Homogeneous Dielectric Slabs Research Articles

Homogeneous Two-Layer Structures for Broadband and Wide-Angle RCS Reduction

A two-layer stacked resonance absorber is proposed to achieve broadband and wide-angle radar cross section reduction. The most prominent structure feature is its simple geometry, which comprises two homogeneous dielectric slabs only. The two slab heights are chosen as one-sixth of a guided wavelength to form a three-way cancellation mode via interface reflections while a high permittivity dielectric is used to achieve low profile. The total thickness is less than 0.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda_{L}$</tex-math></inline-formula> . Because it involves no periodic structure, both monostatic and bistatic scattering levels can be suppressed in a broad frequency band and a wide angular region. A bouncing ray-based high frequency method was developed to perform parametric studies and design optimization in a timely fashion. Measured and simulated results are compared in spectral and angular domains. An 83% 10 dB RCS reduction band is observed. More than 10 dB back scattering suppression is achieved up to ±75° from the surface's normal.

High-efficiency microwave heating method based on impedance matching technology

In a microwave heating process, part of microwave power may be reflected on the air-heating sample interface due to the impedance mismatching, caused by the relative dielectric permittivity difference between the air and the heating sample. To overcome this problem, a novel impedance matching based technology is proposed. It designs and inserts an impedance matching structure between the air and the heating sample, which mitigates the drastic permittivity difference on both sides. This realizes good impedance matching and obtains the minimized reflection on the interface, hence highly efficient microwave heating is achieved. In this work, the mechanism of the proposed technology and the synthesis of such an impedance matching structure is illustrated, and then verified by an experiment using 500W 2.45GHz microwave power to heat a box of water. An impedance matching structure consisting of three homogeneous dielectric slabs is designed, and the heating cases with and without the impedance matching structure are compared. Both simulation and measurement results show that the impedance matching structure decreases the reflection coefficient of the microwave power from -1.8 dB to -25.6 dB, and correspondingly improves the heating efficiency from 34% to above 86.15%. A further analysis estimates the technology’s robustness and effectiveness for various heating sampling, showing that the heating efficiency maintains above 80% even when the heating sample’s relative permittivity ranges from 27 to 75.

A Wire-Metamaterial-Loaded Resonant Cavity Antenna Using 3-D Printing Technology

Homogeneous dielectric slabs (HDSs) are widely used as the superstrate of resonant cavity antennas (RCAs). For HDS-loaded RCAs, the achievable directivity increases with the relative permittivity ( ϵ ${}_{r}$ ) of the HDS. Commercial printed circuit boards with removed copper cover are usually used as the HDS superstrate, but their ϵ ${}_{r}$ and thickness are limited in the market. Three-dimensional (3-D) printed HDS has the advantage of fast prototyping, process simplicity, and rigidity and can alleviate the limitation of thickness. However, the low ϵ ${}_{r}$ makes it unattractive in the application of RCA serving the superstrate. In this letter, the wire metamaterial (WMM) with 3-D-printed host dielectric is used as the superstrate of RCA. The effective relative permittivity and effective thickness of the WMM are investigated. It is found that the WMM slab can be equivalent to an HDS with higher ϵ ${}_{r}$ and thinner thickness. To validate the equivalence, the WMM-loaded RCA operating at 14 GHz is simulated, fabricated, and measured. The gain of 12.9 dBi, sidelobe level of −18 dB, and cross polarization of −24 dB have been achieved.

A Compact Wideband Circular Polarized Fabry-Perot Antenna Using Resonance Structure of Thin Dielectric Slabs

A wideband small-footprint circularly polarized Fabry-Perot resonator antenna is presented in this paper. The design employs a partial reflecting surface (PRS) consisting of thin homogeneous dielectric slabs separated by a small air gap. The PRS is analyzed using transmission line theory, which gives more insightful into the theoretical concept and provides a simple tool for design and optimization. An antenna prototype with overall size of $1.2\lambda _{f_{\text {min}}} \times 1.2\lambda _{f_{\text {min}}} \times 0.6\lambda _{f_{\text {min}}}$ has been fabricated and experimentally validated ( $\lambda _{f_{\text {min}}}$ being the free-space wavelength at the minimum operating frequency). The measured impedance matching and axial ratio bandwidths are approximately 55.7% and 47.7%, respectively. In addition, the antenna achieved 3-dB gain bandwidth of 50.9% with a peak gain of 15 dBi. Compared to previous designs targeting similar applications, the antenna proposed here possesses a simple geometry with light weight, ease of optimization and fabrication, as well as enhanced axial ratio bandwidth and 3-dB gain bandwidth.

Extracting Complex Dielectric Properties From Reflection-Transmission Mode Spectroscopy

Material characterization of homogeneous dielectric slabs using reflection–transmission mode spectroscopy can be problematic due to the ambiguity from a phasor term. A comprehensive analytical review of methods for calculating the normalized power spectra, to extract the effective complex dielectric properties of a sample, is undertaken. Three generic power response models (zero-order, power propagation, and electric-field propagation) are derived; these models act as a consolidated mathematical framework for the whole paper. With our unified engineering approach, the voltage-wave propagation, transmission line, and telegrapher’s equation transmission line models are then independently derived; the first two giving the same mathematical solutions, whereas the third generates the same numerical results, as the exact electric-field propagation model. Mathematically traceable simulation results from the various models are compared and contrasted using an arbitrarily chosen data set (window glass) from 1 to 100 THz. We show how to extract the approximate effective complex dielectric properties using time-gated time-domain spectroscopy and also the exact values with our theoretical graphical techniques from the first-order reflectance and transmittance. Our approach is then taken further by considering all the Fabry–Perot reflections with the frequency- and space-domain spectroscopy. With the scalar reflection–transmission mode infrared spectroscopy, we model the threshold conditions between the solution space that gives the single (exact) solution for the complex refractive index and the solution space that gives multiple mathematical solutions. By knowing threshold conditions, it is possible to gain a much deeper insight, in terms of the sample constraints and metrology techniques that can be adopted, to determine the single solution. Finally, we propose a simple additional measurement/simulation step to resolve the ambiguity within the multiple solution space. Here, sample thickness is arbitrary and no initial guesses are required. In theory, the result from this paper allows for the exact extraction of complex dielectric properties using simpler and lower cost scalar reflection–transmission mode spectroscopy.

CONTROLLED RADIATION FROM DIELECTRIC SLABS OVER SPOOF SURFACE PLASMON WAVEGUIDES

The radiation characteristics of dielectric slabs over a transmission waveguide, based on the concept of spoof surface plasmons, are studied in this paper. The proposed structure can be used to control the radiation over a wide band of operation, whilst retaining low Side Lobe Levels (SLLs) and cross-polarization. Leaky modes, broadside radiation and directive beams at flxed angles can all be obtained using various conflgurations (utilising homogeneous or gradient index dielectric slabs). The proposed antenna design has attractive performance for THz detectors and transmitters.

Efficient characterization of microwave applicator loaded with multilayer dielectric based on neural networks

In this paper, new neural network approach for characterization of cylindrical metallic cavity loaded with multilayer dielectric is suggested. Such cavity configuration has a great importance in realization of microwave resonant applicators, widely used for thermal processing of multilayer materials. Proposed neural model is based on representation of multilayer cavity load via several planparallel homogeneous dielectric slabs and superposition of separate influence of each dielectric slab parameters on the cavity resonant frequency. Model is incorporated into MLP neural network enabling its efficient implementation on modest hardware platform. Suggested approach is verified on the example of circular cylindrical metallic cavity loaded with two-layer dielectric.

Casimir Energy of the Evanescent Field between Inhomogeneous Dielectric Slabs

Using the mode-by-mode summation technique, we study the contribution of the evanescent modes to the Casimir energy between semi-infinite dielectric slabs whose plasma frequencies change linearly near their surfaces. The vacuum energy is defined as the summation of the zero-point energy over all possible TM-polarized evanescent modes, and the dispersion relation is numerically obtained using multilayer approximation and the transfer matrix method. In contrast to homogeneous dielectric slabs, the number of allowed modes between inhomogeneous dielectric slabs diverges in the continuous limit; however, our computational method affords the convergent Casimir energy. Furthermore, the effect of inhomogeneity in dielectric permittivity on the Casimir energy is evaluated.

Self-consistent inhomogeneous dielectric response profiles from a nonlocal continuous Lifshitz formulation of van der Waals interactions

We show that a nonlocal formulation of the Lifshitz theory of van der Waals interactions valid for continuous dielectric profiles can be used as a starting point to calculateself-consistent dielectric profiles. The general approach advocated here is very close to the formulation of electrostatic self-consistent charge density profiles obtained from the Poisson - Boltzmann theory. The calculated dielectric profiles obtained from minimization of the free energy containing a lattice gas entropy contribution on top of the Lifshitz interaction energy give quite surprising and interesting results. The ramifications of these results for the distribution of polarizable matter between two homogeneous dielectric slabs will be discussed in detail.

Analysis of the effects arising from the near-field optical microscopy of homogeneous dielectric slabs

The term “near-field optical microscopy” refers to a range of techniques capable of producing images of surfaces at a resolution far exceeding the Rayleigh criterion for conventional microscopes. In the illumination mode scanning near-field optical microscope, the sample is placed in the near field of an optical source formed from the aperture at the end of a fibre-optic taper. In normal operation, the aperture is raster scanned over the sample and, by recording the resultant far-field intensity transmitted through the sample as a function of the source coordinates, an image is assembled whose appearance derives from a non-trivial combination of the optical and topological composition of the sample. In this paper a numerical analysis of the near-field interaction of the source and sample is performed using a three-dimensional finite-difference time-domain method. A propagation technique is applied to the near-field values to form a far-field signal that is representative of the signal generated by a real instrument. In this way, the microscope output for a variety of very simple samples is simulated. It is shown that the magnitude of the transmitted signal can be increased both by decreasing the separation between source and sample and, in some circumstances, by increasing the sample permittivity. Simulated microscope output from samples containing topological variation is compared with that produced by samples containing a variation in refractive index and it is demonstrated that contrast inversion may occur at different scan heights in the latter case, leading to the possibility of distinguishing between the different contrast mechanisms experimentally.

Transmission characteristics of one‐dimensional image fiber‐strong coupling case

AbstractWhen an image is transmitted over a long distance by an image fiber in which a great number of homogeneous fibers is bundled, cross‐talk (mode coupling) is easily introduced and the resolution is decreased by blurring. In this paper, an exact numerical analysis of the transmission characteristics (cross‐talk characteristics) of one‐dimensional image fiber, constructed by arranging a finite number of homogeneous dielectric slabs at equal spacing, is carried out by means of wave‐optics for the strong coupling as well as the weak coupling case. The main conclusions drawn from this research are:(1) The weak coupling approximation theory on the cross‐talk characteristics of the image fiber is applicable to the case, which is usually considered as strong coupling.(2) When N element fibers are bundled in an image fiber, the mode generation is removed and each mode of single fiber separates into N different modes, some of which, if coupling is too strong, may turn into leaky modes.(3) The girth of the image fiber has an effect similar to that of a perfect conductor.(4) If the mode coupling in an image fiber is used deliberately, the structure can be used as a spatial image processing element such as planar lens, spatial filter, etc.