Substrate Permittivity Research Articles

Theoretical analysis and spatial–temporal dynamic simulation of radiation from graphene

In this paper, the terahertz (THz) radiation based on the graphene is studied by theoretical analysis and particle-in-cell (PIC) simulation. We present the transition radiation (TR) of the charged particle traversing a monoatomic graphene layer under arbitrary incidence. By theoretical analysis, we find strongly tunable effects of the radiation field distribution related to the electron’s incident angle, substrate permittivity and graphene conductivity. In the case of the normal incidence, the transition radiation exhibits strong symmetry and considerably more intensive than that at oblique incidence. Furthermore, the plasmon and radiation are simulated by a PIC code with the method of equivalent permittivity to deal graphene. With the PIC code, we observe the spatial–temporal dynamics of plasmon and transition radiation from graphene. Meanwhile, the PIC simulation results of radiation are in good agreement with those by theoretical analysis.

Diverse Substrate-Mediated Local Electric Field Enhancement of Metal Nanoparticles for Nanogap-Enhanced Raman Scattering.

The localized surface plasmon resonance of plasmonic nanoparticles (NPs) can be coupled with a noble metal substrate (S) to induce a localized augmented electric field (E-field) concentrated at the NP-S gap. Herein, we analyzed the fundamental near-field properties of metal NPs on diverse substrates numerically (using the 3D finite-difference time-domain method) and experimentally [using surface-enhanced Raman scattering (SERS)]. We systematically examined the effects of plasmonic NPs on noble metals (Ag and Au), non-noble metals (Al, Ti, Cu, Fe, and Ni), semiconductors (Si and Ge), and dielectrics (TiO2, ZnO, and SiO2) as substrates. For the AgNPs, the Al (11,664 times) and Si (3969 times) substrates produced considerable E-field enhancements, with Al in particular generating a tremendous E-field enhancement comparable in intensity to that induced by a Ag (28,224 times) substrate. Notably, we found that a superior metallic character of the substrate gave rise to easier induction of image charges within the metal substrate, resulting in a greater E-field at the NP-S gap; on the other hand, the larger the permittivity of the nonmetal substrate, the greater the ability of the substrate to store an image charge distribution, resulting in stronger coupling to the charges of localized surface plasmon resonance oscillation on the metal NP. Furthermore, we measured the SERS spectra of rhodamine 6G (a commonly used Raman spectral probe), histamine (a biogenic amine used as a food freshness indicator), creatinine (a kidney health indicator), and tert-butylbenzene [an extreme ultraviolet (EUV) lithography contaminant] on AgNP-immobilized Al and Si substrates to demonstrate the wide range of potential applications. Finally, the NP-S gap hotspots appear to be widely applicable as an ultrasensitive SERS platform (∼single-molecule level), especially when used as a powerful analytical tool for the detection of residual contaminants on versatile substrates.

Resonant Characteristics of Split Ring Resonator and Unit Cell for Periodic Metamaterial Devices

The resonant characteristics of single split ring resonator-based metamaterial devices with single gap are presented using the analytical formulation developed for the lumped element equivalent circuit model. The characteristics of the metamaterial resonators have been investigated for different ring sizes, gap widths and substrate permittivity. Equivalent circuit model is developed for two ring structures. The analytical, and simulation results are compared and verified. The prototype has been then built and measured. It has been observed that all the results agree. The results presented in this paper can be used to develop devices at the THz range that can operate as sensors, antennas or tuning elements.

Design method for radar absorbing structures using reliability-based design optimization of the composite material properties

Functional composites dispersed with nanofillers are used to enhance the characteristics of radar absorbing structures (RAS). In this study, the relative permittivity of the RAS substrate is measured under various environmental conditions of temperature and humidity. The probability distributions of the dielectric materials used for RAS are determined using maximum likelihood estimation based on raw data. With the determined distributions, 107 samples are generated and sampling-based reliability-based design optimization (RBDO) for reflection loss of a double slab RAS is performed based on transmission line theory. Two objectives considering absorption properties and bandwidth that represent the performance of the RAS are presented. The results of deterministic optimization (DO) and RBDO are compared considering the failure probability of the application system under operating conditions. Although the total thickness increases slightly, the RBDO provides a significant reduction in the failure probability.

Extracting the Infrared Permittivity of SiO2 Substrates Locally by Near-Field Imaging of Phonon Polaritons in a van der Waals Crystal.

Layered materials in which individual atomic layers are bonded by weak van der Waals forces (vdW materials) constitute one of the most prominent platforms for materials research. Particularly, polar vdW crystals, such as hexagonal boron nitride (h-BN), alpha-molybdenum trioxide (α-MoO3) or alpha-vanadium pentoxide (α-V2O5), have received significant attention in nano-optics, since they support phonon polaritons (PhPs)―light coupled to lattice vibrations― with strong electromagnetic confinement and low optical losses. Recently, correlative far- and near-field studies of α-MoO3 have been demonstrated as an effective strategy to accurately extract the permittivity of this material. Here, we use this accurately characterized and low-loss polaritonic material to sense its local dielectric environment, namely silica (SiO2), one of the most widespread substrates in nanotechnology. By studying the propagation of PhPs on α-MoO3 flakes with different thicknesses laying on SiO2 substrates via near-field microscopy (s-SNOM), we extract locally the infrared permittivity of SiO2. Our work reveals PhPs nanoimaging as a versatile method for the quantitative characterization of the local optical properties of dielectric substrates, crucial for understanding and predicting the response of nanomaterials and for the future scalability of integrated nanophotonic devices.

General Design Technique for High-Gain Traveling-Wave Endfire Antennas Using Periodic Arbitrary-Phase Loading Technique

This article proposes a general design technique for high-gain traveling-wave endfire antennas using a periodic arbitrary-phase loading technique. This work extends the periodic phase-reversal techniques reported in previous work to application scenarios where phase reversals are difficult to realize (e.g., single-layer circuits). It also relaxes the substrate permittivity requirement in phase-reversal techniques. To verify the proposed technique and theory, two design examples using different phases other than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> are provided. Double-side parallel stripline and coplanar stripline are employed to provide vertical and horizontal polarizations, respectively. Double-side parallel stripline antenna achieves a maximum endfire gain of 15.6 dBi with vertical polarization. The coplanar-stripline antenna exhibits a maximum endfire gain of 16.6 dBi with horizontal polarization. The experimental results of these two examples validate the proposed technique.

Graphene Rectangular Loop Antenna for Terahertz Communications

Rectangular graphene loop antenna for terahertz (THz) band is proposed and analyzed. Symmetry properties of the antenna in terms of currents and the radiated fields are investigated. By means of numerical simulations, the antenna is analyzed in free space and on a semi-infinite substrate. It is shown that the maximum total efficiency and minimum input reflection coefficient occur around the second resonance frequency of the antenna. Depending on the relative permittivity of the semi-infinite substrate considered in this article (from 1 to 3.8), the maximum total efficiency of the proposed antenna ranges from 50.1% to 57.4% and its fractional bandwidth (BW) is between 85.2% and 96.6%. Fractional BW of loop antenna can be approximately twice that of graphene dipole antenna and maximum total efficiency can be about 14% higher. BW augmentation is explained using developed equivalent circuit models for dipole and loop antennas. It is also shown that under specific conditions, the radiation pattern of the proposed device can be approximated by an equivalent scheme of two coupled small electric dipole antennas. Finally, we demonstrate that the proposed antenna tolerates moderate variations of graphene parameters.

Bandwidth limit and synthesis approach for single resonance ultrathin metasurfaces

Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.

Permittivity of 3D-Printed Nylon Substrates with Different Infill Patterns and Densities for Design of Microwave Components

Printed circuit boards, chemical etching, and computer numerical control milling currently dominate industrial processes for manufacturing microwave components. However, these manufacturing methods do not provide the flexibility for customization possible with additive manufacturing. Additive manufacturing (AM) has the potential to fabricate microwave components for desired frequency ranges with less effort in prototyping and fabrication. Relative permittivity (εr) of materials is a critical parameter in microwave component design, yet the value changes during the AM process. This article investigates how relative permittivity for nylon substrates, created with AM, changes with different infill densities and infill patterns. The measurement method and procedure can be used to design AM microwave components like antennas or dielectric-filled waveguides with desired characteristics. The two-microstrip-line method was used and improved for the accurate and convenient measurement of the relative permittivity of AM nylon substrates. Several nylon substrates with different infill patterns, including rectangular, hexagonal, triangular, and solid, were fabricated with AM to demonstrate how the relative permittivity value changes as the infill density increases. A linear relationship between the infill density of the rectangular pattern and the substrate permittivity was found. The permittivity data were applied to the design of a rectangular patch antenna for use in the 2.5-GHz WiMAX band. The fabricated antenna with AM, which was tested using a vector network analyzer, showed good agreement with simulation results. The method and procedure of permittivity measurements are specially designed to be applied in the design of microwave components with AM dielectric substrates.

Study of resonant microstrip antennas on artificial neural networks

This paper presents a new model based on the backpropagation multilayered perception network to find accurately the bandwidth of both electrically thin and thick rectangular microstrip antennas. This proposed neural model does not require complicated Green's function methods and integral transformation techniques. The method can be used for a wide range of substrate thickness and permittivities and is useful for the computer-aided design of microstrip antennas. The results obtained by using this new method are in conformity with those reported elsewhere. This method may find wide applications in high-frequency printed antennas, especially at the millimeter-wave frequency range.

Kriging metamodels and design re‐utilization for fast parameter tuning of antenna structures

AbstractThe paper addresses the problem of computationally efficient electromagnetic (EM)‐driven design closure of antenna structures. The foundations of the presented approach are fast kriging interpolation metamodels, utilized for two purposes: (a) producing a good starting point for further parameter tuning, and (b) yielding a reasonable Jacobian matrix estimate to jump‐start the optimization procedure. The models are rendered using available designs, for example, obtained from the previous design work with the same antenna structure. The low cost of design closure is ensured by employing Broyden‐based trust‐region gradient search along with the aforementioned Jacobian initialization. Our methodology is demonstrated using two planar antennas, a dual‐band uniplanar dipole and a quasi‐Yagi with a parabolic reflector, both optimized within wide ranges of operating conditions (center frequencies, the dielectric permittivity of the antenna substrate). The re‐design process requires only a handful of EM analyses of the respective structure. The presented framework can be viewed as a convenient algorithmic tool that capitalizes on the existing information on the structure at hand to enable warm‐start parameter tuning.

Impact of circuit metallization on dielectric permittivity measurement by scanning microwave microscopy

This study demonstrates a dielectric permittivity measurement technique using scanning microwave microscopy (SMM). The proposed technique can be used to evaluate the local dielectric permittivity of a circuit substrate with circuit metallization. The impact of the circuit metallization was reduced by analyzing the positional dependence of the resonance frequencies. Although the evaluated dielectric permittivity of the alumina substrate was in the range of 18–25 without the modification, the values were modified to 9.3. The modified dielectric permittivity was compared to those of other local dielectric measurement techniques, a probe-backside reflection (PBR) method. Dielectric permittivity was evaluated as 9.5 ± 0.35 using the PBR method. Therefore, the SMM result corresponded to the accuracy range. This validated the reliability of the proposed measurement technique using the SMM method.

LTCC-Based Fluidic Tuners for Low Microwave Frequency Reconfigurable Circuits

This article presents a frequency reconfigurable RF impedance tuner using eight cascaded fluidic cells to cover the frequency band [0.9, 2.4] GHz. The tuner is made of eight fluidic fillable cavities on top of a coplanar waveguide transmission line in a single low-temperature cofired ceramic (LTCC) substrate. Each cavity is either kept empty or filled with deionized (DI) water to alter the substrate's permittivity and so change the tuner's response. Closed-form expressions are derived and used to dimension the tuner's individual cells, while 3-D field simulations are used to complete the design of a single cell and an entire eight-cell tuner. Both components are fabricated, and good agreement between simulation and measured is observed. The fabricated tuner is used to build a fluidic reconfigurable RF amplifier.

Circuit model for curved coupled microstrip line AMC structure based on quasi-static analysis

A novel equivalent circuit model for an artificial magnetic conductor (AMC) made of curved coupled microstrip line (CCML) wideband resonators is presented. In the proposed method, the effects of substrate thickness and permittivity is explicitly accounted in both the ground-terminated line and frequency selective surface of the AMC, which leads to a more accurate model compared to the previous ones. In addition, closed-form formulas predicting the behaviour of the AMC versus geometrical parameters are provided. The dependence of the equivalent circuit parameters on the excitation frequency is discussed and the wideband behaviour of the CCML based AMC is justified. The validity and application of the model is demonstrated via full-wave electromagnetic simulations and design examples.

Determination of dielectric properties of unknown substrates using hybrid approach

Unknown dielectric properties of RF/microwave substrates cause headaches for microwave circuit and antenna designers. Most commercial reinforced dielectric substrates made from layers of woven-glass and/or ceramic fillings lead to dielectric anisotropy. Often, re-optimization and even re-fabrication of designed prototypes are required when measuring results are found to be departed from the prediction. These are time consuming and costly. In order to determine the unknown complex permittivity of any substrates, this paper describes a hybrid approach composed of two measurement stages. The first stage is the coaxial-probe method for predicting an initial complex permittivity of unknown dielectric substrates, which is known as the perpendicular permittivity. The second stage is the microstrip open-circuit stub (OCS) method that regulates the complex permittivity of the dielectric laminate by mapping the input impedance curves of OCS via electromagnetic simulation and actual measurement. Three dielectric substrates without prior dielectric properties were selected to validate the proposed method. The equivalent dielectric constants, dissipation factors and dielectric anisotropies were determined and compared. The proposed method is found to be reliable for determining the unknown properties of dielectric substrates.

Ultraminiature Antennas Combining Subwavelength Resonators and a Very-High-ε Uniform Substrate: The Case of Lithium Niobate

Combining the effects of subwavelength resonators and very-high-permittivity substrates enables a high extent of miniaturization, even for very simple, split-loop resonators. Here, we demonstrate how requirements to the substrate's permittivity are connected with the desired extent of miniaturization and why materials with a relative permittivity of 30 <; ε <; 100, like lithium niobate, may offer a real possibility to miniaturize. For demonstration purposes, we designed, in line with this approach, an ultraminiature dual-band antenna to operate at 2.8 and 4.2 GHz. The antenna is fabricated using microfabrication techniques and studied experimentally. There is good agreement between the measurement and simulation results. The realized gain is about -5 dB for the first resonance, at which the size of the substrate-resonator block is λ/24. The obtained results demonstrate the potential of the suggested approach, which is expected to be applicable to a very wide class of subwavelength resonators and a wide variety of substrates with high permittivity.

Triboelectric Nanogenerator With Enhanced Performance via an Optimized Low Permittivity Substrate

With electrical power generated from mechanical contact, triboelectric nanogenerators (TENGs) offer a promising route to realizing self-powered sensors. For effective usage, it is important to improve their limited power range (0.1–100 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and this can be achieved by optimizing the output performance. Among the factors that confer higher performance are materials with a strong triboelectric effect together with low permittivity, but it is challenging to optimize both within a single material. This paper presents a solution to this challenge by optimizing a low permittivity substrate beneath the tribo-contact layer. Results are simulated over a range of substrate permittivities. The open circuit voltage is found to increase by a factor of 1.6 in moving from PVDF to the lower permittivity PTFE. Two TENG devices have been fabricated with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100\boldsymbol {\mu m}$ </tex-math></inline-formula> PET and PTFE substrates to compare performance. The experiments confirm that lowering the substrate dielectric constant (i.e. PET to PTFE) raises the open circuit voltage in line with simulation predictions.

Experimental analysis of fabric substrate on a moisture sensor

In this work, an embroidered textile moisture sensor is proposed and characterized with three different fabric substrates. The sensors are based on a capacitive interdigitated structure embroidered on a textile substrate with a conductive yarn. The performance of three different type of substrates has been addressed. In order to evaluate the sensor behavior, the impedance of the sensor has been measured by means of an LCR meter from 20 Hz to 200 kHz on a climatic chamber with a sweep of the relative humidity from 30% to 80% at 20 °C. The results show that the fabric substrate defines the electrical properties of the sensor due to both, the substrate permittivity and size yarn thickness increase. This sensor dependence determines the optimum substrate to be used to develop wearable applications for moisture measurement.

SIW Horn Antenna with a Special Coplanar Transition and Dielectric Lens for Feeding a Leaky-Wave Antenna

The paper describes the main phases and features in designing a feeder for a leaky-wave antenna in the Ka-band. The problem of feeding can be effectively solved using a horn antenna SIW technology-based and single substrate-integrated with the radiating aperture of the leaky-wave antenna. Unlike most similar studies, where an aperture exciting from a closed-type structure is analyzed, in our work the open-type antenna feeder is synthesized.In the paper, we give a brief overview of the main achievements in designing traditional SIW-structures and present relations for their calculation. Basing on results of electromagnetic modeling, determine the influence of the substrate permittivity on the efficiency of SIW transmission line by the method of finite Weiland integrals. Consider the most popular types of the waveguide transitions to SIW, including micro-strip lines and grounded coplanar waveguides (GCPW). Depending on the required bandwidth and substrate dimensions specify conditions for selecting a design type of the waveguide transition.A special attention is paid to the non-standard coplanar transition with an elevated grounded surface (EGCPW), which is worth using if the substrates are thick. It was used as a basis for designing a new, more efficient EGCPW transition with the elevated grounded surface, which contains specially made slots (SEGCPW). Recommendations for its adjustment are given.The dependences of the gain on the aperture width for the H-plane SIW horn antenna, which are used to select its optimal size, are obtained basing on simulation. It was found that for matching the feeder with the leaky-wave antenna and decreasing the side lobes level, a dielectric lens with through holes worth using. Basing on the simulation of the unit cell of the lens the dependence of the effective permittivity on the diameter of the through hole is obtained. An influence of the non-uniformity degree of the lens with the holes of different diameters on the radiation efficiency of a horn antenna is determined. The complete design of the synthesized feeder and its radiation pattern is presented to prove the antenna efficiency in the wide frequency band.

Tunable broadband multiresonance graphene terahertz sensor

We proposed a tunable multiresonance graphene sensor consisting of a graphene disk and three non-overlapping graphene rings on a dielectric substrate. Compared to the previous reports, our proposed graphene sensor acts in an ultrawide spectral range from mid-to far-infrared ranges. The resonance of the graphene disk is in the mid-infrared range, while the resonance of graphene rings is in the far-infrared range. The multiple plasmonic resonance of the proposed structure is highly sensitive to the change of the substrate permittivity such that the frequency sensitivity of the graphene disk, ring 1, ring 2, and ring 3 are 2.72, 0.878, 0.582, and 0.529 THz (THz) per substrate permittivity unit (THz/SPU), respectively. Interestingly, by increasing the radii of each graphene subarray ring a red-shift of the transmission resonance dips toward the THz frequencies can be observed. It is found that by changing the Fermi energy of graphene, each resonance dip of the proposed structure can be flexibly controlled without re-manufacturing the structure. The performance of our proposed structure makes it a powerful tool for using in the optical devices with operating in the mid-to far-infrared range.