Number Of Absorption Peaks Research Articles

Infrared optical spectra of a silicene quantum dot modulated by spin-orbit coupling and uniaxial strain

Abstract We theoretically study the electronic structure and light absorption spectra of a triangular silicene quantum dot (TZSQD) modulated by spin-orbit coupling (SOC) and the uniaxial strain from a symmetry point of view. After considering the SOC, the symmetry of the system decreases and the degenerate energy level split, creating the absorption peaks in the far infrared region and some multimodal structures in the near-infrared region and visible region. In addition, the intensity of the SOC also affects the degree of splitting of the energy levels, which makes the absorption peaks in the far infrared region are blueshifted to the mid-infrared region, and absorption edge in the near infrared region is redshifted to the mid-infrared region. We also find that when a uniaxial strain and SOC coexist at the same time, the symmetry of the system is further reduced, and the number of absorption peaks is increased. Moreover, the uniaxial strain intensity can efficiently regulate the frequency position of the absorption edge in the near-infrared region, and make its spectral range greatly expanded.

Design of ultra-thin multi-band sub-terahertz metamaterial absorber using three concentric ring resonators with same opening direction

In this paper, an ultra-thin sub-terahertz multi-band terahertz metamaterial absorber is proposed, its cell consists of three concentric ring resonators with the same opening direction, which can realize three discrete absorption peaks in the sub-terahertz frequency domain. The absorption peaks are mainly formed when the three open circular resonators interact with each other, and the superposition of the three discrete peaks creates a three-band absorption. The physical mechanisms of three-band absorption are illustrated by the electric fields and surface current distributions of three absorption peaks. Adjustment of the absorption effect could be obtained by altering the dimensions of the cleft circular resonator. By fixing the dimensions and changing the opening direction, these resonators will couple to each other and play an important role in regulating the frequency, intensity, and number of absorption peaks. Moreover, the multi-band metamaterial structure could be combined with vanadium dioxide, a phase change material, to actively modulate the absorption. In addition, the three-band metamaterial absorber has the characteristics of polarization-sensitive and wide-angle, which has extensive applications in the domains of security detection, biomedicine and communication.

A new hybrid metamaterial absorber composed of ceramic resonators stacked on thin conductive rubber layer

In this letter, a hybrid metamaterial absorber made of ceramic resonators stacked on the traditional thin conductive rubber layer (RAT) has been developed. While RAT has been proven to be an effective absorber with good absorption around 10.5 GHz, its ability to absorb electromagnetic (EM) waves is inconsistent due to its flexible characteristic. Meanwhile, the absorption bandwidth above 90 % for RAT is constrained by its dispersion, limiting the coverage across the entire X-band (8–12 GHz). To address these limitations, we have incorporated the ceramic artificial nanostructure to enhance absorption intensity and broaden the absorption bandwidth by leveraging the strong magnetic Mie resonance of the ceramic resonator. Simultaneously, the number of absorption peaks and the bandwidth of each absorption peak can be controlled by adjusting the offset distance of the ceramic resonators and by varying the sintering temperature of the ceramics. Through the integration of metamaterial design and ceramic materials optimization, we have experimentally demonstrated the feasibility of multi-band, high-absorption devices by utilizing the hybrid structure of ceramic resonators and RAT, which opens up new possibilities for the further development of absorber devices.

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.

Ultrasensitive and tunable multi-narrowband metamaterial absorber

In recent years, metamaterials have been widely used as a new type of optoelectronic device materials, such as optical stealth, holographic imaging, and metamaterial absorbers. In particular, the research of metamaterial absorbers with multi-narrowband, high absorption and tunability has attracted much attention. However, it is a great challenge to integrate multi-narrowband, high absorption, polarization independence, high sensitivity and tunability into a simple absorption structure. In this paper, we use FDTD to design a multi-narrow band, polarimetric independent, high sensitivity and tunability of metamaterial perfect absorber, which is a metal-dielectric-metal three-layer structure. From the bottom to the top of the structure, they are Au layer, SiO2 layer and Au pattern layer. The simulation results show that three resonance absorption peaks are obtained under the coaction of local surface plasmon resonance, magnetic resonance and cavity resonance. The absorption peaks are located at 878 nm, 1020 nm and 1276 nm, and the absorption rates are 98.7%, 99.3% and 99.5%, respectively. In addition, it is found that the number of absorption peaks can be changed by adjusting the top pattern layer structure. Further, the position of the absorption peak and the absorption rate can be altered by adjusting the geometric parameters of the structure. Besides its simple structure, the analysis also show that the structure has high sensitivity under different refractive index environment. Finally, it can also be used for optical conversion devices, such as optical absorption switches, splitters, optical modulators.

Tunable bidirectional perfect THz absorber realized by graphene-based one-dimensional photonic crystals

In this paper, a perfect absorption structure of graphene-based one-dimensional photonic crystals (1DPC) with tunable absorption channels and absorptivity is proposed. The proposed structure can achieve four perfect absorption peaks with the absorptivity of 99.31%, 99.88%, 99.74% and 99.32% at the same time, and the absorptivity of all absorption peaks is more than 95%. By adjusting the period number of 1DPC, the number of absorption peaks and absorption efficiency can be changed. By changing the chemical potential and relaxation rate of graphene, the absorption of the proposed structure can be dynamically tuned. And the influence of structural layer thickness on absorption property is also explored. In addition, we use this structure to design two different bidirectional absorbers. The designed bidirectional absorber can tailor the perfect absorption frequency with the absorptivity of more than 99.51%, and can change the absorption channel from single channel to double channel and double channel to multi-channel under the forward and backward incidence. This work not only fills the gap in the design of bidirectional perfect absorbers for 1DPC, but also provides a scheme for the design of multifunctional devices.

Quad-band perfect absorption of the terahertz light using the simple metamaterial resonator

Multiple-band light absorbers have been proved to have important applications in many fields. The structures of them, however, usually require multiple different dimensions of sub-units, which will inevitably lead to time-consuming optimization and complex manufacturing processes. Herein, we show that simple metamaterial design (analogous to a modified T-shaped resonator) can be utilized to achieve quad-band absorption at terahertz frequency. The structure possesses the ability to concentrate the incident wave around the resonator using the localized resonance responses. Four clearly distinguishable absorption peaks with narrow bandwidth and near perfect absorption are realized. Moreover, the number of absorption peaks can be controlled from the original quad-band to triple-band, dual-band and even single-band by varying some specific parameters. Multiple-band device with advantages of simple structure design and controllable absorption peak number is promising in practical applications.

Guided Bloch surface wave resonance for multispectral enhancement of absorption in mono-layer grapheme

A multispectral absorber based on Bloch surface wave (BSW) resonance with distributed Bragg reflectors (DBRs) coated with monolayer graphene is proposed. The guided Bloch surface wave resonance (GBSWR) is realized by coupling the diffraction of a subwavelength grating with the BSW at the interface between a 1D-PhC and graphene layer. Strong absorption phenomenon occurs induced by the coupling of the incident light and GBSWR. The highly efficient absorption and spectral selectivity with designing structural parameters in the visible range are verified through the rigorous coupled-wave analysis (RCWA) method. It is found that the number of absorption peaks is strongly related with the stacked period number of two dielectric materials in DBR. Compared with previous studies, not only the complete absorption of single-atomic-layer graphene can be achieved, but also the above 90 % absorption for all of resonance positions is maintained. These characteristics open up new methods of enhancing the light-graphene interaction, which may find potential application in high-performance modulators and sensors.

Microwave synthesis of worm-like SiC nanowires for thin electromagnetic wave absorbing materials

Abstract Thin thickness is always the pursuit of excellent electromagnetic wave absorbing materials. Herein, SiC nanowires with worm-like morphology were synthesized by microwave heating the mixture of expanded graphite and silica. The worm-like SiC nanowires exhibit an excellent microwave absorption ability at a thin thickness. With the filling ratio of SiC nanowires increases in the matrix, the dielectric loss and microwave absorbing ability are significantly enhanced; meanwhile the number of absorption peaks is gradually increased, and the absorption peaks also move toward a thinner thickness. When the nanowires filling ratio was 40 wt%, the minimum reflection loss reached down to −35.2 dB and the effective absorption (RL

Infrared absorption spectra of plagioclase feldspar: Dependencies of composition and temperature

We investigated the absorption spectra of crystalline plagioclase feldspar, using natural and synthetic samples with a wide compositional range, in the infrared region. And we also carried out cryogenic measurements for synthetic anorthite. In the absorption spectra, systematic spectroscopic variations that depend on the chemical composition were observed, in terms of characteristic features such as the number of absorption peaks, shift of peak location and variety of peak width and intensity. Since, plagioclase feldspar is considered as a reasonable mineral species formed in oxygen rich circumstellar environments from the viewpoint of the equilibrium condensation sequence, our spectral data obtained in this work is expected to be used widely in the field of astronomical mineralogy.

Modelling vibro-acoustic coupling in flexible micro-perforated plates by a patch-impedance approach

This study proposes a Finite Element (FE)-based efficient numerical model of the vibro-acoustic coupling in flexible micro-perforated plates (f-MPPs) where each perforation is described as an imposed impedance boundary condition (uniform impedance patch) on the plate. This approach opens the possibility of predicting the influence of perforation distribution on the acoustic performance of f-MPP. Micro perforated plates have been a topic of interest as a promising sound absorber in a wide range of applications, from room acoustics to combustion systems. One great advantage of these plates is that it gives the designer the freedom of choice on material in production. Depending on the material and the dimensions, the acoustical modes of the medium can couple with the structural modes of the plate. This coupling changes the number of absorption peaks, frequency and amplitude of the Helmholtz resonance of the system, therefore the coupling becomes an extra parameter in the design process. Current analytical models superpose the mechanical impedance of the plate with the acoustic impedance of the perforations to compute this coupling. This approach works fairly well for plates with uniform perforation distribution. This study proposes a numerical method which assumes perforations as discrete impedance patches on the flexible plate so that they can be considered separately. The method couples the solution of the Helmholtz equation in air with shell plate theory to model the vibro-acoustic effects and the impedance patches are represented as imposed transfer impedance boundary conditions. The assessment of the method is performed in terms of comparing the calculated absorption coefficient values from the simulations of several test cases, fundamental theories and measurement results from the literature. The simulation results agree both with these theoretical limits and measurement results. The use of the method is illustrated by considering an example of the influence of modification of the spatial distribution of perforations on the sound absorption of a membrane.

Performance Evaluation of Palm Kernel Shell Adsorbents for the Removal of Phosphorus from Wastewater

Studies were carried out on Palm Kernel Shell, an agricultural waste available in large quantity in Nigeria, to evaluate its ability to remove phosphorus from wastewater. The adsorbents, which were prepared from Palm Kernel Shells (PKN), were characterized using Fourier Transform Infrared (FT-IR), Energy dispersive X-ray (EDX) and Scanning Electron Microscopy (SEM). Batch mode experiments were conducted to study the effects of adsorbent dosage and contact time on phosphorus adsorption. Equilibrium and Kinetic studies of the process were also carried out. Results obtained show that, FT-IR spectrum of the activated carbon displays a number of absorption peaks, reflecting the complex bio-mass structure and a variety of functional groups which explains its improved adsorption behaviour on the colloidal particles. SEM shows the spherical shape of the carbon particles with a wide range of sizes, EDX indicated the constituent elements in the adsorbent in which C and O were found to be the most abundant. Equilibrium data fitted well to the Freundlick and Langmuir models but the data were best described by Langmuir Isotherm model at the temperature of 313 K. Pseudo second order best described the kinetics of the adsorption process. Removal efficiency (E%) of 97% was attained within 120 minutes at 50 g/l adsorbent concentration, pH6 and 0.2mm particle size of the adsorbent.

FT-NIR analysis of different garlic cultivars

Garlic represents a source of various biologically active phytomolecules. The content of phenolic compounds in garlic varies greatly with genetic, agronomic, and environmental factors, and the cultivar is the primary factor which determines this variation. The aim of this study was to analyse by Fourier transform near infrared spectroscopy (FT-NIR) different garlic cultivars, organically grown or commercial ones, coming from four different geographical areas. Garlic samples were collected from four different locations, and then analyse in the spectral range of the NIR radiation, which extends from 800 to 2500 nm, using three different modules (integrating sphere, transmission and fibre optic modules) that provide tools for analysing most liquid and solid samples. For all the measurements, no sample preparation was performed, since this analytical technique is non-invasive and non-destructive, and each sample was subject to repeated measurements. All the FT-NIR garlic spectra were undergone to a post processing analysis and compared between them. The comparison enhanced a clear distinction between biological and commercial garlic spectra. In particular, the last ones were characterised by a greater number of absorption peaks. This different trend in the FT-NIR spectra could be related to the possible contamination by toxic waste of the cultivation soil. Consumers are gradually looking for quality seals and trust marks on food products, and expected manufacturers and retailers to provide products of high quality. In this context, the FT-NIR represents a valid instrument for food classification and an important task for quality control and identification of foodstuff product adulteration.

A DFT study on structures, frontier molecular orbitals and UV–vis spectra of RuX(PPh3)(NHCPh2)L (X = Tp and Cp; L = Cl and N3)

Geometry optimization for RuX(PPh3)(NHCPh2)(L) (X=hydridotris(pyrazolyl)borate (Tp) and cyclopentadiene (Cp); L=Cl and N3) are investigated by using density functional theory (DFT) with DZVP2/DZVP all-electron mixed basis sets and compared with available experimental values, and the calculated structures are in very good agreement with experimental data. The frontier molecular orbitals (FMOs) and electronic transitions have been investigated as well. Our calculations show that the π electron-rich ligand (N3) may increase the energies of occupied orbitals and reduce the energy gap of the HOMO–LUMO (ΔEL–H) in these ruthenium based complexes. The simulated UV–vis spectra of these complexes in methanol have been studied with time-dependent density functional theory (TD-DFT), and conductor-like polarizable continuum model (CPCM) was employed to account for the solvent effects. Our results show that a number of absorption peaks are found in the visible region (400–800nm) with non-zero oscillator strengths. The strongest adsorption feature is associated to a transition from HOMO−2 to LUMO, which is assigned to metal-to-ligand charge transfer (MLCT) or metal/ligand-to-ligand charge transfer (MLCT/LLCT) depending on co-ligands. In addition, the Cp group increases electron-accept ability and results in red shift due to its π electron-rich and π donor characters. According to our results, these ruthenium based complexes are good candidates for dye-sensitized solar cell owing to their absorption intensities and rich absorption bands in the visible region.

Position-dependent absorption-dispersion spectrum of a Λ-type three-level atom embedded in a defective photonic crystal

The position-dependent optical spectra of a Λ-type three-level atom embedded in a photonic crystal are studied by considering the double-band photonic band gap reservoir with defect modes in the band gap. It is shown that the number of absorption peaks, transparency windows and the slope of the dispersion curve can be modulated by adjusting the position of the embedded atom.

Multiple responses of TPP-assisted near-perfect absorption in metal/Fibonacci quasiperiodic photonic crystal

Absorption properties in one-dimensional quasiperiodic photonic crystal composed of a thin metallic layer and dielectric Fibonacci multilayers are investigated. It is found that a large number of photonic stopbands can occur at the dielectric Fibonacci multilayers. Tamm plasmon polaritons (TPPs) with the frequencies locating at each photonic stopband are excited at the interface between the metallic layer and the dielectric layer, leading to almost perfect absorption for the energy of incident wave. By adjusting the length of dielectric layer with higher refractive-index or the Fibonacci order, the number of absorption peaks can be tuned effectively and enlarged significantly.

Tunneling time correction to the intersubband optical absorption in a THz laser-dressed GaAs/Al x Ga1- x As quantum well

Tunneling effect on the intersubband optical absorption in a GaAs/Al x Ga1- x As quantum well under simultaneous presence of intense non-resonant laser and static electric fields is theoretically investigated. Based on the shooting method the quasi-stationary energy levels and their corresponding linewidths are obtained. By considering the joint action of the two external fields the linear absorption coefficient is calculated by means of Fermi’s golden rule and taking into account the intersubband relaxation. We found that: (i) the linewidth broadening due to the electron tunneling has an appreciable effect on the absorption spectrum; (ii) a constant relaxation time adopted in the previous studies could not be justified even for moderate electric fields, especially in the laser dressed wells. Our model predicts that the number of absorption peaks can be controlled by the external applied fields. While in the high-electric fields the excited states become unbounded due to a significant tunneling of the electrons, for high laser intensities and low/moderate electric fields the absorption spectrum has a richer structure due to the laser-generated resonant states. The possibility of tuning the resonant absorption energies by using the combined effects of the static electric field and the THz coherent radiation field can be useful in designing new optoelectronic devices.

Temperature-dependent terahertz spectroscopy of D-, L- and DL-ornidazole

Terahertz time-domain spectroscopy （THz-TDS） has been used to measure the vibrational spectra of polycrystalline D-, L- and DL-ornidazole over the temperature range 6—298 K. The large differences of the spectra are observed in the frequency range 0.3 THz—2.5 THz, which indicates that THz-TDS is an effective tool to distinguish between enantiomers （D-, L-ornidazole） and its racemic compound （DL-ornidazole）. A number of absorption peaks which could not be observed at room temperature are well resolved in the low temperature range. This provides more experimental details for mode assignment. The evolution of these absorption features between 6 K and 298 K was continuously mapped. The absorption frequencies shift to lower values as temperature increases. The measured temperature dependence can be well fitted if we consider the phonon modes follow a Bose-Einstein distribution. Finally, the quantum chemical calculations were carried out to simulate the experimental THz spectra of the three samples in the range of 0.2—2.5 THz. Compared with the calculated results, the observed characteristic absorptions are explained at molecular level.

Theoretical analysis on the structure and properties of thiourea dioxide crystal

The equilibrium geometry, electronic structure and optical properties of thiourea S, S-dioxide crystal have been studied using DFT within generalized gradient approximation (GGA) and the local density approximation (LDA), implemented using ultrasoft pseudo-potentials. The optimum bulk geometry is in good agreement with crystallographic data. An analysis of electronic structure, charge and bond order is presented. The energy gap of thiourea dioxide with GGA and LDA calculation is 3.217 or 3.210 eV, respectively, indicating that the compound is an insulator. The calculated absorption spectrum shows a number of absorption peaks, which are believed to be associated with different exciton states, in the fundamental absorption region.

Structural, electronic and optical properties of ultrathin thallium nanowires – an ab initio study

The energetics and structural, electronic and optical absorption properties of thallium nanowires, Tl n with n = 1–18, have been investigated by employing a first-principles density functional theory in the local density approximation. The spin–orbit (SO) interaction has also been considered. We study four types of stable structures: planar, caged, pyramidal and helical. In general, the binding energy increases with the coordination number except in a few cases where the nearest-neighbours lie at comparatively larger separations. The maximum stability is seen for the helical configurations containing pentagons, hexagons, heptagons and octagons. Nanowires containing a core linear chain of atoms on the tube axis are more stable than the corresponding nanotubes having no such chains. All the wires or tubes are found to be metallic with or without consideration of the SO interaction. The electronic structures of the pentagonal-, hexagonal- and octagonal-configuration wires provide a large number of channels, which may give rise to large quantum ballistic conduction. One finds large differences between the optical absorption calculated with and without the SO interaction. Consideration of the SO interaction enhances the number of absorption peaks by approximately a factor of two. A strong and multi-peaked optical absorption, extending up to 4.0 eV including the visible region, appears for wires containing pentagons and octagons. These wires may thus be useful as a source of white radiation.