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Resonance and Sensing Properties of Single-Walled Carbon Nanotube Terahertz Metasurface
Abstract
In this study, we extracted key parameters such as dielectric constant, conductivity, and refractive index from THz time-domain spectra of single-walled carbon nanotubes (CNTs) film. We propose a novel THz metasurface sensor characterized by etching a subwavelength array onto a single-walled CNT film. Both experimental and simulation results show that the metasurface device has pronounced resonance transmission enhancements at 0.727 and 0.914 THz. Furthermore, this novel terahertz (THz) metasurface for refractive index sensing has a maximum sensitivity of 423 GHz/RIU. The Fano model is employed to elucidate the resonance peak at 0.914 THz. Then, the physical mechanism of resonant transmission peak has been investigated according to multiple scattering theory, revealing that both resonance peaks are predominantly influenced by toroidal dipole. This research provides a practical solution for achieving heightened sensitivity in THz sensors, emphasizing the potential of the proposed metasurface device in advancing sensing technologies.
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By depositing the trapezoidal dielectric stripe on top of the 3D Dirac semimetal (DSM) hybrid plasmonic waveguide, the thermal tunable propagation properties have been systematically investigated in the terahertz regime, taking into account the influences of the structure of the dielectric stripe, temperature and frequency. The results manifest that as the upper side width of the trapezoidal stripe increases, the propagation length and figure of merit (FOM) both decrease. The propagation properties of hybrid modes are closely associated with temperature, in that when the temperature changes in the scope of 3-600 K, the modulation depth of propagation length is more than 96%. Additionally, at the balance point of plasmonic and dielectric modes, the propagation length and FOM manifest strong peaks and indicate an obvious blue shift with the increase of temperature. Furthermore, the propagation properties can be improved significantly with a Si-SiO2 hybrid dielectric stripe structure, e.g., on the condition that the Si layer width is 5 µm, the maximum value of the propagation length reaches more than 6.46 × 105 µm, which is tens of times larger than those pure SiO2 (4.67 × 104 µm) and Si (1.15 × 104 µm) stripe. The results are very helpful for the design of novel plasmonic devices, such as cutting-edge modulator, lasers and filters.
Metasurfaces are planar or 2D forms of metamaterials made up of arrays of antennas with a subwavelength thickness. They have been rapidly developed in the recent years due to their ability to manipulate light–matter interaction in both linear and non‐linear regimes at the nanoscale. Various metasurfaces display remarkable optical features, such as acute resonance, significant near‐field enhancement, and suitable capacity to support electric and magnetic modes, on account of the strong light–matter interaction and the low optical loss. Due to these important properties, they can be used in several advanced optoelectronic applications, like surface‐enhanced spectroscopy, photocatalysis, and sensing. This review reports on the recent progress of metamaterials and metasurfaces in molecular optical sensors. The principles that govern plasmonic and dielectric metasurfaces along with their features are outlined, supported by numerous examples. Then, the factors that result in a high Q‐factor are presented in order to show that metamaterials and metasurfaces can be used for label‐free sensing in a variety of detection mechanisms, including surface‐enhanced spectroscopy, refractometric sensing, and surface‐enhanced thermal emission spectroscopy via infrared absorption and Raman scattering, as well as chiral sensing. Finally, the challenges for future development are outlined.
Terahertz technology has broad application prospects in biomedical detection. However, the mixed characteristics of actual samples make the terahertz spectrum complex and difficult to distinguish, and there is no practical terahertz detection method for clinical medicine. Here, we propose a three-step one-way terahertz model, presenting a detailed flow analysis of terahertz technology in the biomedical detection of renal fibrosis as an example: 1) biomarker determination: screening disease biomarkers and establishing the terahertz spectrum and concentration gradient; 2) mixture interference removal: clearing the interfering signals in the mixture for the biomarker in the animal model and evaluating and retaining the effective characteristic peaks; and 3) individual difference removal: excluding individual interference differences and confirming the final effective terahertz parameters in the human sample. The root mean square error of our model is three orders of magnitude lower than that of the gold standard, with profound implications for the rapid, accurate and early detection of diseases.
In this paper, a terahertz (THz) biosensor based on all-metal metamaterial is theoretically investigated and experimentally verified. This THz metamaterial biosensor uses stainless steel materials that are manufactured via laser-drilling technology. The simulation results show that the maximum refractive index sensitivity and the figure of merit of this metamaterial sensor are 294.95 GHz/RIU and 4.03, respectively. Then, bovine serum albumin was chosen as the detection substance to assess this biosensor’s effectiveness. The experiment results show that the detection sensitivity is 72.81 GHz/(ng/mm ² ) and the limit of detection is 0.035 mg/mL. This THz metamaterial biosensor is simple, cost-effective, easy to fabricate, and has great potential in various biosensing applications.
Neurostimulant drugs or magnetic/electrical stimulation techniques can overcome attention deficits, but these drugs or techniques are weakly beneficial in boosting the learning capabilities of healthy subjects. Here, we report a stimulation technique, mid-infrared modulation (MIM), that delivers mid-infrared light energy through the opened skull or even non-invasively through a thinned intact skull and can activate brain neurons in vivo without introducing any exogeneous gene. Using c-Fos immunohistochemistry, in vivo single-cell electrophysiology and two-photon Ca ²⁺ imaging in mice, we demonstrate that MIM significantly induces firing activities of neurons in the targeted cortical area. Moreover, mice that receive MIM targeting to the auditory cortex during an auditory associative learning task exhibit a faster learning speed (~50% faster) than control mice. Together, this non-invasive, opsin-free MIM technique is demonstrated with potential for modulating neuronal activity.
With the non-ionizing, non-invasive, high penetration, high resolution and spectral fingerprinting features of terahertz (THz) wave, THz spectroscopy has great potential for the qualitative and quantitative identification of key substances in biomedical field, such as the early diagnosis of cancer, the accurate boundary determination of pathological tissue and non-destructive detection of superficial tissue. However, biological samples usually contain various of substances (such as water, proteins, fat and fiber), resulting in the signal-to-noise ratio (SNR) for the absorption peaks of target substances are very small and then the target substances are hard to be identified. Here, we present recent works for the SNR improvement of THz signal. These works include the usage of attenuated total reflection (ATR) spectroscopy, the fabrication of sample-sensitive metamaterials, the utilization of different agents (including contrast agents, optical clearing agents and aptamers), the application of reconstruction algorithms and the optimization of THz spectroscopy system. These methods have been proven to be effective theoretically, but only few of them have been applied into actual usage. We also analyze the reasons and summarize the advantages and disadvantages of each method. At last, we present the prospective application of THz spectroscopy in biomedical field.
Intensive research in photonics has discovered many original and fascinating optical phenomena associated with the physics of Fano resonances. Here we extend the research of Fano resonance to on-chip multiple modes and high-order mode, which can be called multimode Fano resonance. By exploiting both wavelength and mode degrees of freedom, we demonstrate a compact structure to engineer wavelength- and shape-controlled multimode Fano resonances on an integrated photonic platform. Both simulated and experimental results indicate that the unambiguous asymmetric spectra line shapes for both transverse electric modes ( and ) can be further applied to low-power mode switching. The demonstrated on-chip multimode Fano resonances scheme may provide a new perspective for exploring the advantages and applications of multimode Fano resonances in integrated photonics.
The myelin sheath enables dramatic speed enhancement for signal propagation in nerves. In this work, myelinated nerve structure is experimentally and theoretically studied using synchrotron‐radiation‐based Fourier‐transform infrared microspectroscopy. It is found that, with a certain mid‐infrared to terahertz spectral range, the myelin sheath possesses a ≈2‐fold higher refraction index compared to the outer medium or the inner axon, suggesting that myelin can serve as an infrared dielectric waveguide. By calculating the correlation between the material characteristics of myelin and the radical energy distribution in myelinated nerves, it is demonstrated that the sheath, with a normal thickness (≈2 µm) and dielectric constant in nature, can confine the infrared field energy within the sheath and enable the propagation of an infrared signal at the millimeter scale without dramatic energy loss. The energy of signal propagation is supplied and amplified when crossing the nodes of Ranvier via periodic relay. These findings provide the first model for explaining the mechanism of infrared and terahertz neurotransmission through myelinated nerves, which may promote the development of biological‐tissue label‐free detection, biomaterial‐based sensors, neural information, and noninvasive brain–machine interfaces. The myelin sheath, which serves as a dielectric waveguide for signal propagation, is experimentally confirmed using Fourier‐transform infrared microspectroscopy. The high contrast of reflectivity/refractivity between the myelin sheath and inner axon and outer medium at certain mid‐infrared to terahertz spectral range realize energy concentrates in myelin, and signal propagation is amplified when crossing the nodes of Ranvier via periodic relay.
Conventional imaging systems comprise large and expensive optical components that successively mitigate aberrations. Metasurface optics offers a route to miniaturize imaging systems by replacing bulky components with flat and compact implementations. The diffractive nature of these devices, however, induces severe chromatic aberrations, and current multiwavelength and narrowband achromatic metasurfaces cannot support full visible spectrum imaging (400 to 700 nm). We combine principles of both computational imaging and metasurface optics to build a system with a single metalens of numerical aperture ~0.45, which generates in-focus images under white light illumination. Our metalens exhibits a spectrally invariant point spread function that enables computational reconstruction of captured images with a single digital filter. This work connects computational imaging and metasurface optics and demonstrates the capabilities of combining these disciplines by simultaneously reducing aberrations and downsizing imaging systems using simpler optics.
We have investigated numerically toroidal dipolar excitation at optical frequency in metamaterials whose unit cell consists of three identical Ag nanodisks and a SiO2 spacer on Ag substrate. The near-field plasmon hybridization between individual Ag nanodisks and substrate forms three magnetic dipolar resonances, at normal incidence of plane electromagnetic waves. The strong coupling among three magnetic dipolar resonances leads to the toroidal dipolar excitation, when space-inversion symmetry is broke along the polarization direction of incident light. The influences of some geometrical parameters on the resonance frequency and the excitation strength of toroidal dipolar mode are studied in detail. The radiated power from toroidal dipole is also compared with that from conventional electric and magnetic multipoles.
The demand for advanced bacterial detection tools is continuously increasing, promoted by its significant benefits in various applications. For instance, in the medical field, these tools would facilitate decision making about more tailored therapies once the infection source has been identified. In the past few years, terahertz (THz = 10(12) Hz) spectroscopy has also shown potential as a novel bacterial detection modality due to its unique advantages. Impressive breakthroughs have been achieved in this field related to bacterial component characterization, spore identification, and cell detection. However, some intrinsic limitations and technical bottlenecks have led to some debates about the practicability of its clinical adoption. In this review, we summarize the progress achieved in this field and discuss some challenges and strategies for future implementation of practical applications.
We demonstrated sensitive detection of individual yeast cells and yeast films by using slot antenna arrays operating in the terahertz frequency range. Microorganisms located at the slot area cause a shift in the resonant frequency of the THz transmission. The shift was investigated as a function of the surface number density for a set of devices fabricated on different substrates. In particular, sensors fabricated on a substrate with relatively low permittivity demonstrate higher sensitivity. The frequency shift decreases with increasing slot antenna width for a fixed coverage of yeast film, indicating a field enhancement effect. Furthermore, the vertical range of the effective sensing volume has been studied by varying the thickness of the yeast film. The resonant frequency shift saturates at 3.5 μm for a slot width of 2 μm. In addition, the results of finite-difference time-domain simulations are in good agreement with our experimental data.
Fano resonances with a characteristic asymmetric line shape can be observed in light scattering, transmission and reflection spectra of resonant optical systems. They result from interference between direct and indirect, resonance-assisted pathways. In the nanophotonics field, Fano effects have been observed in a wide variety of systems, including metallic nanoparticle assemblies, metamaterials and photonic crystals. Their unique properties find extensive use in applications, including optical filtering, polarization selectors, sensing, lasers, modulators and nonlinear optics. We report on the observation of a Fano resonance in a single semiconductor nanostructure, opening up opportunities for their use in active photonic devices. We also show that Fano-resonant semiconductor nanostructures afford the intriguing opportunity to simultaneously measure the far-field scattering response and the near-field energy storage by extracting photogenerated charge. Together they can provide a complete experimental characterization of this type of resonance.
Fano resonances and their strong doping dependence are observed in Raman
scattering of single-layer graphene (SLG). As the Fermi level is varied by a
back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near
the charge neutrality point as a manifestation of a Fano resonance, whereas the
line shape is symmetric when the graphene sample is electron or hole doped.
However, the G band of bilayer graphene (BLG) does not exhibit any Fano
resonance regardless of doping. The observed Fano resonance can be interpreted
as interferences between the phonon and excitonic many-body spectra in SLG. The
absence of a Fano resonance in the Raman G band of BLG can be explained in the
same framework since excitonic interactions are not expected in BLG.
Recently reported metamaterial analogues of electromagnetically induced transparency enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors and nonlinear devices. In particular, optical control of electromagnetically induced transparency in metamaterials promises essential application opportunities in optical networks and terahertz communications. Here we present active optical control of metamaterial-induced transparency through active tuning of the dark mode. By integrating photoconductive silicon into the metamaterial unit cell, a giant switching of the transparency window occurs under excitation of ultrafast optical pulses, allowing for an optically tunable group delay of the terahertz light. This work opens up the possibility for designing novel chip-scale ultrafast devices that would find utility in optical buffering and terahertz active filtering.
Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New
degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array
of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities
on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed
in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface,
which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great
flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer
metallic interfaces.
In this paper, periodic peaks in a terahertz absorption spectrum are confirmed to be induced from interference effects. Theoretically, we explained the periodic peaks and calculated the locations of them. Accordingly, a technique was suggested, with which the interference peaks in a terahertz spectrum can be eliminated and therefore a real terahertz absorption spectrum can be obtained. Experimentally, a sample, Methamphetamine, was investigated and its terahertz fingerprint was successfully extracted from its interference destruction spectrum. This technique is useful in getting samples’ terahertz fingerprint spectra, and furthermore provides a fast nondestructive testing method using a large size terahertz beam to identify materials.
Chemical sensors based on individual single-walled carbon nanotubes (SWNTs) are demonstrated. Upon exposure to gaseous molecules such as NO(2) or NH(3), the electrical resistance of a semiconducting SWNT is found to dramatically increase or decrease. This serves as the basis for nanotube molecular sensors. The nanotube sensors exhibit a fast response and a substantially higher sensitivity than that of existing solid-state sensors at room temperature. Sensor reversibility is achieved by slow recovery under ambient conditions or by heating to high temperatures. The interactions between molecular species and SWNTs and the mechanisms of molecular sensing with nanotube molecular wires are investigated.
Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy
storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and
nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products.
Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost,
polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications
of single-walled nanotubes.
A novel tunable terahertz hybrid plasmonic waveguide based on a 3D Dirac semimetal (DSM) elliptical fiber separated from a silicon layer by a dielectric gap is explored. Interestingly, the results manifest that as the elliptical ratio increases, the real part of the effective mode index and the propagation length both monotonically increase. Importantly, the figure of merit strikingly reaches more than 1500. The propagation length shows a peak near the interface between the near filed and dissipation regions. The near field region is significantly affected by the Fermi level, in that the critical transition thickness is from 10.8 μm to 0.92 μm when the Fermi level varies in the range from 0.03 eV to 0.10 eV. The propagation properties of hybrid modes can be modulated in a wide scope via the Fermi level of the 3D DSM fiber, the propagation length can be modulated from 6.27×10
3
μm to 1.16×10
5
μm on the condition that the Fermi level changes in the range of 0.04 eV-0.15 eV, and the corresponding modulation depth is about 95%. The results are very useful to aid the design of novel plasmonic devices, e.g. low threshold lasers, modulators and polarizer.
Sensing technologies based on terahertz (THz) waves have significant application prospects in fast imaging, free label, and non-invasive inspection methods. However, the main drawback limiting the performance of THz-based bio-chemical sensors is the weak interaction between the optical field and the analyte on account of whose characteristic scale mismatched with the THz wavelength, leading to low sensitivity. Herein, we present an ultra-sensitive THz metasensor based on electric Fano resonant metasurface which consists of three gold microrods arranged periodically. The designed electric Fano resonance provides a strong near-field enhancement near the surface of the microstructure, significantly boosting the light-analyte interactions and thus the sensitivity. Such an electric Fano resonance is formed by the interference between a leaky electric dipole resonance and a bounded toroidal dipole mode which is a symmetry-protected bound state in the continuum supported by the sub-diffractive periodic system here. Owing to the strong electric fields generated near the interface of our microstructure around the toroidal dipole BIC, the proposed structure can distinguish extremely dilute concentrations (nM) of solutions. Importantly, by controlling the degree of geometry asymmetry, the BIC-inspired mechanism provides an important and simple tool to engineer and tailor the linewidth and Q-factor of our proposed electric Fano resonance, indicating the ability to realize different biosensors for different optical regimes. Our results open new possibilities to realize a nondestructive and non-contactare quantitative inspection of low-concentration solutions, providing a useful sensing approach for disease prevention and diagnosis.
The development of terahertz (THz) technology is creating a demand for devices that can modulate THz beams. Here, we report the design and characterization of three THz modulators. One uses graphene and a metal-microstructure-integrated metamaterial, another uses a bare graphene film, and the third uses graphene-based all-dielectric metamaterials. Ultrasensitive dynamic THz modulation is achieved by shifting the quasi-Fermi level between the Dirac point, the conduction band, and the valence of graphene via continuous-wave optical illumination or bias voltages. When the Fermi level is close to the Dirac point, the modulation is ultrasensitive to the external stimuli. The modulation depth can reach the maximum value of 346% in the current public publication, breaking through the bottleneck of modulation inefficiency, and is expected to realize practical applications for the first time. For the range 0.2–2 THz, the modulation depth initially increases, then decreases. These results will enable potential designs for ultrasensitive THz devices.
A deficiency of Ca2+ fluxes arising from dysfunctional voltage-gated calcium channels has been associated with a list of calcium channelopathies such as epilepsy, hypokalemic periodic paralysis, episodic ataxia, etc. Apart from analyzing the pathogenic channel mutations, understanding how the channel regulates the ion conduction would be instructive to the treatment as well. In the present work, in relating the free energetics of Ca2+ transport to the calcium channel, we demonstrate the importance of bridging Ca2+ hydration waters, which form hydrogen bonds with channel -COO- and -C═O groups and enable a long-distance effect on the Ca2+ permeation. By firing a terahertz wave which resonates with the stretching mode of either the -COO- or the -C═O group, we obtain significantly enhanced selectivity and conductance of Ca2+. The Ca2+ free energy negatively grows nearly 5-fold. The direct evidence is the reinforced hydrogen bonds. In addition, thanks to forced vibrations, -COO- contributes to raised permeation as well even under a field in resonance with -C═O, and vice versa. Since the resonant terahertz field could manipulate the conduction of calcium channels, it has potential applications in therapeutic intervention such as rectifying a Ca2+ deficiency in degraded calcium channels, inducing apoptosis of tumor cells with overloaded calcium etc.
With the rapid advances in functional optoelectronics, the research on carbon-based materials and devices has become increasingly important at the terahertz frequency range owing to their advantages in terms of weight, cost, and freely bendable flexibility. Here, we report an effective material and device design for a terahertz plasmonic metasurface sensor (PMS) based on carbon nanotubes (CNTs). CNT metasurfaces based on silicon wafers have been prepared and obvious resonant transmission peaks are observed experimentally. The enhanced resonant peaks of transmission spectra are attributed to the surface plasmon polariton resonance, and the transmission peaks are further well explained by the Fano model. Furthermore, the different concentration gradients of pesticides (2,4-dichlorophenoxyacetic and chlorpyrifos solutions) have been detected by the designed PMSs, showing the lowest detection mass of 10 ng and the sensitivities of 1.38 × 10-2/ppm and 2.0 × 10-3/ppm, respectively. Good linear relationships between transmission amplitude and pesticide concentration and acceptable reliability and stability have been obtained. These materials and device strategies provide opportunities for novel terahertz functional devices such as sensors, detectors, and wearable terahertz imagers.
Unwinding the double-helix of DNA molecule is the basis of gene duplication, gene editing and the acceleration of this unwinding process is crucial to the rapid detection of genetic information. Based on the unwinding of 6-basepairs DNA duplexes, we demonstrate that terahertz stimulus at a characteristic frequency (44.0-THz) can be served as an efficient, nonthermal, and long-range method to accelerate the unwinding process of DNA duplexes. The average speed of the unwinding process increased by 20 times at least, and its temperature was significantly reduced. The mechanism was revealed to be the resonance between the terahertz stimulus and the vibration of purine connected by the weak hydrogen bond, and the consequent break in hydrogen bond connections between these basepairs. Our findings potentially provide a promising application of terahertz technology for the rapid detection of nucleic acid, biomedicine and therapy.
The introduction of cellular structure in conductive polymer composites is supposed to be an effective way to ameliorate the electromagnetic interference (EMI) shielding properties. Moreover, the wetting behavior should be taken into consideration when exposed to a moist environment. Herein, oleophilic conductive poly(vinylidene fluoride) (PVDF)/multiwall nanotube (MWCNT) composite foams were successfully fabricated through a simple and effective batch foaming process. PVDF/MWCNT nanocomposite foams with various microcellular structures were obtained by controlling the foaming parameters (MWCNT content and impregnation temperature). The electrical conductivity (EC) and EMI shielding properties critically depended on their microcellular structure. Interestingly, the EC and EMI shielding properties declined with a higher degree of foaming in the range of 37 %–67 % for PVDF/1 wt% MWCNT (FC1) and PVDF/2 wt% MWCNT (FC2) nanocomposite foams. Conversely, for the PVDF/5 wt% MWCNT (FC5) and PVDF/8 wt% MWCNT (FC8) nanocomposite foams, the EC and EMI shielding properties gradually increased with an increased degree of foaming in the range of 27 wt%–40 wt%. The optimal EMI shielding properties of PVDF based nanocomposite foams containing MWCNT contents of 1 wt%, 2 wt%, 5 wt% and 8 wt%, reached 18.6 dB, 31.5 dB, 88.3 dB and 132.6 dB, respectively, at a sample thickness of 4.0 mm. Analysis of the EMI shielding mechanism found that absorption resulting from multiple reflections and scattering, and strong conduction band polarization loss were the main contributing factors. Moreover, the PVDF/MWCNT foams were hydrophobic, indicating that they could be competitive candidates for EMI shielding and oil/water separation multi-functional applications.
Graphene-enabled metamaterials have emerged as promising photoelectric devices for dynamically controlling the polarization of electromagnetic waves. However, it is still a key technological challenge to design a tunable metamaterial possessing more sufficient freedom for polarization manipulation. Here, we propose a graphene-enabled tunable multifunctional metamaterial consisting of metallic strips arranged on a grounded polymer substrate embedded with a graphene sandwich structure to electrically control the polarization states of terahertz waves. By adjusting the Fermi energy of graphene through voltage biasing, the electromagnetic responses of the presented metamaterial can be tailored, resulting in dynamical tunable polarization manipulation in reflection mode. All expected performances are demonstrated through full wave simulation. The results reveal that this metamaterial device can operate as a switchable quarter-wave plate and a tunable half-wave plate for both linear- and circular-polarization incidences. Furthermore, the polarization conversion ratio, of which incident waves are converted to their cross-polarized reflection mode, can be continuously regulated from 2% to 95% in a wide band from 1.46 to 2.26 THz. The proposed metamaterial continues the process of developing tunable polarizers and polarization switchers with graphene materials, and may be applied in various areas such as wireless communication, terahertz sensing and imaging.
A planar terahertz electromagnetically induced transparency-like (EIT-like) metamaterial, comprising an outer circular ring resonator (CRS) made by graphene, an inner vertical two-gap circular split ring resonator (VSRR) and a smaller horizontal two-gap circular split ring resonator (HSRR), is proposed. A distinct transparency window can be observed in two perpendicular polarization directions, respectively. By tuning the Fermi energy, the maximum modulation depths in two perpendicular polarization directions respectively can reach up to 83.54% and 94.39%. Meanwhile, the EIT-like effect, amplitude of transparency window, group delay and delay bandwidth product (DBP) can be adjusted dynamically without shift of frequency. The surface currents and three-level Λ-type system can explain the physical mechanism of EIT-like effect. The analytical results based on a two-particle model are in good agreement with the numerical results. Our work provides a new method for dynamically adjustable terahertz slow light devices, which makes the graphene more controllable in metal-graphene hybrid structure.
In this paper, we use the theory of quantum optics and electrodynamics to study the electromagnetic field problem in the nervous system based on the assumption of an ordered arrangement of water molecules on the neuronal surface. Using the Lagrangian of the water molecule-field ion, the dynamic equations for neural signal generation and transmission are derived. Perturbation theory and the numerical method are used to solve the dynamic equations, and the characteristics of high-frequency signals (the dispersion relation, the time domain of the field, the frequency domain waveform, etc.) are discussed. This model predicts some intrinsic vibration modes of electromagnetic radiation on the neuronal surface. The frequency range of these vibration modes is in the THz and far-infrared ranges.
A bound state in the continuum (BIC) is a localized state of an open structure with access to radiation channels, yet it remains highly confined with, in theory, an infinite lifetime and quality factor. There have been many realizations of such exceptional states in dielectric systems without loss. However, realizing BICs in lossy systems such as those in plasmonics remains a challenge. In this Letter, we explore the possibility of realizing BICs in a hybrid plasmonic-photonic structure consisting of a plasmonic grating coupled to a dielectric optical waveguide with diverging radiative quality factors. The plasmonic-photonic system supports two distinct groups of BICs: symmetry-protected BICs at the Γ point and off-Γ Friedrich-Wintgen BICs. The photonic waveguide modes are strongly coupled to the gap plasmons in the grating, leading to an avoided crossing behavior with a high value of Rabi splitting of 150 meV. Moreover, we show that the strong coupling significantly alters the band diagram of the hybrid system, revealing opportunities for supporting stopped light at an off-Γ wide angular span.
Imaging technologies based on terahertz (THz) waves have great potential for use in powerful non-invasive inspection methods. However, most real objects have various three-dimensional curvatures and existing THz technologies often encounter difficulties in imaging such configurations, which limits the useful range of THz imaging applications. Here, we report the development of a flexible and wearable THz scanner based on carbon nanotubes. We achieved room-temperature THz detection over a broad frequency band ranging from 0.14 to 39 THz and developed a portable THz scanner. Using this scanner, we performed THz imaging of samples concealed behind opaque objects, breakages and metal impurities of a bent film and multi-view scans of a syringe. We demonstrated a passive biometric THz scan of a human hand. Our results are expected to have considerable implications for non-destructive and non-contact inspections, such as medical examinations for the continuous monitoring of health conditions.
In this paper, we present a simple trimeric metasurface consisting of three dipolar resonators in each unit cell, to achieve the independent controlling over both the broad bright mode and the sharp dark mode of Fano resonances. Through both the finite difference time domain simulation and microwave experiment, we find that spectral positions of the bright and dark modes are linearly dependent on, respectively, the global spacing between adjacent unit cells and the local spacing between adjacent dipoles within each unit cell. The dependence of the spectral position of bright (dark) mode on the global (local) spacing is independent without mutual influence, which provides a facile pathway to control the Fano resonance with large flexibility. Our proposed scheme to control Fano resonance is highly desired in various fields including lasing spaser and biosensing with improved performance.
Anisotropic transmission of ultrathin freestanding multi-walled carbon nanotubes (MWCNTs) film has been investigated using terahertz (THz) time-domain spectroscopy. The super-aligned MWCNTs films were fabricated by a novel method—drawing and winding carbon nanotubes on square-shaped polytetrafluoroethylene (PTFE) frame. The MWCNTs film exhibited anisotropic transmittance of THz wave depending on the alignment direction of carbon nanotubes relative to the THz wave polarization orientation, which proved that such MWCNTs film acts as an ideal polarizer in the THz regime. The degree of polarization (DOP) of 99.4% and the extinction ratio (ER) of 25.3 dB (average value) has been demonstrated for a MWCNTs film of 9- μm thickness in a wide frequency range from 0.1 to 2.5 THz. Furthermore, the value of DOP and ER can be engineered expediently by controlling the film thickness. Our approach opens the possibility of MWCNTs film in application of THz optoelectronic devices.
Fano-type transmission, of which the specifics lie in continuum and resonant transmission, has been exploited in various areas since Fano published his study [1]. In this paper, we show macroscopic transmission fitted well with Fano analysis, using a sample composed of Al 2O 3 and multi-wall carbon nanotube. In addition to fitting, the properties of Fano-type transmission were shown to be its resonance and the interference of continuum and resonant transmission, of which the quantity was also estimated, in a similar way to Genet's result [9], and it was found that the resonance was made by carbon nanotubes (CNTs); from this, we can also conclude that CNTs have good conductivity for cavity.
The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nano-scale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this article, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultra-fast and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1-GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19-rad phase shift was realized. A wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2-Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultra-fast dynamic devices for THz wireless communication and imaging systems.
Surfaces covered by ultrathin plasmonic structures-so-called metasurfaces-have recently been shown to be capable of completely controlling the phase of light, representing a new paradigm for the design of innovative optical elements such as ultrathin flat lenses, directional couplers for surface plasmon polaritons and wave plate vortex beam generation. Among the various types of metasurfaces, geometric metasurfaces, which consist of an array of plasmonic nanorods with spatially varying orientations, have shown superior phase control due to the geometric nature of their phase profile. Metasurfaces have recently been used to make computer-generated holograms, but the hologram efficiency remained too low at visible wavelengths for practical purposes. Here, we report the design and realization of a geometric metasurface hologram reaching diffraction efficiencies of 80% at 825 nm and a broad bandwidth between 630 nm and 1,050 nm. The 16-level-phase computer-generated hologram demonstrated here combines the advantages of a geometric metasurface for the superior control of the phase profile and of reflectarrays for achieving high polarization conversion efficiency. Specifically, the design of the hologram integrates a ground metal plane with a geometric metasurface that enhances the conversion efficiency between the two circular polarization states, leading to high diffraction efficiency without complicating the fabrication process. Because of these advantages, our strategy could be viable for various practical holographic applications.
A carbon nanotube free-standing linearly dichroic polariser is developed using solid-state extrusion. Membrane cohesion is experimentally and numerically demonstrated to derive from inter-tube van der Waals interactions in this family of planar metastable morphologies, controlled by the chemical vapour deposition conditions. Ultra-broadband polarisation (400 nm – 2.5 mm) is shown and corroborated by effective medium and full numerical simulations.
The terahertz spectra of four kinds of vitamins are presented. The refractive index and absorption spectra of these vitamins are obtained by the terahertz time-domain spectroscopy. The full-geometry optimizations and frequency calculations using the density functional theory are applied to obtain the structure and vibration frequencies of these vitamin molecules. The calculated vibration frequencies are compared with the experimental data. The results show that there are terahertz fingerprint absorptions for all of four kinds of vitamins. The terahertz absorbance spectra of vitamins result from not only the intramolecular vibration modes, but also the intermolecular interaction or phonon modes.
A highly sensitive and selective dopamine sensor was fabricated with the unique 3D carbon nanotube nanoweb (CNT-N) electrode. The as-synthesised CNT-N was modified by oxygen plasma to graft functional groups in order to increase selective electroactive sites at the CNT sidewalls. This electrode was characterized physically and electrochemically using HRSEM, Raman, FT-IR, and cyclic voltammetry (CV). Our investigations indicated that the O(2)-plasma treated CNT-N electrode could serve as a highly sensitive biosensor for the selective sensing of dopamine (DA, 1 μM to 20 μM) in the presence of ascorbic acid (AA, 1000 μM).
We show that the capacitance of single-walled carbon nanotubes (SWNTs) is highly sensitive to a broad class of chemical vapors and that this transduction mechanism can form the basis for a fast, low-power sorption-based chemical sensor. In the presence of a dilute chemical vapor, molecular adsorbates are polarized by the fringing electric fields radiating from the surface of a SWNT electrode, which causes an increase in its capacitance. We use this effect to construct a high-performance chemical sensor by thinly coating the SWNTs with chemoselective materials that provide a large, class-specific gain to the capacitance response. Such SWNT chemicapacitors are fast, highly sensitive, and completely reversible.
Electromagnetic metasurfaces and information metasurfaces
- L Zhang
L. Zhang et al., "Electromagnetic metasurfaces and information
metasurfaces," Chin. J. Radio Sci., vol. 36, no. 6, pp. 817-828,
Dec. 2021, doi: 10.12265/j.cjors.2021218.
Research progress of terahertz metamaterial biosensors
- J Yang
J. Yang et al., "Research progress of terahertz metamaterial biosensors,"
Spectrosc. Spectr. Anal., vol. 41, no. 6, pp. 1669-1677, Jun. 2021.
Collective antenna effects in the terahertz and infrared response of highly aligned carbon nanotube arrays
- L Ren
L. Ren et al., "Collective antenna effects in the terahertz and infrared
response of highly aligned carbon nanotube arrays," Phys. Rev. B,
Condens. Matter, vol. 87, no. 16, Apr. 2013, doi: 10.1103/physrevb.87.161401.