Terahertz fiber devices

Hou G., Liu S., Zha Z., Yu S., Su Z., Liu S., Chen S., Jing C., Chu J.

Chen B., Zhou K., Shao J., Zhao X., Xu B., Zhu J., Xue B., Tan Z., Cao J., Li H., Wu C.

Ghazialsharif M., Dong J., Bongiovanni D., Vorobiov A., Wang Z., Chen Z., Kip D., Morandotti R.

Abstract Innovative terahertz waveguides are in high demand to serve as a versatile platform for transporting and manipulating terahertz signals for the full deployment of future six-generation (6G) communication systems. Metal-wire waveguides have emerged as promising candidates, offering the crucial advantage of sustaining low-loss and low-dispersion propagation of broadband terahertz pulses. Recent advances have opened up new avenues for implementing signal-processing functionalities within metal-wire waveguides by directly engraving grooves along the wire surfaces. However, the challenge remains to design novel groove structures to unlock unprecedented signal-processing functionalities. In this study, we report a plasmonic signal processor by engineering topological interface states within a terahertz two-wire waveguide. We construct the interface by connecting two multiscale groove structures with distinct topological invariants, i.e., featuring a π-shift difference in the Zak phases. The existence of this topological interface within the waveguide is experimentally validated by investigating the transmission spectrum, revealing a prominent transmission peak in the center of the topological bandgap. Remarkably, we show that this resonance is highly robust against structural disorders, and its quality factor can be flexibly controlled. This unique feature not only facilitates essential functions such as band filtering and isolating but also promises to serve as a linear differential equation solver. Our approach paves the way for the development of new-generation all-optical analog signal processors tailored for future terahertz networks, featuring remarkable structural simplicity, ultrafast processing speeds, as well as highly reliable performance.

Li X., Wang Z., Huiqi J., Deng M., Yin L., Gong C., Liu W.

A terahertz metamaterial waveguide (meta-waveguide) and a meta-waveguide-based lens-free imaging system are presented. The meta-waveguide not only inherits the low-loss transmission performance of a waveguide but also breaks through the diffraction limit under the action of the metamaterial, achieving subwavelength focusing. The focusing distance is far greater than the Rayleigh length, thus enabling far-field scanning imaging. For verification, a metal ring-based meta-waveguide was fabricated by 3D printing and metal cladding technology. Then, a transmission scanning imaging system working at 0.1 THz was built. High quality terahertz images with a resolution of 1/3 of the wavelength were obtained by placing the imaging targets at the focus and performing two-dimensional scanning. The focusing and transmission of terahertz wave in the meta-waveguide were simulated and analyzed.

Viratikul R., Hong B., Janpugdee P., Oberhammer J., Robertson I.D., Somjit N.

Hou G., Liu S., Zhu Y., Yu S., Zha Z., Zhao Q., Liu S., Jing C., Chu J.

This work proposed a vacuum evaporation method in preparing an Ag/polypropylene (PP) terahertz (THz) hollow waveguide (HWG). Rray model and COMSOL were used to optimize the geometric structure parameters of the HWG. Accordingly, a 5 mm bore PP tube with a wall thickness of 360 μm was selected as the structural and dielectric tube, and a silver reflective layer was deposited onto the outer surface of the PP tube using vacuum evaporation process. The as-deposited Ag layer consists of small Ag particles (about 100 nm) which contribut to form a more uniform and smoother metal reflective layer than that could be prepared via the wet chemical method. Terahertz time-domain spectroscopy (THz-TDS) analysis confirms that the Ag/PP THz HWG exhibits a low-loss window centered at the frequency of 0.1 or 0.3 THz. The HWG sample shows low transmission losses of 2.26 dB/m at 0.1 THz and 1.23 dB/m at 0.3 THz, respectively. The THz waves could be stably transmitted through the as-fabricated Ag/PP waveguide at low (−78.5 °C) and high (65 °C) temperatures. This research opens a simple and environmentally friendly avenue for the fabrication of low-loss metal/dielectric THz HWG. It is helpful for promoting the applications of THz HWG in future THz imaging, sensing and 6G communication.

Chen B., Xu B., Zhu J., Tang P., Shao J., Yang S., Ding G., Wu C.

Cao Y., Nallappan K., Xu G., Skorobogatiy M.

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.

Carter J., Lees H., Wang Q., Chen S.J., Atakaramians S., Withayachumnakul W.

AbstractTraditional microwave design and fabrication techniques have been adopted into the terahertz domain. Understanding the properties of microwave dielectric materials at terahertz frequencies is critical for accurate component design. Nevertheless, terahertz properties for common microwave dielectric materials are largely unknown. Hence, this paper presents the relative permittivity, loss tangent, refractive index, and extinction coefficient for such materials, including microwave substrates and low-temperature co-fired ceramics (LTCCs), within the 0.1 to 3.5 THz range. Terahertz time-domain spectroscopy (THz-TDS) and a vector network analyzer produce accurate material parameter results. The material parameters presented in this paper serve as a valuable resource for component design at terahertz frequencies.

Chen D., Zhang L., Gao C., Shi Q., He S., Wang Z., Lei Y., Li G., Gong P.

Gupta M., Kumar A., Singh R.

AbstractElectromagnetic filtering is essential for emerging integrated photonic technologies and widely adaptable processing of high‐bandwidth signals. The conventional electro‐optic modulator‐based photonic approach for signal filtering at microwave frequencies cannot be implemented at terahertz (THz) frequencies due to the unavailability of the THz signal‐driven optical modulator. Here, an electrically tunable on‐chip THz photonic notch filter based on the topologically protected valley hall waveguide‐cavity platform is demonstrated. The device shows a significantly large notch suppression depth of more than 20 dB with a return loss of 13 dB in the entire tuning range of notch frequency. The feedback control circuit enables precise control of the notch frequency shift with a minimum step size of 7 MHz. This work extends the application of topological photonic crystals in developing THz‐integrated photonic devices for transformative technologies, including sixth‐generation (6G) communication and high‐resolution spectral sensing.

Headland D., Fujita M., Carpintero G., Nagatsuma T., Withayachumnankul W.

The absence of a suitable standard device platform for terahertz waves is currently a major roadblock that is inhibiting the widespread adoption and exploitation of terahertz technology. As a consequence, terahertz-range devices and systems are generally an ad hoc combination of several different heterogeneous technologies and fields of study, which serves perfectly well for a once-off experimental demonstration or proof-of-concept, but is not readily adapted to real-world use case scenarios. In contrast, establishing a common platform would allow us to consolidate our design efforts, define a well-defined scope of specialization for “terahertz engineering,” and to finally move beyond the disconnected efforts that have characterized the past decades. This tutorial will present arguments that nominate substrateless all-silicon microstructures as the most promising candidate due to the low loss of high-resistivity float-zone intrinsic silicon, the compactness of high-contrast dielectric waveguides, the designability of lattice structures, such as effective medium and photonic crystal, physical rigidity, ease and low cost of manufacture using deep-reactive ion etching, and the versatility of the many diverse functional devices and systems that may be integrated. We will present an overview of the historical development of the various constituents of this technology, compare and contrast different approaches in detail, and briefly describe relevant aspects of electromagnetic theory, which we hope will be of assistance.

Xue L., Sheng X., Mu Q., Kong D., Wang Z., Chu P.K., Lou S.

A single-mode hollow-core anti-resonant (HC-AR) waveguide designed for low-loss terahertz (THz) wave propagation is fabricated by three-dimensional (3D) printing. Compared to similar structures reported recently, the rotating-nested semi-elliptical tubes (SETs) in the HC-AR THz waveguide cladding suppress multiple high-order modes (LP11, LP21, and LP02 modes) at the same time giving rise to enhanced single-mode transmission and low losses. Three HC-AR THz waveguides with different wall thicknesses are produced using two photosensitive resins and analyzed by THz time-domain spectroscopy (THz-TDS). The experimental results show that the electric field distributions at the output end of these waveguides have a Gaussian-like distribution reflecting that of the single mode. The smallest transmission losses determined by the ‘cut-back’ method are 0.03 cm−1 at 0.31 THz for sample A, 0.02 cm−1 at 0.4 THz for sample B, and 0.01 cm−1 at 0.23 THz for sample C. The consistent experimental and simulated results reveal that the HC-AR THz waveguide has many advantages over current ones by achieving low losses and single-mode operation simultaneously.

Thackston K., Doane J., Anderson J., Chrayteh M., Hindle F.

Xu G., Skorobogatiy M.