Broadband tunable terahertz metamaterial absorber having near-perfect absorbance modulation capability based on a patterned vanadium dioxide circular patch.

A new tunable broadband terahertz metamaterial absorber has been designed based on patterned vanadium dioxide (V O 2). The absorber consists of three simple layers, the top V O 2 pattern layer, the middle media layer, and the bottom metal layer. Based on phase transition properties of V O 2, the designed device has excellent absorption modulation capability, achieving the functional transition from broadband absorption to near-perfect reflection. When V O 2 is in the metallic state, there are two absorption peaks observed at frequencies of 4.16 and 6.05 THz, exhibiting near-perfect absorption characteristics; the combination of these two absorption peaks gives rise to the broadband phenomenon and the absorption bandwidth, where the absorbance exceeds 90% and spans from 3.40 to 7.00 THz, with a corresponding relative absorption bandwidth of 69.23%. The impedance matching theory, near-field patterns, and surface current distributions are provided to analyze the causes of broadband absorption. Furthermore, the broadband absorption could be completely suppressed when V O 2 presents the dielectric phase, and its absorbance could be dynamically adjusted from 100% to less than 0.70%, thereby achieving near-perfect reflection. Owing to its symmetrical structure, it exhibits excellent performance in different polarization directions and at large incidence angles. Our proposed absorber may have a wide range of promising applications and can be applied in a variety of fields such as communications, imaging, sensing, and security detection.

Design and experimental realization of triple-band electromagnetically induced transparency terahertz metamaterials employing two big-bright modes for sensing applications.

Multi-band electromagnetically induced transparency (EIT) effects have attracted widespread attention due to their great application prospects. However, their realization is mainly based on the coupling of multiple sub-resonators that typically exceed the number of transparency peaks, resulting in complex structural designs and cumbersome preparation procedures. This paper reports a simple design of a terahertz metamaterial that can produce the triple-band EIT effect using two "big-bright" mode coupling of two sub-resonators. The design adopts the classical two-layer structure. A U-shaped split-ring resonator and a fork-shaped resonator form a periodic array on the surface of the flexible organic polymer material. Three transparency peaks around 0.59 THz, 1.07 THz, and 1.34 THz are experimentally realized, and their formation mechanisms are explored. Furthermore, the triple-band EIT metamaterial was prepared by the photolithography technology and characterized by terahertz time-domain spectroscopy. Theoretical simulation results agree well with experimental results. Sensing characteristics and slow light effects of the terahertz metamaterial are further discussed experimentally. The proposed triple-band EIT metamaterial having excellent properties, including thin size, good flexibility, simple and compact structure, and high sensing sensitivity, could provide guidance for the subsequent design and implementation of multifunctional multi-band EIT metamaterials.

Four-band terahertz metamaterial absorber based on Dirac semimetal for a refractive index sensing application.

We design a four-band narrow-band near-perfect absorber based on bulk Dirac semimetal (BDS) metamaterial in the terahertz region. The absorber has a top-to-bottom three-layer structure of a BDS layer, an insulating dielectric slab, and a gold layer. The BDS is flexible and tunable, allowing the Fermi energy level to be adjusted by changing the applied bias voltage, thus changing the absorption characteristics of the absorber. We use the time-domain finite-difference method to simulate the absorption characteristics of the absorber, which could achieve four discrete near-perfect absorption peaks at 0.98 THz, 1.70 THz, 2.02 THz, and 2.36 THz. The absorber is polarization sensitive, and the conversion between four-band absorption and three-band absorption is achieved by changing the incident polarization angle. We also change the structure of the absorber to study the absorption characteristics and break the structural symmetry to achieve a larger number of absorption peaks. Besides, the sensing performance of four-band narrow-band absorption is analyzed, and the maximum sensitivity of the absorber is 112.78 GHz/RIU. The device should have vast application prospects for bio-detection and high-sensitivity biosensing detection.

Triple-Band and Ultra-Broadband Switchable Terahertz Meta-Material Absorbers Based on the Hybrid Structures of Vanadium Dioxide and Metallic Patterned Resonators.

A bifunctional terahertz meta-material absorber with three layers is designed. The surface of the bifunctional meta-material absorber is a periodically patterned array composed of hybrid structures of vanadium dioxide (VO2) and metallic resonators; the middle layer is a nondestructive TOPAS film, and the bottom layer is a continuous metallic plane. Utilizing the phase-transition property of VO2, the responses of the meta-material absorber could be dynamically switched between triple-band absorption and ultra-broadband absorption. When VO2 is in the metallic state, an ultra-broadband absorption covering the bandwidth of 6.62 THz is achieved over the range from 4.71 THz to 11.33 THz. When VO2 is in the di-electric state, three absorption peaks resonated at 10.57 THz, 12.68 THz, and 13.91 THz. The physical mechanisms of the bifunctional meta-material absorber were explored by analyzing their near-field distributions. The effects of varying structural parameters on triple-band and ultra-broadband absorption were investigated. It is revealed that by optimizing the structure parameters, the number of absorption peaks could be increased for a certain sacrifice of absorption bandwidth. FDTD Solutions and CST Microwave Studio were used to simulate the data of the absorber, and similar results were obtained.

Realization of a multi-band terahertz metamaterial absorber using two identical split rings having opposite opening directions connected by a rectangular patch.

A multi-band metamaterial absorber in the terahertz regime using a periodically arranged surface structure placed on an ultra-thin insulating dielectric slab backed by a metallic ground plane is demonstrated in this paper. Its surface structure consists of two identical split rings having opposite opening directions connected by a rectangular patch. The surface structure can have a strong electromagnetic interaction with incident terahertz waves, thereby generating two localized resonance absorption peaks with different frequencies, and the superposition effect of these two absorption peaks gives rise to dual-band absorption. With the aid of the near-field distributions of the two absorption peaks, the physical mechanism of the dual-band absorption is revealed. The dimension changes of the surface structure, including the split rings and the rectangular patch, play a key role in controlling and adjusting the resonance performance of dual-band absorption. Further optimization of the surface structure without increasing the number of sub-resonators provides the ability to increase the number of absorption peaks, which is different from prior multi-band absorption devices that typically require more sub-resonators in their surface structures. Multi-band metamaterial absorbers designed in this paper should have great application prospects in the field of terahertz absorption.

Flexible surface-enhanced Raman scatting substrates: recent advances in their principles, design strategies, diversified material selections and applications.

Surface-enhanced Raman scattering (SERS) is widely used as a powerful analytical technology in cutting-edge areas such as food safety, biology, chemistry, and medical diagnosis, providing ultra-fast, ultra-sensitive, nondestructive characterization and achieving ultra-high detection sensitivity even down to the single-molecule level. Development of Raman spectroscopy is strongly dependent on high-performance SERS substrates, which have long evolved from the early days of rough metal electrodes to periodic nanopatterned arrays building on solid supporting substrates. For rigid SERS substrates, however, their applications are restricted by sophisticated pretreatments for detecting solid samples with non-planar surfaces. It is therefore essential to reassert the principles in constructing flexible SERS substrates. Herein, we comprehensively review the state-of-the-art in understanding, preparing and using flexible SERS. The basic mechanisms behind the flexible SERS are briefly outlined, typical design strategies are highlighted and diversified selection of materials in preparing flexible SERS substrates are reviewed. Then the recent achievements of various interdisciplinary applications based on flexible SERS substrates are summarized. Finally, the challenges and perspectives for future evolution of flexible SERS and their applications are demonstrated. We propose new research directions focused on stimulating the real potential of SERS as an advanced analytical technique for commercialization.

Bioavailability evaluation of the intestinal absorption and liver accumulation of torularhodin using a rat postprandial model.

Torularhodin, as a new functional carotenoid, possesses great application potential in disease intervention. However, its absorption process and corresponding mechanism have not been studied. In this study, a rat postprandial model was established to explore the absorption and mechanism of torularhodin by investigating the bioavailability of torularhodin in different tissues, the expression of related enzymes and several transporters in the intestine. The results showed that torularhodin entered the intestine faster from micelles (45.21 ± 2.61% was absorbed in the duodenum), and part of it was metabolized into retinol in the anterior segment of the intestine. The expression of genes indicated that absorption of torularhodin in the intestine might require transporter CD36 and SR-B1. The special structure and target organ might be speculated to be the main reason for the low bioavailability of torularhodin in the serum and liver. The results could lay a theoretical foundation for the chemical modification, carrier and subsequent development of torularhodin.

Miniaturized and Actively Tunable Triple-Band Terahertz Metamaterial Absorber Using an Analogy I-Typed Resonator.

Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator-metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications.

An Overview on Recent Progress of the Hydrogels: From Material Resources, Properties, to Functional Applications.

Hydrogels, as the most typical elastomer materials with three-dimensional (3D) network structures, have attracted wide attention owing to their outstanding features in fields of sensitive stimulus response, low surface friction coefficient, good flexibility, and bio-compatibility. Because of numerous fresh polymer materials (or polymerization monomers), hydrogels with various structure diversities and excellent properties are emerging, and the development of hydrogels is very vigorous over the past decade. This review focuses on state-of-the-art advances, systematically reviews the recent progress on construction of novel hydrogels utilized several kinds of typical polymerization monomers, and explores the main chemical and physical cross-linking methods to develop the diversity of hydrogels. Following the aspects mentioned above, the classification and emerging applications of hydrogels, such as pH response, ionic response, electrical response, thermal response, biomolecular response, and gas response, are extensively summarized. Finally, this review is done with the promises and challenges for the future evolution of hydrogels and their biological applications.

Broadband achromatic metalens for linearly polarized light from 450 to 800 nm.

Metalens is a planar optical component that uses nanostructures with a thickness on the order of the wavelength to manipulate the wavefront of the incident light. A key problem, especially in color imaging and display applications, is the correction of chromatic aberration, which is an inherent effect caused by the dispersion of periodic lattices and resonance modes. However, the current achromatic metalenses either use the PB phase method that is only valid for circularly polarized light or nanostructures with complex cross sections that are difficult to manufacture. Here, we designed a broadband achromatic metalens for linearly polarized light from 450 to 800 nm. Rectangular titanium dioxide nanofins of various lengths and widths were applied to modulate the phase and dispersion of the incident light. The metalens can fulfill three target phases simultaneously by using an optimization method. The designed metalens has a stable focus from 450 to 800 nm with an average focusing efficiency of 64%. It can be potentially applied in microscopes, lithography machines, sensors, and displays.

Multi-band terahertz superabsorbers based on perforated square-patch metamaterials.

This paper presents a multi-band terahertz superabsorber with a surface structure that consists of a square metallic patch with a very small rectangular hole whose area is only 3.94% of the square patch. The introduction of a rectangular hole in the square patch plays an important role in achieving multi-band absorption. Three resonant bands with very high absorption (>95%) were observed in the terahertz range. Different from the near-field distributions of the traditional square patch with no modification, the introduction of a rectangular hole in the square patch can break the near-field distributions of the traditional square patch with no modification or can rearrange them to form some new or extra resonance modes, thereby generating multi-band absorption. Considering the fact that the introduced rectangular hole plays the key role in the rearrangement of the near-field and the introduction of some new resonance modes, the parameter changes of the rectangular hole introduced in the square patch provide considerable freedom in controlling the number of absorption peaks, and the resonant bands can be tuned to quad- or dual-band absorption. The multi-band superabsorbers given here should have potential applications in numerous areas.

Surface-Enhanced Infrared Absorption of Ligands on Colloidal Gold Nanowires through Resonant Coupling.

Pushing the detection limit of infrared absorption (IR) through surface-enhanced (SEIRA) approaches have far-reaching prospect for related applications in molecular analysis and detection. Specifically engineered Au nanowires (NWs) can be applied as the surface-enhancing substrates in colloidal solution, given their longitudinal surface plasmon resonance (SPR) being aspect-ratio dependent and extendable into the infrared region. Through carefully designed control experiments, we realized resonant coupling between the longitudinal surface plasmons of Au NWs and the vibration modes of the bonded oleylamine (OA) ligands. In our system, after deliberately tuning thickness of the OA ligands and ratio of the detached/attached ligands in the solution, the apparent enhancement factor of IR signal from ligands around Au NWs could be pushed up to 5.29 × 104. Given the facile tuning of SPR properties of Au NWs in the colloidal solution and the performance demonstrated in the report, our work could be an intriguing platform for SEIRA implementations in a broad spectrum of circumstances.

Design of a dual-band terahertz metamaterial absorber using two identical square patches for sensing application.

A dual-band terahertz metamaterial absorber composed of two identical square metallic patches and an insulating medium layer on top of a continuous metallic ground is demonstrated. Two resonance peaks (labeled A and B) with near 100% absorbance are obtained, of which peak A derived from the localized resonance of the two square patches has a line-width of 0.2571 THz and quality factor of 6.9156, while peak B which resulted from the hybrid coupling of the localized resonance of the two square patches and surface lattice resonance of the device has a very narrow line-width of 0.0083 THz and large quality factor of 296.2771. Narrow line-width and large quality factor have important prospects in sensing application. Based on this, the sensing performance of the device is explored; it is revealed that peak B exhibits highly sensitive sensing ability (including a sensing sensitivity of 1.9010 THz per RIU and figure of merit of 229.04) in terms of the surrounding index. In addition, the influence of structural parameters on the absorption performance is discussed to further verify the formation mechanism of these two absorption peaks.

Bifunctional resonance effects of classical electromagnetically induced transparency and Fano response using a terahertz metamaterial resonator.

A terahertz metamaterial resonator consisting of two rectangular strips embedded in a closed-ring resonator is designed and investigated in this paper that can simultaneously generate the classical electromagnetically induced transparency effect and two Fano resonance modes. The formation of the transparent window and the Fano dips can be attributed to the coupling of the localized resonance modes between the closed-ring resonator and the two embedded rectangular strips. The effect of embedded rectangular strip sizes on the transmitted performance is discussed, and it is found that the resonant performance of the transparent window and Fano effect depends mainly on the length of the rectangular strips in the longitudinal direction. Based on this, two different device applications related to the embedded rectangular strips are discussed. The metamaterial proposed here could open up new avenues toward the control of terahertz waves in many technology-related areas.

Performance comparison of the electromagnetic induction transparency effects for two U-shaped resonators having different opening directions.

This paper gives the design of electromagnetically induced transparency effect using two U-shaped resonators with different opening directions (same direction and opposite direction). It is revealed that extremely similar transparency effect can be found for the two cases. The reason is that the two structures have the same sizes. However, the change in position of the two U-shaped resonators in the opposite opening has a significant effect on the transparent peak which is mainly reflected in the broadening of the broadband and the frequency shift of the working frequency, while there is almost no change in resonance performance for the same opening. We provide the field distributions for analyzing the causes of these different results. We believe these performance can guide future research.

Design of Narrow Discrete Distances of Dual-/Triple-Band Terahertz Metamaterial Absorbers.

Various kinds of structure designs have been proposed to achieve the multiple-band metamaterial absorbers. However, the discrete distance of adjacent frequencies of multiple absorbers is considerably large, which will inevitably overlook a large amount of information hidden in the off-resonance absorption areas. Herein, a narrow discrete distance of dual-band terahertz absorber based on two pairs of an Au strip/dielectric layer backed by Au film is designed. Two nearly 100% absorptivities of resonance peaks having the discrete distance of only 0.30 THz are realized. The relative discrete distance of the device is 13.33%, and this value can be adjusted via the length change of an Au strip. Furthermore, we present two narrow discrete distances of a triple-band absorber through stacking one more pair of an Au strip and dielectric layer. Results prove that two discrete distances of only 0.14 THz and 0.17 THz in adjacent absorption modes of the first two and the last two are achieved, respectively; the relative discrete distances of them are respectively 6.57% and 7.22%, which are far from previous reports. Narrow discrete distances (or low values of relative discrete distance) of the multiple-band absorbers have a large number of applications in the investigation of some hidden information in very near frequencies.

A broadband terahertz metamaterial absorber enabled by the simple design of a rectangular-shaped resonator with an elongated slot.

Broadband metamaterial absorbers are of critical importance in practical applications, but their obtainment approaches are quite complex at present. We demonstrate here that a fairly simple structure design formed by a rectangular-shaped resonator having an elongated slot can be utilized to achieve a broadband absorption response at terahertz frequencies. More than 50% absorption in a continuous frequency range of 1.62 THz (with a central frequency of 2.05 THz) can be gained, and its relative absorption bandwidth is 79.02%, which is superior to that of previous broadband absorption devices. The basic principle of the broadband absorption originates from the superposition of four different but narrowly separated resonance peaks that resulted from different response positions of the suggested resonator. Results further reveal that the broadband terahertz absorption performance (or its four resonance peaks) can be controlled by the resonator dimensions. The suggested method can provide a new type of design strategy to realize broadband integrated terahertz absorption devices.

High-Q Fano Resonance in Terahertz Frequency Based on an Asymmetric Metamaterial Resonator.

We propose a planar metamaterial formed by four-strip metallic resonators, which can achieve high-Q Fano resonance in terahertz regime. This terahertz planar metamaterial supports a sharp Fano resonance at 0.81 THz with 25% transmission. The resonance bandwidth of the dip is 0.014 THz with the Q-factor of 58. The interference between the bright mode and dark mode leads to the Fano line shape. This sharp Fano profile is explained by the electromagnetic theory of Fano resonance. Moreover, multiple Fano resonances can be realized by adding more strips into the original structure. As an example, two Fano dips with Q-factors of 61 and 65 can be achieved via a five-strip structure.

Plasmon-Induced Transparency Based on Triple Arc-Ring Resonators.

This paper presents a plasmon-induced transparency (PIT) using an easy-fabricating metamaterial composed of three pieces of metallic arc-rings on top of a dielectric substrate. The transmission of the transparent peak of 1.32 THz reaches approximately 93%. The utilization of the coupled Lorentzian oscillator model and the distribution of electromagnetic fields together explain the cause of the transparent peak. The simulation results further demonstrate that the bandwidth of the transmission peak can be narrowed by changing the sizes of the arc-rings. Moreover, an on/off effect based on the transparent peak is discussed by introducing photosensitive silicon into the air gaps of the suggested metamaterial structure.

Design of Quad-Band Terahertz Metamaterial Absorber Using a Perforated Rectangular Resonator for Sensing Applications.

Quad-band terahertz absorber with single-sized metamaterial design formed by a perforated rectangular resonator on a gold substrate with a dielectric gap in between is investigated. The designed metamaterial structure enables four absorption peaks, of which the first three peaks have large absorption coefficient while the last peak possesses a high Q (quality factor) value of 98.33. The underlying physical mechanisms of these peaks are explored; it is found that their near-field distributions are different. Moreover, the figure of merit (FOM) of the last absorption peak can reach 101.67, which is much higher than that of the first three absorption modes and even absorption bands of other works operated in the terahertz frequency. The designed device with multiple-band absorption and high FOM could provide numerous potential applications in terahertz technology-related fields.