Terahertz Technology

Background and Objectives: The design of the circuit analog absorbers including resistive and conductive patterns on a dielectric substrate placed above the ground plane with a free spacer is interesting for researchers in the microwave regime. Broad absorption band can be achieved by appropriately designing the structure parameters that lead to matching the input impedance of the structure with the impedance of free space over a wide operating band. In this study, a wideband circuit analogue absorber including double-layer of resistive frequency selective surfaces (FSS) is proposed. Methods: The proposed structure is composed of two layers of periodic arrays of strips loaded with lumped resistors deposited on dielectric substrates and separated by an air spacer. Strips of each layer are orthogonal to each other. The structure is placed on a metallic back reflector with an air spacer. The bottom resistive FSS including resistor-loaded strips directed in the x-direction plays the effective role of producing the resonant frequencies with exciting TM polarization waves and leads to a wide high-frequency absorption band, while the top resistive FSS, including resistor-loaded strips directed in the y-direction plays the effective role in exciting the resonances for TE polarization that can produce a broad low frequency absorption band. Indeed, in each polarization, one of the resistive FSS acts as a resonator while the other resistive FSS acts as a transparent layer and transmits the wave. A circuit model for characterizing the proposed structure is presented for both TE and TM polarizations in the subwavelength regime, which shows good agreement with the full-wave simulations. Results: The results demonstrate that the reflectivity below-10 dB (absorption above 90%) obtains from 3.55 to 9.82 GHz (fractional bandwidth of 93%) under normal incidence for TE polarization while with TM incident wave excitation, the absorption above 90% from 9.44 to 20.85 GHz (fractional bandwidth of 75%) can be achieved. Conclusion: The proposed structure leads to a wideband absorber with various bandwidths corresponding to exciting TE and TM incident waves. Most of the proposed structures in the literature produce similar bandwidths for both polarizations. Therefore, a polarization-controlled wideband absorber is designed in this task.

Впервые разработано бортовое устройство ТГц-детектирования для наблюдения солнечных вспышек из стратосферы. Прибор предназначен для определения загадочных и до сих пор не раскрытых спектральных форм терагерцевого излучения солнечных вспышек. Построенная система SOLAR-T, содержащая модули сбора данных и телеметрии, была успешно протестирована. В конструкции SOLAR-T использованы два детектора в виде ячеек Голея, расположенные за фильтрами низких частот, полосовыми фильтрами на 3 и 7 ТГц и резонансными камертонными модуляторами. SOLAR-T позволяет определить относительные изменения температуры, меньшие, чем 1K, с субсекундным временным разрешением. Длительные стратосферные полетные испытания на борту двух воздушных шаров над Антарктидой и Россией планируются к проведению в 2014-2016 годах. Н аземные наблюдения солнечных вспышек позволили обнаружить новые спектральные составляющие их излучения в субтерагерцевом диапазоне, а также на частоте 30 ТГц [1, 2]. Пока физическая природа этого излучения остается загадкой [3]. Полное описание ТГц-спектра, необходимое для дальнейших исследований, может быть получено из наблюдений, проводимых с помощью детекторов, расположенных за пределами земной атмосферы. На основе концепций, материалов, фильтров и детекторов, полученных на разных стадиях предыдущих разработок лаборатории, была создана опытная установка [4-6] двойной терагерцевой фотометрической системы для измерения температуры переходных процессов в континууме солнечных вспышек на частотах 3 и 7 ТГц. На практике недостаточная

This paper reports the design, the microfabrication and the experimental characterization of an ultra-thin narrow-band metamaterial absorber at terahertz frequencies. The metamaterial device is composed of a highly flexible polyimide spacer included between a top electric ring resonator with a four-fold rotational symmetry and a bottom ground plane that avoids misalignment problems. Its performance has been experimentally demonstrated by a custom polarization-maintaining reflection-mode terahertz time-domain spectroscopy system properly designed in order to reach a collimated configuration of the terahertz beam. The dependence of the spectral characteristics of this metamaterial absorber has been evaluated on the azimuthal angle under oblique incidence. The obtained absorbance levels are comprised between 67% and 74% at 1.092 THz and the polarization insensitivity has been verified in transverse electric polarization. This offers potential prospects in terahertz imaging, in terahertz stealth technology, in substance identification, and in non-planar
applications. The proposed compact experimental set-up can be applied to investigate arbitrary polarization-sensitive terahertz devices under oblique incidence, allowing for a wide reproducibility of the measurements.

Very few real easel paintings have been scanned by means of terahertz time-domain imaging (THz-TDI) up to now. In this study, four different areas of an eighteenth century easel painting have been scanned by THz-TDI with the aim of proving useful information to the conservators for its imminent restoration treatment, owing to the complexity of its structure. Despite the unevenness of the layers composing the painting (lining canvas, original canvas, patching materials, and paint layers), the recorded THz dataset has been processed so that the THz images of the single layers composing the multilayered object have been clearly obtained. Relevant information about the painting structure, the presence of structural defects, overpainting, and the existence and extension of the original remains could have been determined. Infrared photography and X-ray radiography have been used for integrating the collected THz data and comparing the different outputs.

This paper hypothesizes the theoretical development of a novel quantum-photonic metamaterial named "Luxonex-9," capable of capturing, storing, and recycling high-energy photons in a stabilized internal loop. The material's unique composite structure combines quantum dots, photonic crystals, and plasmonic elements within a dielectric matrix, providing unprecedented control over lightmatter interactions. If realized, Luxonex-9 would offer transformative potential in propulsion, energy storage, directed-energy systems, and magnetic field interaction. This paper outlines the proposed chemical architecture, fabrication approach, and potential applications, while identifying critical challenges in synthesis and stability.

The rapid evolution of wireless communication technologies is propelling the development of sixth generation (6G) networks, envisioned to surpass 5G in speed, latency, and connectivity. 6G aims to deliver data rates exceeding 1 Tbps, sub-millisecond latency, and support for massive device connectivity across diverse sectors. These advancements, however, demand intelligent spectrum utilization to address spectrum scarcity and network complexity. This paper presents a comprehensive overview of 6G network architecture and highlights the transformative role of Artificial Intelligence (AI) in dynamic spectrum optimization. It explores how AI enables real-time spectrum allocation, interference mitigation, and adaptive resource management, making 6G networks more resilient, secure, and efficient. Furthermore, it examines the integration of cutting-edge technologies such as terahertz communication, edge computing, quantum security, and blockchain to enhance performance and trust in future networks

Photonic Crystal Fiber Technology (PCF) based terahertz (THz) communication is widely considered the key component of future generation high capacity communication and biomedical engineering. The most difficult aspect of employing this technology is minimizing material losses and dispersion over longer distances. In PCF modeling, a wide variety of materials are considered (Zeonex, Silica, Cyclic Olefin Copolymer, etc.). The Cyclic Olefin Copolymer (COC) stands out as a highly flexible material that is both easily fabricable and inexpensive. This study proposed hexagonal porous cladding and core using the Finite Element Method (FEM). A fiber with an optimal core diameter of 300 µm, a low Effective Material Loss (EML) of 0.039456 cm-1 , a low Confinement Loss (CL) of 2.264 × 10-5 dBcm-1 , dispersion of 0.8594 psTHz-1 cm-1 and V-parameter of 1.42 are obtained for 1 THz operating frequency. The suggested fiber's propagation parameters have been diligently analyzed and obtained promising values. A significant porosity of 65% and power fraction of 29.88% was recorded.

THz technology has a rich history of use in the field of interstellar molecule identification where a variety of molecule specific vibrational and rotational spectroscopic signatures exist and has been aggressively investigated for use in advanced radar applications because of the immediate improvement in object resolution obtained at higher frequencies. Traditionally, high power THz systems have relied upon millimeter wave sources and frequency multiplication techniques to achieve acceptable output power levels, while lower power, table top spectroscopic systems, have relied on broadband incoherent light sources. With the advent of high power lasers, advances in non-linear optics, and new material systems, a number of promising techniques for the generation, detection and manipulation of THz radiation are currently under development and are considered the enabling technologies behind a variety of advanced THz applications. This work presents a programmatic overview of current trends in THz technology of interest to a variety of government organizations. It focuses on those techniques currently under investigation for the generation and detection of THz fields motivated, for example, by such diverse applications as metamaterial spectroscopy, TH imaging, long standoff chem/bio detection and THz communications. Examples of these new techniques will be presented which in turn will motivate the need for the characterization of application specific active and passive THz components.

Interparticle Void Vol. "External Void" volume Absolute powder volume: (also called Absolute volume): The volume of the solid matter after exclusion of all the spaces (pores and voids) (BSI). X Apparent particle volume: The total volume of the particle, excluding open pores, but including closed pores (BSI). X X Apparent powder volume: The total volume of solid matter, open pores and closed pores and interstices (BSI).

Resonant absorbers are used to ensure that all resonant light is neither reflected nor transmitted but is turned completely into heat and get absorbed. From transmission line theory resonant absorbers which depends on the material’s way to interact with wave at particular frequency. Resonant absorbers are also called as quarter wavelength absorbers. Absorber design is an impedance match problem. MTM with its strong EM responses is used for designing a resonant absorber. MTM are material with negative value of permeability and permittivity made up of periodic array of magnetic inclusion. These properties are derived from their precisely designed structure. Metamaterials are unique material’s which exhibits negative index of refraction. Metamaterials are used as cloaks, miniature antennas, concentrators and so on. MTM based absorber is best suited for THZ regime (Far-IR region). Design of a metamaterial based resonant absorbers providing absorption of EM wave at resonance frequency is...

There are several different sources of electromagnetic radiation (EMR), including home appliances, Wi-Fi routers, and mobile towers. The growing usage of these technologies in India has sparked worries about the possible impacts of EMR on the environment and public health. This review addresses potential health impacts like headaches, sleep difficulties, and an increased risk of cancer. It also examines the many sources of EMR, such as cell towers and Wi-Fi routers. It also emphasizes the regulatory controls in place to guarantee that radiation levels stay within safe bounds, as well as the significance of public awareness campaigns and monitoring programs to safeguard the environment and public health.

This study aims to introduce a simple equivalent circuit model of circular loop shaped frequency selective surface (FSS), predicting the plane-wave transmission characteristics for oblique angles of incidence. The equivalent circuit model is based on a series of basic equations in order to calculate the inductance and capacitance of strip gratings. Through provided relations, the values of circuit elements can be calculated as well. The proposed relations are applied to many circular loop FSSs with different dimensions, and the results are compared with the full-wave simulations obtained from computer simulation technology microwave studio. Moreover, some comparison graphs between the simulated and analytical resonant frequency for different values of design parameters have been carried out to show the circuit model follows the trend of the simulated responses. Finally, both the FSS structures have been fabricated and measured in an anechoic chamber. A good agreement between the experimental results and the simulated responses demonstrates the validity of the circuit model.

The Event Horizon Telescope Consortium is on the verge to provide the first telescopic image of massive black holes SgrA* and M87* surrounded by accretion disks, at a resolution scale comparable to the size of their event horizons. Well before this remarkable achievement made possible by VLBI radio astronomy, many researchers used the computer to reconstruct how a black hole surrounded by luminous material would look from close-up views. The images must experience extraordinary optical deformations due to the deflection of light rays produced by the strong curvature of the space-time in the vicinity. General relativity allows the calculation of such effects, both on a surrounding accretion disk and on the background star field. This article is an exhaustive and illustrated review of the numerical work on black hole imaging done during the first thirty years of its history.

We design and demonstrate a flexible, ultrathin and double-sided metamaterial perfect absorber (MPA) at 2.39 terahertz (THz), which enables excellent light absorbance under incidences from two opposite sides. Herein, the MPA is fabricated on a λ0/10.1-thick flexible polyethylene terephthalate substrate of εr = 2.75 × (1 + 0.12i), sandwiched by two identical randomized metallic patterns by our stochastic design process. Such an MPA provides tailored permittivity and permeability to approach the impedance of free space for minimizing reflectance and a great imaginary part of the refractive index for reducing transmittance and finally results in high absorbance. Both experimental measurement and numerical simulation are in a good agreement. The flexible, ultrathin and double-sided MPA significantly differs from traditional quarter-wavelength absorbers and other single-sided perfect absorbers, paving a way toward practical THz applications in thermal emission, sensing and imaging, commu...

The design process of a photoconductive antenna (PCA), which emits efficiently in the electromagnetic terahertz range, demands some considerations that are discussed through this work. In this work, several essential characteristics of a photoconductive antenna made with LT-GaAS are studied by means of well established commercial software (COMSOL 5.3 [1]). An approach to the efficiency is also made through the study of geometry, the laser illumination position, the substrate doping distribution, the direction of the bias applied to the semiconductor, the matching impedance at the laser operating frequency and, finally, the plasmonics effects or penetration laser enhancement due to the use of nano antennas. We study and compare two kinds of structures, one which is quasi-bidimensional or planar and the other which is vertical. Additionally, the photoconductive antennas are also modeled by using a simplified equivalent circuit which helps to understand the antennas' performance. Therefore some fundamental parameters, like the transient capacitance between the metal contacts are also studied. Furthermore, we introduce an optimized vertical design which achieves the best results.

A novel fiber design based on hexagonal shaped holes incorporated within the core of a Kagome lattice photonic crystal fiber (PCF) is presented. The modal properties of the proposed fiber are evaluated by using a finite element method (FEM) with a perfectly matched layer as boundary condition. Simulation results exhibit an ultra-low effective material loss (EML) of 0.029 cm −1 at an operating frequency of 1.3 THz with an optimized core diameter of 300 μm. A positive, low, and flat dispersion of 0.49 ± 0.06 ps/THz/cm is obtained within a broad frequency range from 1.00 to 1.76 THz. Other essential guiding features of the designed fiber such as power fraction and confinement loss are studied. The fabrication possibilities are also investigated to demonstrate feasibility for a wide range of terahertz applications.

Very few real easel paintings have been scanned by means of terahertz time-domain imaging (THz-TDI) up to now. In this study, four different areas of an eighteenth century easel painting have been scanned by THz-TDI with the aim of proving useful information to the conservators for its imminent restoration treatment, owing to the complexity of its structure. Despite the unevenness of the layers composing the painting (lining canvas, original canvas, patching materials, and paint layers), the recorded THz dataset has been processed so that the THz images of the single layers composing the multilayered object have been clearly obtained. Relevant information about the painting structure, the presence of structural defects, overpainting, and the existence and extension of the original remains could have been determined. Infrared photography and X-ray radiography have been used for integrating the collected THz data and comparing the different outputs.

We demonstrate the results of on THz emission from hybrid THz photoconductive antenna loaded with silver nanoantenna. The results of experimental investigations are in a good agreement with numerical simulations presented in our recent work. The conversion efficiency reveals over 5-fold improvement at certain frequencies, if compared with similar photoconductive antenna without silver nanoparticles, while previous results for this type of antenna barely exceeded 2-fold conversion efficiency gain. We propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

This paper reports the design and fabrication of a novel all-metal antenna operating in the millimeter-wave band. Based on the resonant cavity antenna (RCA) concept, the principle of antenna operation is explained, and parametric studies of several key design parameters are provided. A novel impedance matching technique is introduced to broaden the antenna return loss bandwidth. Two gain enhancement methods have been employed to achieve a more directive beam with reduced side lobes and back lobes. The D-band antenna prototypes are produced using i) all-metal printing without any postprocessing; ii) dielectric printing with copper metallization applied later. Comparisons of the simulated and measured results amongst the antennas fabricated using the two additive manufacturing techniques are made. Measurement results of the two antenna prototypes show that the proposed design can achieve a 14.2% bandwidth with a maximum gain of 15.5 dBi at 135 GHz. The present work is the first D-band resonant cavity antenna fabricated using two different 3D printing methods.

Fan-out elements create an array of beams radiating at particular angles along the propagation axis. Therefore, they are able to form a matrix of equidistant spots in the far-field diffraction region. In this work, we report on the first fan-out structures designed for the THz range of radiation. Two types of light-dividing fan-out structures are demonstrated: (i) the 3×1 matrix fan-out structure based on the optimized binary phase grating and (ii) the 3×3 fan-out structure designed on the basis of the well-known Dammann grating. The structures were generated numerically and manufactured using the 3D printing technique with polyamide PA12. To obtain equal powers and symmetry of diffracted beams, the computer-aided optimization algorithm was used. Diffractive optical elements designed for 140 and 282 GHz were evaluated experimentally at both these frequencies using illumination with the wavefront coming from the point-like source. Described fan-out elements formed uniform intensity and equidistant energy distribution in agreement with the numerical simulations.

A switchable metasurface composed of plasmonic split ring resonators and a polymer-stabilized liquid crystal blue phase is developed. Owing to field-induced birefringence (electro-optic Kerr effect), the state of polarization of the incident near infrared radiation changes, when a voltage is applied to the liquid crystal. Thus, different resonant modes of the split ring resonators can be addressed and the transmission spectrum changes accordingly. In comparison with other liquid crystal phases, blue phases have several advantages. For example, they are optically isotropic in the field-off state, so that no alignment layer is required. The results of the present study indicate that the advantages of these mesophases can be utilized for switchable metasurfaces.

Repetition rate tuning enables the fast acquisition of THz pulse profiles [1]. By using this method we demonstrate a compact and broadband terahertz time domain spectroscopy system (THz TDS) driven by ytterbium doped fiber laser source. The importance of this method is realized in that Yb:doped fiber lasers can be amplified to sub-millijoule pulse strengths more easily than other types of fiber lasers [2]. Hence, it has the potential to be used in excite-THz probe experiments. Furthermore, the repetition rate-tuning adds flexibility in the excite-probe techniques. These attributes as well as THz generation and detection are investigated with the laser that was developed.

Scientific analysis of panel paintings could provide key information to art historians and conservators about the composition and condition of the constituent layers. Knowledge of the structure, stratigraphy, and condition of the subsurface layers of these objects is highly relevant to conservation measures, when stability problems such as delamination or internal cracking are considered. Terahertz time-domain imaging (THz-TDI) in reflection geometry is capable of highlighting interfaces between layers in a stratigraphic buildup, and thus makes structural information available in the form of subsurface 3D images and stratigraphic images (B-scans). The technique is contactless and non-invasive, and has been successfully applied to investigation of panel paintings. Regarding gilded panel paintings, the extremely high reflectivity of metals at submillimeter wavelengths generally precludes the transmission of electromagnetic waves through metallic films. Therefore, imaging of subsurface features below gilded finishes may be regarded as Very challenging. A small but non-negligible direct transmission through metal films occurs if the film thickness is of the order of the skin depth of the metal in the terahertz range, due to the skin effect. The thickness of gold leaf, a fraction of a micrometer, matches the skin depth of gold in the terahertz frequency range covered by THz-TDI devices. We therefore investigated and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI.

A second-order bandpass frequency selective surface (FSS) for terahertz applications is experimentally demonstrated in this paper. The proposed FSS is designed using miniaturized element unit cells that enable analysis of their frequency by an equivalent circuit model. A transmission zero is introduced in the upper stopband of the transmission response for improving the out-of-band performance. Numerical and experimental results show a harmonic-free response up to three times the passband center frequency.

Assuring the safety of crowded public places like e.g. airports is one of the most serious social issues. One of the aspects of the problem is the detection of potentially dangerous objects hidden under a person's clothing. There are various solutions from simple metal detectors to X-ray scanners. Some of the available solutions are harmful to humans. There is a great potential in using multispectral surveillance imagers in security systems thus we investigate the possibilities of concealed object detection by screening people with non-invasive cameras working in various spectral ranges.

Contents vii Conference Committee ix SESSION 3 MODELLING OF METAMATERIALS 7353 0C Analysis of metallic nanostructures via a discontinuous-Galerkin time-domain approach (Invited Paper) [7353-11] K. Stannigel, M. König, J. Niegemann, K. Busch, Univ. Karlsruhe (Germany) 7353 0D Analytical modelling of linear and nonlinear properties of metamaterials based on multipole expansion [7353-12]

The papers included in this volume were part of the technical conference cited on the cover and title page. Papers were selected and subject to review by the editors and conference program committee. Some conference presentations may not be available for publication. The papers published in these proceedings reflect the work and thoughts of the authors and are published herein as submitted. The publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon.

A novel approach for designing Metamaterial (MTM) based substrates for Stacked Microstrip Patch Antenna (SMPA) has been analyzed with an aim to design most compact antenna ever resonating at dual band frequencies in L5 and S band. The reported antenna has a miniaturized electric size of 50 mm∗ 50 mm∗9.69 mm(0.196λ0×0.196λ0×0.038λ0) making it suitable for compact antenna to be used in IoT devices and/or as Global Navigational Satellite system (GNSS) receivers. The cross polar discrimination (XPD) value of 16 dB and 12 dB at L5 & S band respectively provide reliable circular polarization (CP) operation with global bandwidth of 30 MHz at both frequency bands. The design of SMPA over traditional substrate, over Reactive Impedance Surface (RIS) substrate and over substrate with MTM inclusion have been compared in term of radiation performance and in context to antenna size for targeted frequency bands.

Photonic-crystal-fiber (PCF) based sensors in the terahertz spectrum have been immensely studied and implemented due to their unique advantages and high sensitivity. At an early stage, conventional and hybrid structured porous core PCFbased sensors were proposed, but the sensitivity was not so high. With the advancement of PCF fabrication technology, hybrid structured hollow-core PCFs have been reported and offer superior sensing characteristics than the previous types. In this chapter, both porous core and hollow-core PCF-based THz sensors are analyzed and the propagation characteristics are explained using terahertz spectrum. Finally, some promising terahertz sensors are studied and compared at the end of this chapter.

Rutin is a flavonoid used in clinical practice to treat capillary fragility and prevent bleeding due to its wide variety of pharmacological actions, including antioxidant, anti-inflammatory, anti-allergic, antiproliferative, and anticarcinogenic activities. In this study, the biopharmacotechnical and physical properties of film-coated tablets containing rutin marketed in drugstores were evaluated. Using samples from three batches called A, B and C, we determined the average weight, disintegration time, hardness, content and dissolution profile, and kinetics of the tablets. The samples demonstrated average weight of 457.45 ± 12.32 to 449.15 ± 8.95 mg; disintegration time, 30.17 ± 2.14 to 15.17 ± 2.14 min; hardness, 1.92 ± 0.55 to 1.69 ± 0.36 Kgf; and rutin content, 18.34 ± 1.21 to 15.66 ± 1.29 mg. After 90 min, the dissolution profile showed 52.65, 41.80, and 79.2% for A, B, and C, respectively. The results imply that the non-conformities of the tested products can significantly comp...

In this research, we have proposed two novel types of Kagome PCFs one is slotted core PCF (S-KPCF) and another one is circular PCF (C-KPCF). FEM has been applied with boundary condition to investigate the optical properties of Proposed PCFs. HRS material along with Epsilon Near Zero (ENZ) material has been applied in core and TOPAS was used as background material. Numerical investigation takes place a wider frequency range for 0.8 THz to 2 THz. Slotted core PCF (S-KPCF) shows eminent birefringence of 2.98×10− 2, Lower EML of 0.232 cm− 1, lower confinement loss of 2.76×10− 8, Bending loss 10.76×10− 9, Total loss of 0.232cm− 1 at frequency 1THz. Besides circular core PCF (C-KPCF) shows eminent birefringence of 8.69×10− 2, Lower effective material loss of 0.746 cm− 1, lower confinement loss of 2.80×10− 7, Bending loss 7.76×10− 9, Total loss of 0.746cm− 1 at 1THz. Both PCFs show flat dispersion near zero (for S-KPCF is -0.997ps/nm/Km and for C-KPCF is -0.797ps/nm/Km) range. V-Parameter ...

We present the results of terahertz pulsed imaging (TPI) experiments to measure the 'hardness' of pharmaceutical solid dosage forms (tablets) compacted at various compression forces. Results of TPI measurements are compared to those from diametric compression tests as well as finite element analysis (FEA) simulations of density distributions on the surface of the tablet. In both cases strong correlation with TPI results was found. In particular, radially symmetric spatial distributions in tablet density due to the shape of the punch used in tablet manufacture were observed. The results of these experiments show that TPI is suitable for non-destructive monitoring and control of pharmaceutical manufacturing processes.

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In this paper, we present the design and simulation of a single and multi-band perfect metamaterial absorber (MA) in the THz region base on Jerusalem cross (JC) and metamaterial load in unit cells. The structures consist of dual metallic layers for allowing near-perfect absorption with absorption peak of more than 99%. In this novel design, four-different shape of Jerusalem cross is presented and by adding L, U and W shape loaded to first structure, we tried to achieve a dual-band absorber. In addition, by good implementation of these loaded, we are able to control the absorption resonance at second resonance at 0.9, 0.7 and 0.85 THz respectively. In the other hand, we achieved a semi stable designing at first resonance between 0.53 and 0.58 THz. The proposed absorber has broadband polarization angle. The surface current modeled and proved the broadband polarization angle at prototype MA. The LC resonance of the metamaterial for Jerusalem cross and modified structures are extracting from equivalent circuit. As a result, proposed MA is useful for THz medical imaging and communication systems and the dual-band absorber has applications in many scientific and technological areas.

Scientific analysis of panel paintings could provide key information to art historians and conservators about the composition and condition of the constituent layers. Knowledge of the structure, stratigraphy, and condition of the subsurface layers of these objects is highly relevant to conservation measures, when stability problems such as delamination or internal cracking are considered. Terahertz time-domain imaging (THz-TDI) in reflection geometry is capable of highlighting interfaces between layers in a stratigraphic buildup, and thus makes structural information available in the form of subsurface 3D images and stratigraphic images (B-scans). The technique is contactless and non-invasive, and has been successfully applied to investigation of panel paintings. Regarding gilded panel paintings, the extremely high reflectivity of metals at submillimeter wavelengths generally precludes the transmission of electromagnetic waves through metallic films. Therefore, imaging of subsurface features below gilded finishes may be regarded as Very challenging. A small but non-negligible direct transmission through metal films occurs if the film thickness is of the order of the skin depth of the metal in the terahertz range, due to the skin effect. The thickness of gold leaf, a fraction of a micrometer, matches the skin depth of gold in the terahertz frequency range covered by THz-TDI devices. We therefore investigated and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI.

The cross polarization analysis of the proposed design done in "Comment on 'A Wideband Wide-angle Ultra-thin Metamaterial Microwave Absorber" is similar to studies already conducted in [1, 2]. However, this analysis reveals that the design provides quite low absorption due to its diagonally symmetric geometry, but such various design configurations reported by different researchers [3-9] create the path to understand the importance of the requirement of polarization insensitive wideband/broadband metamaterial absorbers. Further, this importance enhances the in depth analysis of polarization sensitive multiband and wideband absorbers (so called) that are designed based on the studies and findings done at that time. Moreover, the very first perfect metamaterial absorber design reported in [7] is polarization sensitive, but it spawned great interest of researchers at the time due to its potential applications, and therefore, it becomes the motivation for today's absorbers. Similarly, the dual-band designs reported in [8, 9] are also polarization sensitive, but they give an idea to design multiband and wideband metamaterial absorbers. Hence, there is a requirement to develop new techniques for the design of polarization insensitive multiband and broadband absorbers which is observed missing in this study of comment on the proposed design. Moreover, it also proves that the proposed design is a good metamaterial based wideband rotator which makes it useful for other potential applications such as spatial filters [10, 11] and radomes [12]. Thus, it is concluded that the design of monolayer polarization insensitive metamaterial based multiband and wideband microwave absorbers is still a challenge, and the readers should take this as an opportunity to improve the existing polarization sensitive designs or develop some techniques to reduce cross polarization effects as reported in [13] rather than simply proving that which one is an absorber or not as reported in [1, 2, 14]. ACKNOWLEDGMENT We sincerely thank the editors for giving us a chance to reply on this issue.

A quad band polarisation-insensitive metamaterial absorber comprising electric field-driven LC resonator has been proposed in this study. The unit cell of the proposed structure is designed with a metallic pattern on the top of a grounded dielectric substrate. The geometric parameters of the unit cell are optimised in such a way that the proposed absorber exhibits quad band absorption at C, X and Ku bands. The absorber exhibits polarisation insensitivity under normal incidence and provides high absorption for wide angles of the incident wave up to 60° for both TE and TM polarisation. The absorption mechanism has been studied by analysing the surface current and field distributions for all the four absorption frequencies. The proposed design is compact in its unit cell size and ultra-thin with a thickness of 0.019λ 0. Finally, a prototype of the proposed absorber structure has been fabricated and its performance is experimentally verified. The measured responses are observed in agreement with simulation results.

A novel design of wideband, ultra-thin, wide-angle metamaterial microwave absorber has been presented. The unit cell of the proposed structure is designed by using parametric optimization in such a way that absorption frequencies come closer and give wideband response. For normal incidence, the simulated FWHM bandwidth of the proposed structure is 1.94 GHz, i.e., from 5.05 GHz to 6.99 GHz and −10 dB absorption bandwidth is 1.3 GHz from 5.27 GHz to 6.57 GHz. The proposed structure has been analyzed for different angles of polarization, and it gives high absorption (more than 50%) for oblique angles of incidence up to 60 •. The designed absorber is in low profile with a unit cell size of λ 0 /6 and ultrathin with a thickness of λ 0 /32 at the center frequency of 5.92 GHz corresponding to 10dB absorption bandwidth. The current and electromagnetic field distributions have been analyzed to understand the absorption mechanism of the absorber. An array of the proposed absorber has been fabricated and experimentally tested for various polarization angles and oblique incidences of electromagnetic wave. The proposed absorber is well suited for surveillance and other defense applications.

A novel fiber design based on hexagonal shaped holes incorporated within the core of a Kagome lattice photonic crystal fiber (PCF) is presented. The modal properties of the proposed fiber are evaluated by using a finite element method (FEM) with a perfectly matched layer as boundary condition. Simulation results exhibit an ultra-low effective material loss (EML) of 0.029 cm −1 at an operating frequency of 1.3 THz with an optimized core diameter of 300 μm. A positive, low, and flat dispersion of 0.49 ± 0.06 ps/THz/cm is obtained within a broad frequency range from 1.00 to 1.76 THz. Other essential guiding features of the designed fiber such as power fraction and confinement loss are studied. The fabrication possibilities are also investigated to demonstrate feasibility for a wide range of terahertz applications.

The design process of a photoconductive antenna (PCA), which emits efficiently in the electromagnetic terahertz range, demands some considerations that are discussed through this work. In this work, several essential characteristics of a photoconductive antenna made with LT-GaAS are studied by means of well established commercial software (COMSOL 5.3 [1]). An approach to the efficiency is also made through the study of geometry, the laser illumination position, the substrate doping distribution, the direction of the bias applied to the semiconductor, the matching impedance at the laser operating frequency and, finally, the plasmonics effects or penetration laser enhancement due to the use of nano antennas. We study and compare two kinds of structures, one which is quasi-bidimensional or planar and the other which is vertical. Additionally, the photoconductive antennas are also modeled by using a simplified equivalent circuit which helps to understand the antennas' performance. Therefore some fundamental parameters, like the transient capacitance between the metal contacts are also studied. Furthermore, we introduce an optimized vertical design which achieves the best results.

The deployment of additive manufacturing processes relies on part quality, specifically the absence of internal defects. Some of those defects have been associated with porosities in the powder feedstock. Since the level of porosity in the powder is generally very low, standard characterisation techniques such as pycnometry and metallography are not suitable for quantification. However, the quantification of such micro sized porosity in metallic powders is crucial to better understand the potential source of internal defects in final components and for quality control purposes. X-ray tomography with a 3 μm resolution offers the possibility to visualise pores in large volume of powder and to quantify their geometrical features and volume fraction using image analysis routines. This combination is unique and demonstrates the power of the approach in comparison to standard powder characterisation techniques. Results presented show the prospects and limits of this technique depending on the imaging device, material and image analysis procedure.

This paper explores the possibility of the precise determination of mechanical vibrations using metamaterial split ring resonator (SRR) structure. The amplitude of the interacting electromagnetic wave in the range of GHz frequency is directly varied in accordance with the amplitude of mechanical vibration, using a Broadside Coupled SRR (BCSRR) acting as a vibration sensor. Dependence of the spacing between the two rings on the resonance frequency of the BCSRR is used for the detection of vibration and it is achieved by allowing the spacing to change in accordance with the amplitude of mechanical vibration. For the effective sensing of mechanical vibration, the electromagnetic wave frequency is chosen at the center of the linear portion of the rising or the falling slope of the resonant curve of the BCSRR. By properly choosing the parameters of the BCSRR along with the effective tuning of the operating frequency, it is possible to detect even very weak vibrations. The chances of various distortions in the detected vibration waveform in connection with selection of the operating frequency and intensity of vibrations are also analyzed. The qualitative formulation of the detection process along with its experimental verification is presented. This novel method may find applications in the detection of mechanical vibrations caused by various man made and natural sources and may find manifold possibilities in the field of communication and instrumentation.

In this paper, a third-order band-pass frequency selective surface (FSS) filter is proposed for terahertz applications. The presented FSS filter is designed based on miniaturized unit cells and a fractional bandwidth of 72% around 0.775 THz is achieved. In addition, a high transmission magnitude is obtained with an acceptable flatness in comparison to previous reports. In order to verify the numerical results, an equivalent circuit model of the proposed spatial filter is introduced considering both normal and oblique incident angles. The sensitivity of the filter with respect to the incident wave angle is investigated in the range of 0-60°whereas the obtained performance is acceptable for various incident angles in the mentioned range. Moreover, the effects of altering different geometrical parameters are studied and justified based on the extracted equivalent circuit model.

A switchable metasurface composed of plasmonic split ring resonators and a polymer-stabilized liquid crystal blue phase is developed. Owing to field-induced birefringence (electro-optic Kerr effect), the state of polarization of the incident near infrared radiation changes, when a voltage is applied to the liquid crystal. Thus, different resonant modes of the split ring resonators can be addressed and the transmission spectrum changes accordingly. In comparison with other liquid crystal phases, blue phases have several advantages. For example, they are optically isotropic in the field-off state, so that no alignment layer is required. The results of the present study indicate that the advantages of these mesophases can be utilized for switchable metasurfaces.