Circular dichroism (CD) in terahertz (THz) regions has been widely used in biomonitoring, analytical chemistry, communication sensing, and other fields. Herein, we present a simple design for a dual-band THz chiral metasurface absorber (CMA) with a stronger CD effect based on temperature-tunable InSb for enhanced sensing applications. The proposed dual-band CMA consisted of a periodic array of the evolved C-shaped InSb adhered to a copper substrate. The designed CMA at 305 K achieved a right-handed circular polarization (RCP)-selective absorbance of 98.86% and 97.43% at 1.65 THz and 1.89 THz, respectively, and left-handed circular polarization (LCP) absorbance of 9.98% and 22.46%, respectively, and exhibited stronger CD values of 0.89 and 0.75. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. The simulated electric field and power follow distributions indicate that this dual-band chiral-selective absorption of the designed CMA is due to the different plasma resonance mode excitations for the incident circular polarization (CP) wave. In addition, the CD properties of the designed CMA can be adjusted by changing the geometrical parameters of the unit-cell structure. Furthermore, CD spectra can be dynamically adjusted by varying the outside temperature and refraction index (RI) of the filled analytes. The designed dual-band CMA can function as a high-performance temperature sensor with sensitivities of 4.68 GHz K⁻¹ and 5.52 GHz K⁻¹ and also as an RI sensor with sensitivities of 1080 GHz RIU⁻¹ and 860 GHz RIU⁻¹, respectively. Our proposed tunable dual-band CMA with its exquisite performance has the potential to be widely applied in diverse areas such as detection, sensing, and other related optoelectronic fields.

High-performance terahertz (THz) narrow-band metamaterial absorbers (MMAs) occupy a central position in sensing application. However, the majority of current MMAs for refractive index (RI) sensing typically only perform well within a single or dual-band range, or exhibit relatively low sensing performance. In this work, a tri-band MMA based on tri-cylindrical-shell (TCS) structure Ti ₃ C ₂ T _x MXene is proposed and investigated numerically for the enhanced refraction index (RI) sensing applications in THz region. The fundamental idea behind our approach lies in harnessing the unique electromagnetic (EM) properties of Ti ₃ C ₂ T _x MXene to achieve multi-band absorption and enhanced sensing capabilities. A comprehensive numerical simulation analysis of the designed MMA was conducted using the finite element method (FEM) and the equivalent circuit method (ECM). The simulation results demonstrate that the proposed MMA achieves remarkably high absorbance levels of 99.97%, 94.54%, and 99.65% at 1.20 THz, 1.55 THz, and 1.91 THz, respectively. This strong absorption is attributed to the hybrid coupling effect between waveguide modes and surface plasmon polaritons (SPPs), as evidenced by the simulated EM fields and energy distributions. Notably, the tri-band absorption characteristics of our MMA can be easily tuned by adjusting the geometric parameters of its unit-cell. Moreover, the MMA exhibits exceptional sensing performance, with sensitivities of up to 1.79 THz/RIU, 1.40 THz/RIU, and 1.13 THz/RIU, respectively. This combination of straightforward design, robust absorption capabilities, and heightened sensitivity makes our MMA structure a promising candidate for applications in THz biosensing, material detection, and communication systems.

A novel nano-β-FeOOH/Fe₃O₄/biochar composite with enhanced photocatalytic performance and superparamagnetism was successfully fabricated via an environmentally friendly one-step method. The structural properties of the prepared composite were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and a vibrating sample magnetometer. The XPS spectrum of the as-prepared composites confirmed the presence of Fe-O-C bonds between β-FeOOH and biochar, which could be conducive to transfer photo-generated electrons. UV-vis spectroscopy confirmed the existence of an electron–hole connection between β-FeOOH and biochar, which promoted the rapid interface transfer of photogenerated electrons from β-FeOOH to biochar. These novel structures could enhance the response of biochar to accelerate the photoelectrons under visible light for more free radicals. Electron spin resonance analysis and free radical quenching experiments showed that •OH was the primary active species in the photodegradation process of methyl orange by nano-β-FeOOH/Fe₃O₄/biochar. In the synergistic photocatalytic system, β-FeOOH/Fe₃O₄/biochar exhibited excellent catalytic activity for the degradation of azo dye (methyl orange), which is 2.03 times higher than that of the original biochar, while the surface area decreased from 1424.82 to 790.66 m²·g⁻¹. Furthermore, β-FeOOH/Fe₃O₄/biochar maintained a stable structure and at least 98% catalytic activity after reuse, and it was easy to separate due to its superparamagnetism. This work highlights the enhanced photocatalytic performance of β-FeOOH/Fe₃O₄/biochar material, which can be used in azo dye wastewater treatment.

Metasurfaces (MSs) are being extensively researched owing to their ability to modulate the polarization and wavefront of electromagnetic (EM) waves in a flexible manner, which usually offer significant advantages including ultra-thinness, low losses, and easy fabrication. However, conventional MSs typically operate well only with a single polarization. Here, we propose a novel design strategy for a terahertz (THz) reflective-mode MS that relies on a single unit-cell arrangement combining propagation phase and geometric phase. Our designed MS can achieve multiple wavefront manipulations in reflection mode, not limited to circular polarization (CP) transformation, but also enabling linear polarization (LP) conversion. The MS we propose consists of a periodic array of bilayered metal patterned resonator structures sandwiched by a dielectric substrate. The metallic resonator is made of the outer single-split-ring (SSR) and C-shaped slot (CSS), inner double-split-ring (DSR), and its complementary structure. With this design, the MS is capable of converting a LP wave to its orthogonal counterpart at lower frequency (f₁=0.7THz) after reflection. Additionally, at higher frequency (f₂=1.4THz), the proposed MS can also convert the right-handed CP (RCP) to left-handed CP (LCP) upon reflection or vice versa. The 2π phase full coverage of the orthogonal LP and CP waves can be achieved independently and simultaneously by adjusting the opening and orientation angles of the SSR based on propagation phase, and orientation angle of the DSR based on geometric phase. We numerically demonstrate beam deflection, planar focusing, and the vortex beam for both reflected orthogonal LP and CP waves with three representative MSs to provide proof of concept. These findings reveal the great potential for multifunctional devices for dual-polarization in imaging and communication systems.

Optical neural network (ONN), integrating deep learning algorithms and optical computing (OC) hardware, are gradually emerging as a powerful tool for artificial intelligence (AI) computing. Metasurfaces (MSs) have infused new vitality into the development of ONN by leveraging their nanoscale structures to precisely manipulate optical field parameters. In recent years, MS-based optical neural networks (M-ONNs) have sparked an innovative trend in the OC field, significantly impacting the development of next-generation optical communication and networking technologies. This paper presents an overview of M-ONN, beginning with an introduction to its theoretical framework and various design approaches. Then, the latest research progress in the practical applications of M-ONN is reviewed, emphasizing the crucial role of direct laser writing (DLW) in the manufacturing process. Further discussion covers the potential applications of M-ONN in next-generation optical communication and networking. Lastly, the focus shifts to the industrial progress of M-ONN. With technology maturing and the industrial chain solidifying, M-ONN is transitioning from the laboratory to real-world applications, poised to become a catalyst for transformative developments in the next-generation optical communication and networking field.