Reconfigurable terahertz metamaterials: From fundamental principles to advanced 6G applications

Flexible terahertz phase shifter for optically controlled polydimethylsiloxane-vanadium dioxide composite film

In the terahertz (THz) band, modulation research has become a focal point, with precise control of the phase shift of THz waves playing a pivotal role. In this study, we investigate the optical control of THz phase shift modulation in a polydimethylsiloxane (PDMS)-vanadium dioxide (VO2) flexible material using THz time-domain spectroscopy. Under the influence of an 808-nm continuous wave (CW) laser with power densities ranging from 0 to 2.74 W/cm2, the PDMS-VO2 flexible material exhibits significant phase shift modulation in the frequency range of 0.2 to 1.0 THz. The maximum optical-pumping phase shift reaches 0.27π rad at 1.0 THz in a composite material with a VO2 mass fraction of 5% and a thickness of 360 µm, and the amplitude transmittance from 0.2 THz to 1.0 THz exceeds 70%. Furthermore, the composite material exhibits good stability under at least 640 switching cycle times, as confirmed through repeatability tests. The proposed composite devices offer a new approach for more flexible phase shift modulation owing to the flexibility of the composite material and the non-contact and precise modulation of light control. Additionally, the stress-adjustable characteristics of flexible materials make them highly suitable for use in wearable THz modulators, highlighting their significant application potential.

A self-aligned assembling terahertz metasurface microfluidic sensor for liquid detection

This paper reports a new terahertz metasurface microfluidic sensor, which is actually a kind of reflective terahertz metasurface absorber with a microfluidic-channel structure located in the strong field energy region of the absorber. The metasurface structure is made on an ultra-thin quartz substrate as the cap layer, while a two-step structure is made on a silicon substrate as the pedestal layer. In order to precisely control the thickness of the microfluidic channel, the cap layer is self-aligned assembled with the pedestal layer to form the terahertz metasurface microfluidic sensor. The obtained sensor could enhance the light-matter interaction, resulting in high sensing performance. The measured results show that the sensor has a perfect absorption peak at 2.60 THz and a high Q-factor of 62.59, which are basically consistent with the simulated results. The sensitivity and FOM calculated based on the measured results of different liquids with different refractive indices is 0.733 THz per RIU and 16.4, respectively. Compared with some recently related work, the sensitivity is increased by about 40%, the Q-factor is increased by 3-5 times, and the FOM is increased by 5 times, which make the sensor have great application potential for solution detection in the terahertz frequency band.

A Polarization-Insensitive, Vanadium Dioxide-Based Dynamically Tunable Multiband Terahertz Metamaterial Absorber

A tunable multiband terahertz metamaterial absorber, based on vanadium dioxide (VO2), is demonstrated. The absorber comprises a three-layer metal-insulator-metal (MIM) configuration with a split ring and slots of VO2 on the uppermost layer, a middle dielectric substrate based on silicon dioxide (SiO2), and a gold reflector on the back. The simulation results indicate that, when VO2 is in the metallic state, the proposed metamaterial exhibits nearly perfect absorption at six distinct frequencies. The design achieves an average absorption of 98.2%. The absorptivity of the metamaterial can be dynamically tuned from 4% to 100% by varying the temperature-controlled conductivity of VO2. The proposed metamaterial absorber exhibits the advantages of polarization insensitivity and maintains its absorption over 80% under different incident angle conditions. The underlying physical mechanism of absorption is explained through impedance matching theory, interference theory, and the distribution of electric fields. The ability to achieve multiband absorption with tunable characteristics makes the proposed absorber a promising candidate for applications in terahertz sensing, imaging, communication, and detection. The polarization insensitivity further enhances its practicality in various scenarios, allowing for versatile and reliable performance in terahertz systems.

Advances and Applications of Metal-Organic Frameworks (MOFs) in Emerging Technologies: A Comprehensive Review

Metal-organic frameworks (MOFs) that are the wonder material of the 21st century consist of metal ions/clusters coordinated to organic ligands to form one- or more-dimensional porous structures with unprecedented chemical and structural tunability, exceptional thermal stability, ultrahigh porosity, and a large surface area, making them an ideal candidate for numerous potential applications. In this work, the recent progress in the design and synthetic approaches of MOFs and explore their potential applications in the fields of gas storage and separation, catalysis, magnetism, drug delivery, chemical/biosensing, supercapacitors, rechargeable batteries and self-powered wearable sensors based on piezoelectric and triboelectric nanogenerators are summarized. Lastly, this work identifies present challenges and outlines future opportunities in this field, which can provide valuable references.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Artificial intelligence enhanced sensors - enabling technologies to next-generation healthcare and biomedical platform

The fourth industrial revolution has led to the development and application of health monitoring sensors that are characterized by digitalization and intelligence. These sensors have extensive applications in medical care, personal health management, elderly care, sports, and other fields, providing people with more convenient and real-time health services. However, these sensors face limitations such as noise and drift, difficulty in extracting useful information from large amounts of data, and lack of feedback or control signals. The development of artificial intelligence has provided powerful tools and algorithms for data processing and analysis, enabling intelligent health monitoring, and achieving high-precision predictions and decisions. By integrating the Internet of Things, artificial intelligence, and health monitoring sensors, it becomes possible to realize a closed-loop system with the functions of real-time monitoring, data collection, online analysis, diagnosis, and treatment recommendations. This review focuses on the development of healthcare artificial sensors enhanced by intelligent technologies from the aspects of materials, device structure, system integration, and application scenarios. Specifically, this review first introduces the great advances in wearable sensors for monitoring respiration rate, heart rate, pulse, sweat, and tears; implantable sensors for cardiovascular care, nerve signal acquisition, and neurotransmitter monitoring; soft wearable electronics for precise therapy. Then, the recent advances in volatile organic compound detection are highlighted. Next, the current developments of human-machine interfaces, AI-enhanced multimode sensors, and AI-enhanced self-sustainable systems are reviewed. Last, a perspective on future directions for further research development is also provided. In summary, the fusion of artificial intelligence and artificial sensors will provide more intelligent, convenient, and secure services for next-generation healthcare and biomedical applications.

Electric Split-Ring Metamaterial Based Microfluidic Chip with Multi-Resonances for Microparticle Trapping and Chemical Sensing Applications

In this work, an integration of terahertz (THz) electrical split-ring metamaterial (eSRM) with microfluidic chip is presented. This eSRM-based microfluidic chip exhibits multiple resonances in the THz spectrum and trapping selectively microparticle size characteristics. The arrangement of eSRM array is dislocation. It generates the fundamental inductive-capacitive (LC) resonant mode, quadrupole, and octupolar plasmon resonant modes and then exhibits high sensitivity to the environmental refraction index. The trapping structures of microparticles are elliptical barricades on eSRM surface. Thus, the electric field energy is strongly confined within the gap of eSRM in transverse electric (TE) mode and then the elliptical trapping structures are anchored on both sides of the split gap to ensure the microparticles can be trapped and located on the gap. To imitate the microparticle sensing ambient environment qualitatively and quantitatively in the THz spectrum, the microparticles are designed different feature sizes with different refraction index from 1.0 to 2.0 in ethanol medium. The results show the proposed eSRM-based microfluidic chip possesses the trapping and sensing abilities in single microparticle and high sensitivity for fungus, microorganism, chemical and environmental applications.

Tunable resonance of a graphene-perovskite terahertz metasurface

The combination of graphene and perovskite has received extensive research attention because its photoelectric properties are excellent for the dynamic manipulation of light-matter interactions. Combining graphene and perovskite with a metasurface is expected to effectively improve the metasurface device's performance. Here, we report a terahertz graphene-perovskite metasurface with a tunable resonance. Under 780 nm laser excitation, the device's THz transmission is significantly reduced, and the Fano resonance mode can be manipulated in multiple dimensions. We verify the experimental results using a finite-difference time-domain (FDTD) simulation. Graphene and perovskite interact strongly with the metasurface, resulting in a short-circuit effect, which significantly weakens the resonance intensity of the Fano mode. The photoinduced conductivity enhancement intensifies the short-circuit effect, reducing the THz transmission and resonance intensity of the Fano mode and causing the resonance frequency to redshift. Finally, we provide a reference value for applications of hybrid metasurface-based optical devices in a real environment by investigating the effect of moisture on device performance.

Machine learning-augmented surface-enhanced spectroscopy toward next-generation molecular diagnostics

The world today is witnessing the significant role and huge demand for molecular detection and screening in healthcare and medical diagnosis, especially during the outbreak of COVID-19. Surface-enhanced spectroscopy techniques, including Surface-Enhanced Raman Scattering (SERS) and Infrared Absorption (SEIRA), provide lattice and molecular vibrational fingerprint information which is directly linked to the molecular constituents, chemical bonds, and configuration. These properties make them an unambiguous, nondestructive, and label-free toolkit for molecular diagnostics and screening. However, new issues in molecular diagnostics, such as increasing molecular species, faster spread of viruses, and higher requirements for detection accuracy and sensitivity, have brought great challenges to detection technology. Advancements in artificial intelligence and machine learning (ML) techniques show promising potential in empowering SERS and SEIRA with rapid analysis and automatic data processing to jointly tackle the challenge. This review introduces the combination of ML and SERS/SEIRA by investigating how ML algorithms can be beneficial to SERS/SEIRA, discussing the general process of combining ML and SEIRA/SERS, highlighting the molecular diagnostics and screening applications based on ML-combined SEIRA/SERS, and providing perspectives on the future development of ML-integrated SEIRA/SERS. In general, this review offers comprehensive knowledge about the recent advances and the future outlook regarding ML-integrated SEIRA/SERS for molecular diagnostics and screening.

Tunable MEMS-Based Terahertz Metamaterial for Pressure Sensing Application

In this study, a tunable terahertz (THz) metamaterial using the micro-electro-mechanical system (MEMS) technique is proposed to demonstrate pressure sensing application. This MEMS-based tunable metamaterial (MTM) structure is composed of gold (Au) split-ring resonators (SRRs) on patterned silicon (Si) substrate with through Si via (TSV). SRR is designed as a cantilever on the TSV structure. When the airflow passes through the TSV from bottom to up and then bends the SRR cantilever, the SRR cantilever will bend upward. The electromagnetic responses of MTM show the tunability and polarization-dependent characteristics by bending the SRR cantilever. The resonances can both be blue-shifted from 0.721 THz to 0.796 THz with a tuning range of 0.075 THz in transverse magnetic (TM) mode and from 0.805 THz to 0.945 THz with a tuning range of 0.140 THz in transverse electric (TE) mode by changing the angle of SRR cantilever from 10° to 45°. These results provide the potential applications and possibilities of MTM design for use in pressure and flow rate sensors.

Reassessing Fano Resonance for Broadband, High-Efficiency, and Ultrafast Terahertz Wave Switching

Miniaturized ultrafast switchable optical components with high efficiency and broadband response are in high demand to the development of optical imaging, sensing, and high-speed communication. Sharp Fano-type resonance switched by active materials is one of the key concepts that underpins the control of light in metaoptics with high sensitivity. However, actuating such metasurfaces exhibits a long-standing trade-off between modulation depth and operational bandwidth. Here, the limitations are circumvented by theoretical analysis, numerical simulation, and experimental realization of an achromatic Fano metasurface so that a high contrast of tunability with ultrafast switching rate over a broad range of frequency is achieved. By developing the physics of inter-mode coupling, the Fano metasurface is designed according to a complete phase diagram derived from coupled mode theory. Unlike conventional Fano metasurfaces, the cross-polarized inter-metaatoms coupling is discovered as a superior ability of high-efficiency broadband achromatic polarization conversion. To prove the ultrasensitive nature, a metadevice is constructed by incorporating a thin amorphous Ge layer with a weak photoconductivity perturbation. Transmission modulation over broadband frequency range from 0.6 to 1.1 THz is thus successfully realized, featuring its merits of modulation depth over 90% and On-Off-On switching cycle less than 10 ps.

Tunable anisotropic parameters realized by metal-dielectric hybrid meta-atoms

Rational design of the structure enables metamaterials to go beyond the ingredients and achieve unprecedented material properties. However, the realization of complicated and anisotropic electromagnetic parameters relies on the elaborate design of building blocks, and the mutual coupling between the anisotropic responses makes precise control of material parameters even more difficult. Here, we propose a metal-dielectric hybrid metamaterial, not only realizing the decoupling between anisotropic electromagnetic responses, but also establishing a one-to-one correspondence between independent geometric dimensions and anisotropic parameter components. Moreover, a tuning theoretical paradigm applied to an anisotropic and resonant system is further suggested, which proves that the operating frequency of this hybrid metamaterial can be easily adjusted by changing external fields. As prototypes, two typical and tunable microwave meta-devices, a transformation-optics cloak and a frequency splitter, are constructed with Ba-Sm-La-Ti ferroelectric ceramic and flexible printed circuit board, which successfully demonstrate our proposed design theory. This work provides a simple strategy for the design and fabrication of tunable anisotropic metamaterials, and boost the development of meta-devices toward practical application.

A review of tunable photonics: Optically active materials and applications from visible to terahertz

The next frontier of photonics is evolving into reconfigurable platforms with tunable functions to realize the ubiquitous application. The dynamic control of optical properties of photonics is highly desirable for a plethora of applications, including optical communication, dynamic display, self-adaptive photonics, and multi-spectral camouflage. Recently, to meet the dynamic response over broad optical bands, optically active materials have been integrated with the diverse photonic platforms, typically in the dimension of micro/nanometer scales. Here, we review recent advances in tunable photonics with controlling optical properties from visible to terahertz (THz) spectral range. We propose guidelines for designing tunable photonics in conjunction with optically active materials, inherent in wavelength characteristics. In particular, we devote our review to their potential uses for five different applications: structural coloration, metasurface for flat optics, photonic memory, thermal radiation, and terahertz plasmonics. Finally, we conclude with an outlook on the challenges and prospects of tunable photonics.

Tune the resonance of VO2 joined metamaterial dimers by adjacent cut wires

Two terahertz metamaterials were joined by a conductivity variable VO₂ patch to obtain a metamaterial dimer. By applying voltage or heat to the VO₂ patches, active modulation of terahertz wave could be achieved. A cut-wire metamaterial was placed adjacent to the VO₂ joined dimer to affect its electromagnetic response. It was found that the cut wire could heavily impact the resonance mode of the VO₂ joined dimer, which gives dual resonance dips in transmission spectrum for both insulating and conducting states of VO₂ patches. As a result, by tuning the conductivity of VO₂, active dual band phase modulation could be achieved with high transmission window by this dimer-cut wire coupling system.