Realization of switchable EIT metamaterial by exploiting fluidity of liquid metal

High-FOM Temperature Sensing Based on Hg-EIT-Like Liquid Metamaterial Unit

High-performance temperature sensing is a key technique in modern Internet of Things. However, it is hard to attain a high precision while achieving a compact size for wireless sensing. Recently, metamaterials have been proposed to design a microwave, wireless temperature sensor, but precision is still an unsolved problem. By combining the high-quality factor (Q-factor) feature of a EIT-like metamaterial unit and the large temperature-sensing sensitivity performance of liquid metals, this paper designs and experimentally investigates an Hg-EIT-like metamaterial unit block for high figure-of-merit (FOM) temperature-sensing applications. A measured FOM of about 0.68 is realized, which is larger than most of the reported metamaterial-inspired temperature sensors.

Liquid Metal-Based Devices: Material Properties, Fabrication and Functionalities

This paper reviews the material properties, fabrication and functionalities of liquid metal-based devices. In modern wireless communication technology, adaptability and versatility have become attractive features of any communication device. Compared with traditional conductors such as copper, the flow characteristics and lack of elastic limit of conductive fluids make them ideal alternatives for applications such as flexible circuits, soft electronic devices, wearable stretch sensors, and reconfigurable antennas. These fluid properties also allow for innovative manufacturing techniques such as 3-D printing, injecting or spraying conductive fluids on rigid/flexible substrates. Compared with traditional high-frequency switching methods, liquid metal (LM) can easily use micropumps or an electrochemically controlled capillary method to achieve reconfigurability of the device. The movement of LM over a large physical dimension enhances the reconfigurable state of the antenna, without depending on nonlinear materials or mechanisms. When LM is applied to wearable devices and sensors such as electronic skins (e-skins) and strain sensors, it consistently exhibits mechanical fatigue resistance and can maintain good electrical stability under a certain degree of stretching. When LM is used in microwave devices and paired with elastic linings such as polydimethylsiloxane (PDMS), the shape and size of the devices can be changed according to actual needs to meet the requirements of flexibility and a multistate frequency band. In this work, we discuss the material properties, fabrication and functionalities of LM.

Printed Transformable Liquid-Metal Metamaterials and Their Application in Biomedical Sensing

Metamaterial is becoming increasingly important owing to its unique physical properties and breakthrough applications. So far, most metamaterials that have been developed are made of rigid materials and structures, which may restrict their practical adaptation performances. Recently, with the further development of liquid metal, some efforts have explored metamaterials based on such tunable electronic inks. Liquid metal has high flexibility and good electrical conductivity, which provides more possibilities for transformable metamaterials. Here, we developed a new flexible liquid-metal metamaterial that is highly reconfigurable and could significantly extend the working limit facing current devices. The printed electronics method was adopted to fabricate artificial units and then construct various potential transformable metamaterials. Based on metamaterial theory and printing technology, typical structured flexible liquid-metal electromagnetic metamaterials were designed and fabricated. The electronic and magnetic characteristics of the liquid-metal-based electromagnetic metamaterials were evaluated through simulated analysis and experimental measurement. Particularly, the potential of liquid-metal metamaterials in biomedical sensing was investigated. Further, the future outlook of liquid-metal metamaterials and their application in diverse categories were prospected.

Angstrom-Scale Active Width Control of Nano Slits for Variable Plasmonic Cavity

Nanogap slits can operate as a plasmonic Fabry–Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wavenumber. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blueshift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10⁻⁵ enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

Thermally tunable high-Q metamaterial and sensing application based on liquid metals

Achieving a high Q-factor metamaterial unit for a precision sensing application is highly demanded in recent years, and most of the developed high-performance sensors based on the high-Q metamaterial units are due to the dielectric/magnetic property changes of the substrate/superstrate. In this paper, we propose a completely different sensing metamaterial unit configuration, with good sensing sensitivity and precision properties, based on the thermally tunable liquid metals. Specifically, a basic thermally tunable metamaterial unit, the mercury-inspired split ring resonator (SRR), is firstly presented to theoretically show the magnetic resonance and negative permeability frequency band shift properties under different background temperatures. Then, considering the radiation loss mechanism of the conventional SRR metamaterial unit and based on the physically reliable ability of liquid metals, the modified mercury-inspired Fano and toroidal resonators with a large frequency tuning range and high Q-factor are developed and discussed. The numerical demonstrations have shown that the designed Fano and toroidal resonators have much better sensing precision performances compared to the conventional SRR for the temperature sensing application. The experimental demonstrations have also been used to verify the proposed mercury-based toroidal resonators, and good agreements are achieved.

EIA metamaterials based on hybrid metal/dielectric structures with dark-mode-enhanced absorption

Metamaterial analogue of electromagnetically induced absorption (EIA) has promising applications in spectroscopy and sensing. Here we propose an EIA metamaterial based on hybrid metal/dielectric structures, which are composed of a metallic wire and a dielectric block, and investigate the EIA-like effect by simulations, experiments, and the two-oscillator model. An EIA-like effect emerges in virtue of the near-field coupling between metallic wire and dielectric block, and the dielectric block exhibiting magnetic dipolar resonance makes a major contribution to the resonance absorption. The magnetic flux through the dielectric block engendered by the near filed of the metallic wire determines the coupling between dielectric block and metallic wire. With the variation of the separation between dielectric block and metallic wire, the EIA-like effect is preserved and does not convert into the EIT-like effect although the coupling and consequently the absorbance are altered. Based on the two-oscillator model, the absorption spectrum of the EIA metamaterial is quantitatively analyzed and the parameters of the oscillator system are retrieved.

Simultaneous realizations of absorber and transparent conducting metal in a single metamaterial

By introducing vanadium dioxide film into a multilayer structure, the dual functionalities of perfect absorption and high transmission are presented using the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is in the conducting state, the designed system acts as a narrowband absorber. The proposed absorber is composed of the top metallic ring, silica spacer, and the vanadium dioxide film. The absorption peak is originated from localized magnetic resonance between metallic ring and vanadium dioxide film. When vanadium dioxide is in the insulating state, the designed system acts as a transparent conducting metal. The top metallic ring, the middle dielectric spacer, and the subwavelength metallic mesh are combined together to form an antireflection coating. The influences of incident angle and structure parameter on absorption and transmission are also discussed. This work has demonstrated a new route for developing vanadium dioxide-based switchable photonic devices in the fields of filter and modulator at terahertz frequencies.

Advances in Liquid Metal-Enabled Flexible and Wearable Sensors

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.

Generalized circuit model for all-dielectric narrowband metasurface absorbers

All-dielectric metasurface absorbers have great potential in many scientific and technical applications. The emerging metasurfaces show strong and versatile capabilities in controlling absorptance, reflectance, and transmittance of electromagnetic waves. In this work, we propose and investigate all-dielectric metasurface absorbers with an equivalent circuit model. In the proposed circuit model, we satisfy the first Kerker condition. To verify the accuracy of the proposed model, the obtained results for an all-dielectric cubic metasurface absorber are compared with the existing experimental data. Moreover, using the proposed circuit model, we propose a hemisphere structure and compare the results of the proposed model with those of full-wave simulations. With this novel structure, we achieve higher absorptance and quality factor in comparison to a cubic one. Additionally, our proposed model reduces the calculation time and needs less memory compared to full-wave simulations. The results of the circuit model have an acceptable agreement with the experimental data and those of full-wave simulations. The proposed circuit model is simple yet general. It provides physical insight into the design and operation of various sub-wavelength structures in the broad frequency range, including THz and visible regions.

Controllable coherent perfect absorber made of liquid metal-based metasurface

Coherent perfect absorber (CPA) is a novel strategy proposed and demonstrated for solving the challenge to attain efficient control of absorption by exploiting the inverse process of lasing. The operation condition of CPA results in narrow-band, which is the main limitation obstruct it from practical applications. Here, we demonstrate a CPA with tunable operation frequency employing the liquid metal made reconfigurable metasurface. The flow of liquid metal is restricted with a plastic pipe for realizing a controllable liquid metal cut-wire. The adjustable electric dipolar mode of the reconfigurable cur-wire metasurface ensures that the quasi-CPA point can be dynamically controlled; the measured CPA under proper phase modulation is in good agreement with the simulation results. The proposed CPA system involving liquid metal for dynamic control of operation frequency will have potential applications and may stimulate the exploitation of liquid based smart absorption control of optical waves.