscope based on a hollow-core sapphire waveguide and a solid immersion lens

To address a challenging problem of super-resolution terahertz (THz) endoscopy, in this paper, an antiresonant hollow-core waveguide was coupled with a sapphire solid immersion lens (SIL), aimed at subwavelength confinement of guided mode. The waveguide is formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube, the geometry of which was optimized to ensure high optical performance. SIL was judiciously designed, fabricated of bulk sapphire crystal, and then mounted at the output waveguide end. Study of the field intensity distributions at the shadow side of the waveguide-SIL system revealed the focal spot diameter of ≃0.2λ at the wavelength of λ = 500 μm. It agrees with numerical predictions, overcomes the Abbe diffraction limit, and justifies super-resolution capabilities of our endoscope.

Magnetic induced terahertz modulation characteristics based on ferromagnetic nematic liquid crystals

In recent years, solid state terahertz (THz) modulators have obtained rapid progress with the widespread use of two-dimensional (2D) materials in the field of THz; however, challenges remain in preparing flexible THz modulators. In this study, flexible ferromagnetic nematic materials were prepared by doping thermotropic nematic liquid crystals 5CB into magnetic fluids, and the influence of water was reduced by a self-made cyclic olefin copolymer (COC) microfluidic chip. THz modulation characteristics of magnetic fluid and ferromagnetic nematic liquid crystal (FNLC) under the induction of external magnetic field were compared using a THz time domain spectroscopy system. Under the action of a 91 mT magnetic field, the magnetic fluid has a maximum modulation depth (MD) of 54%. Under the same magnetic field, the maximum MD of the FNLC materials increase to 78% because of the rearrangement of Fe₃O₄ nanoparticles induced by the topological defect of the liquid crystal. We demonstrate that the magneto-optical effect is significantly enhanced in the ferromagnetic nematic liquid crystal hybrid system. This strategy of doping thermotropic nematic liquid crystals to enhance the magneto-optical effect has great potential for THz filtering, modulation, and sensing applications.

Bio-Inspired Nanomembranes as Building Blocks for Nanophotonics, Plasmonics and Metamaterials

Nanomembranes are the most widespread building block of life, as they encompass cell and organelle walls. Their synthetic counterparts can be described as freestanding or free-floating structures thinner than 100 nm, down to monatomic/monomolecular thickness and with giant lateral aspect ratios. The structural confinement to quasi-2D sheets causes a multitude of unexpected and often counterintuitive properties. This has resulted in synthetic nanomembranes transiting from a mere scientific curiosity to a position where novel applications are emerging at an ever-accelerating pace. Among wide fields where their use has proven itself most fruitful are nano-optics and nanophotonics. However, the authors are unaware of a review covering the nanomembrane use in these important fields. Here, we present an attempt to survey the state of the art of nanomembranes in nanophotonics, including photonic crystals, plasmonics, metasurfaces, and nanoantennas, with an accent on some advancements that appeared within the last few years. Unlimited by the Nature toolbox, we can utilize a practically infinite number of available materials and methods and reach numerous properties not met in biological membranes. Thus, nanomembranes in nano-optics can be described as real metastructures, exceeding the known materials and opening pathways to a wide variety of novel functionalities.