A key requirement for achieving high-density integration of terahertz (THz) systems is a strongly confining single-mode and low-loss waveguide. Several waveguide solutions based on technologies from both electronics and photonics have been proposed; among these, hollow-core waveguides are one of the best options for guiding THz radiation due to their very low material absorption of air. However, to minimize reflection losses, hollow-core waveguides typically have a core diameter larger than the operating wavelength, and as a consequence are multimode. Here, we report on a single-mode, single-polarization hollow-core THz fiber with a metamaterial cladding, consisting of subwavelength-diameter metal wires embedded in a dielectric host. The idea of using metal–dielectric hybrid cladding relies on the extreme anisotropy of wire metamaterials, which reflects transverse magnetic (TM) waves and transmits transverse electric waves, leading to a waveguide structure that only confines TM modes—thus halving the number of modes from the outset. Numerical simulations and experimental measurements confirm a wide single-mode single-polarization window ranging from 0.31 to 0.44 THz, with a wavelength-sized core (0.88 mm diameter). Our work overcomes a stumbling block for achieving compact and flexible single-mode THz waveguides, which may be important for future THz systems with high-density integration.

A magnetic field sensor using a dual-frequency optoelectronic oscillator (OEO) incorporating cascaded magnetostrictive alloy-fiber Bragg grating-Fabry Perot (MA-FBG-FP) and FBG-FP filters is proposed and demonstrated. In the OEO resonant cavity, two microwave signals are generated, whose oscillation frequencies are determined by the FBG-FP filter and MA-FBG-FP filter filters with two ultra-narrow notches and two laser sources. Due to the characteristics of MA and FBG, the two generated microwave signals show different magnetic field and temperature sensitivities. By monitoring the variations of two oscillating frequencies and the beat signal using a digital signal processor, the simultaneous measurement for the magnetic field and temperature can be realized. The proposed sensor has the advantages of high-speed and high-resolution measurement, which make it very attractive for practical magnetic field sensing applications. The sensitivities of the proposed OEO sensor for magnetic field and temperature are experimentally measured to be as high as −38.4MHz/Oe and −1.23 or −2.45 GHz/°C corresponding to the MA-FBG-FP filter and FBG-FP filter, respectively.

Metamaterial absorbers (MAs) serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term. Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function. Here, we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method. Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly, including near-complete absorption at resonance as well as strong robustness to wide incident angles. Specifically, the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26% and 8% with this new method, remaining in the ranges of ∼ 0.03 – 0.04 THz and ∼ 20 – 27 for two selected example MAs. More significantly, sensing capacities of this type of MA, in terms of maximum detection range (enhancing at least 9%), observable spectral modulation (increasing at least 6.3%), and refractive index sensitivity, are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs. This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte, which is illustrated by field profiles of the MAs at resonance in this work. Additionally, other desirable absorber features are also explored with such MAs, like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors. Such new-style terahertz MAs shown in the paper, acting as good examples, not only prove that terahertz MAs can be fabricated by the proposed time- and cost-saving route in contrast to the traditional MA fabrication process, but also can serve as novel platforms to explore other intriguing terahertz photonic effects, such as the field enhancement effect.

We explore terahertz (THz) orbital angular momentum (OAM) modes supported in multimode Kagome hollow-core fibers. Numerical models are adopted to characterize the effective indices and confinement losses of vector modes over 0.2–0.9 THz, where two low-loss transmission windows are observed. Linearly combining the vector modes, THz OAM states can be generated. Covering a broad bandwidth of 0.25 THz, the purity values of OAM modes are beyond 0.9. Using numerical simulations, the hollow-core THz fibers with one and two rings of Kagome structures are also comparably investigated. We reveal that the OAM purity is dependent upon the confinement performance of THz fiber.

A novel differential intensity-measurement high-sensitivity refractive index (RI) sensor based on cascaded dual-wavelength fiber laser and single-mode-no-core-hollow-core-no-core-single-mode (SNHNS) structure is proposed and demonstrated. The sensing unit consists of one uniform fiber Bragg grating (FBG) and an SNHNS structure as all-fiber interferometer filter. The dual-wavelength fiber laser has a ring cavity composed of two FBGs with central wavelengths of 1550.10nm and 1553.61nm. Through monitoring the wavelength shift and the output power difference of the dual-wavelength fiber laser, the simultaneous measurement for RI and temperature is realized. In our experiment, the proposed fiber laser sensor exhibits high RI sensitivities of −193.1dB/RIU and 174.8dB/RIU in the range of 1.334-1.384. The relative variation of output power at the two FBG wavelengths shows a higher RI sensitivity of −367.9dB/RIU with better stability, which is greater than the traditional modal interferometer structure. Meanwhile, the temperature sensitivity of the proposed sensor is 8.53 × 10⁻³nm/°C, and the changes of laser output power caused by temperature are −0.223dB/°C and 0.215dB/°C.