Electric Field Amplitude Distributions Research Articles

Analytical approach for calculation of aperture efficiency enhancement of horn antennas using a thin metal cylinder

AbstractThis study aims to increase horn antenna gain and aperture efficiency in a way that are compatible with aperture electric field distribution similar to TE10 rectangular waveguide mode using analytical calculation method. The proposed analytical approach can be useful to calculate the performance of a horn antenna design with thin (r«λ) perfect electric conductor (PEC) cylinder placed on the antenna aperture dividing H‐plane. In the analytical solution, the aperture field distribution is obtained by solving the electromagnetic scattering from an infinitely long cylinder placed in front of the aperture. This structure not only improves the aperture electric field amplitude distribution but also corrects phase errors in the horn antenna. An X‐band pyramid horn antenna is manufactured by using three‐dimensional (3‐D) printer and coated with aluminum tape. The measurement results of the 3‐D printed antenna are also compared with the simulation results. The antenna gain is measured with and without the PEC cylinder on the aperture. It is found that, the antenna gain is increased by 1 dB. The results of the proposed analytical calculation method are compared with the simulation and measurement results and justified.

Circular Polarization Selective Metamaterial Absorber in Terahertz Frequency Range

In this article, we propose a chiral metamaterial absorber based on four rotated twisted closed ring resonators operative in terahertz frequency range. The resonant features are analyzed by electric field amplitude distributions, and surface currents flow generated by the interaction of circularly polarized waves with the chiral structure. The highest field amplitudes at the fundamental resonance are obtained at the gap region of the resonator. Therefore, the efficiency of the proposed absorber is determined through response of absorption spectrum around this resonance with variation of geometrical parameters of the structure. Significant increase of absorption of the left circularly polarized light is obtained with variation of the period size of structure. Consequently, huge circular dichroism, and near-unity polarization selective absorption have been provided. This makes the proposed chiral structure a good candidate for designing circular polarization selective absorber.

Actively tunable polarization-sensitive multiband absorber based on graphene* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11804178 and 11274188) and the Natural Science Foundation of Shandong Province, China (Grant No. ZR2018BA027).

We design an actively tunable polarization-sensitive multiband absorber in the mid-infrared region, which consists of stacked graphene multilayers separated by dielectric layers on a metal mirror. Benefiting from the anisotropic structure, the absorber has dual absorption bands with almost perfect absorption at different wavelengths under the x and y polarizations. Analyzing the electric field amplitude distributions and the surface currents, we find that the absorption peaks under the same polarization are excited in the graphene layers independently. Therefore, more absorption bands can be achieved by increasing the graphene layers. Adjusting the Fermi energy of the graphene layers, the working wavelengths of the polarization-sensitive multiband absorbers can be tuned actively, and thus achieving a wide band regulation range. Besides, the peak number and the peak strength of the multiband absorber can be actively controlled by the polarization angle as well. We also propose a method to design an actively tunable polarization-sensitive multiband absorber, which may have potential applications in mid-infrared devices, such as polarization-sensitive filters and detectors.

Bi-functional switchable broadband terahertz polarization converter based on a hybrid graphene-metal metasurface.

In this letter, we have proposed a bi-functional switchable broadband polarization converter based on the hybrid graphene-metal metasurface. Turning the bias voltage to change Fermi level Ef from 0 to 1.0 eV, the metasurface can switch between quarter-wave plate (QWP) and half-wave plate (HWP) in the frequency band 1.38-1.72 THz. Besides, the metasurface simultaneously works as a broadband QWP and HWP in different frequency range when Ef = 1.0 eV. In addition, when Ef is in the range of 0.3 eV-0.6 eV, the metasurface can work as bi-functional broadband QWP in different frequencies as well. The physical mechanism of the bi-functional polarization converter can be explained by the electric field amplitude distributions. What's more, we find that the metasurface can work well with a tolerance to the incident light polarization angle of about ± 12.5°, which can also change the converted wave from RHCP to LHCP with the incident polarization angle change of 90°. The hybrid metasurface with the advantages of switchable bi-functions, wide operating bandwidth, and ultra-thin thickness, may achieve potential applications in tunable devices for terahertz communications.

Numerical investigation on beam–wave interaction of W-band extended interaction oscillator

Based on the basic principle of energy conservation and electron disk beam–wave interaction equation, combined with the characteristic impedance and quality factor of the oscillator, a self-consistent nonlinear theoretical model of extended interaction oscillators (EIOs) is obtained. Taking the W-band EIO as an example, the effects of operating parameters and structural parameters on the oscillation, output power, and electron conversion efficiency of the extended interaction oscillator are analyzed. The numerical implementation of this model shows that the electron beam–wave synchronization interaction is a basic prerequisite for the oscillator to start, the positive feedback to energy in the resonator is a necessary condition for spontaneous oscillations, the distribution of electric field amplitude on each gap directly affects the beam–wave interaction efficiency and output power, and the electron trajectories in all gaps at steady state illustrate the conversion efficiency. At an operating voltage of 18 kV and a direct current of 0.5 A, a seven-gap EIO model with an output power of 2629 W and electron efficiency over 29% is predicted for gradually increased electric field amplitude distribution on each cavity.

All-dielectric transformation medium mimicking a broadband converging lens.

Radio waves carrying orbital angular momentum (OAM) may potentially increase spectrum efficiency and channel capacity based on their extra rotational degree of freedom. However, due to their divergence characteristics, vortex waves are not suitable to transmit over a long distance in the radio frequency (RF) and microwave domains. In this paper, a transformation optics (TO) based all-dielectric converging lens is proposed. The beam divergence angle of the vortex wave passing through the lens can be decreased from 25° to 9°. The transformed material parameters of the converging lens are determined by solving Laplace's equation subject to specific boundary conditions. Far-field antenna radiation patterns as well as near-field helical phase and electric field amplitude distributions obtained from numerical simulations are reported, demonstrating the broadband characteristics of the proposed microwave lens. Moreover, the all-dielectric compact lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.

Templated fabrication of periodic arrays of metallic and silicon nanorings with complex nanostructures

Here we report a scalable colloidal templating approach for fabricating periodic arrays of metallic and silicon nanorings with complex nanostructures. Non-close-packed monolayer silica colloidal crystal prepared by a simple spin-coating technology is first used as template for making periodic arrays of mushroom-like composite nanostructures consisting of silica spherical caps and polymer stems. Subsequent metal sputtering and reactive ion etching lead to the formation of ordered asymmetric nickel nanorings which can be further utilized as etching masks for patterning periodic arrays of symmetric silicon nanorings. Moreover, periodic arrays of metallic and silicon concentric double nanorings can be fabricated by using the asymmetric nickel nanorings as templates. We have also demonstrated that gold concentric double nanorings show strong surface-enhanced Raman scattering (SERS) with a SERS enhancement factor of ∼9.5 × 107 from adsorbed benzenethiol molecules. The SERS enhancement and the electric field amplitude distribution surrounding gold concentric double nanorings have been calculated by using finite element electromagnetic modeling. This new colloidal templating technique is compatible with standard microfabrication and enables wafer-scale production of a variety of periodic nanorings with hierarchical structures that could find important technological applications in plasmonic and magnetic devices.

Effect of the Interindividual Variability on Computational Modeling of Transcranial Direct Current Stimulation.

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique that delivers low intensity, direct current to cortical areas facilitating or inhibiting spontaneous neuronal activity. This paper investigates how normal variations in anatomy may affect the current flow through the brain. This was done by applying electromagnetic computational methods to human models of different age and gender and by comparing the electric field and current density amplitude distributions within the tissues. Results of this study showed that the general trend of the spatial distributions of the field amplitude shares some gross characteristics among the different human models for the same electrode montages. However, the physical dimension of the subject and his/her morphological and anatomical characteristics somehow influence the detailed field distributions such as the field values.

Array truncation effects in infrared frequency selective surfaces

A metasurface consisting of an infinite array of square loops was designed for maximal absorptivity for s-polarized light at a wavelength of 10.6 µm and 60 degrees off-normal. We investigate the effects of array truncation in finite arrays of this design using far-field FTIR spectroscopy and scattering scanning near-field optical microscopy. The far-field spectra are observed to blue-shift with decreasing array size. The near-field images show a corresponding decrease in uniformity of the local electric field amplitude and phase spatial distributions. Simulations of the far-field absorption spectra and local electric field are consistent with the measured results.

Influence of Electric Field Distribution on High-Power Array Antenna Radiation Pattern with Rectangular Aperture

The antenna element with rectangular aperture is one of the main forms of the array antenna. The electric field amplitude distribution of the rectangular aperture, as well as the phase distribution is the most important parameter that affects the radiation gain and beam direction of the array antenna. In this work, a theoretical study is carried out on array antennae for high-power microwave (HPM) applications. An electric integration method is applied to obtain the far-field radiation pattern with different kinds of electric field distributions. Moreover, the influence of the electric field amplitude and phase on the performance of the array antenna is analyzed. For one antenna element, uniform electric field distribution is not the best choice. However, the uniform distribution has specific advantages for an array antenna consisting of combined antenna elements. The phase deviation has more significant influence on the performance of the array antenna than the amplitude deviation. It indicates that a good working phase shifter with high-power capacity and time-adjusting capability is very important.

Investigation of Positive and Negative Bias Temperature Instability of High-κ Dielectric Metal Gate Metal–Oxide–Semiconductor-Field-Effect-Transistors by Random Telegraph Signal

We investigate the impact of positive and negative bias temperature instability (P/NBTI) on the current degradation by using the drain current random telegraph signal (IdRTS) in high-κ gate dielectric and metal-gate MOSFETs. The samples were stressed at typical BTI oxide electric field (∼7 MV/cm) and IdRTS amplitude (ΔIdRTS) distributions were measured before and after P/NBTI stress. It is shown that IdRTS degradation (ΔIdRTS/IdRTS) in NBTI devices exhibits a wider amplitude distribution than the PBTI one. In addition, we trace trapped charge-induced saturation current IDsat degradation in P/NBTI stress. As a result, the statistical analysis indicates that ΔIDsat/IDsat of the p-MOSFET is larger than ΔIDsat/IDsat of the n-MOSFET. Moreover, the lifetime of the device is numerically estimated based on current degradation with or without RTS. We consequently found that P/NBTI lifetime comes worse by considering the effect of RTS.

Surface plasmon resonance and surface-enhanced Raman scattering sensing enabled by digital versatile discs

We report the simultaneous achievement of sensitive surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS) sensing using digital versatile discs (DVDs). The aluminum-covered data tracks of DVDs can efficiently concentrate local electromagnetic field, resulting in an excellent SPR medium (with sensitivity of ∼850 nm per refractive index unit which is higher than that of most available nanofabricated plasmonic substrates) with a small but measurable SERS enhancement. The SPR sensitivity and the optical spectra, as well as the distribution of electric field amplitude, are simulated using a finite-difference time-domain model. The theoretical predictions agree reasonably well with the experimental results.

A study on the effect of filling factor for laser action in dielectric random media

In a random system composed of dielectric materials, laser action is expected to occur from the light localization caused by multiple scattering and interference effect. The wavelengths at which the laser oscillation occurs in random media depend on the mean free path of continuous scattering. Since the mean free path of continuous scattering is dependent on the filling factor, it is important to investigate systematically the effect of the filling factor for the laser action. In the present study, we calculate the Poynting vectors of the light emitted from two-dimensional random media consisting of dielectric rods with population inversion modeled by the negative imaginary part of relative permittivity. The spectra of the radiated Poynting vectors from the two-dimensional random media are shown for several different filling factors and for various values of population inversion. We try to find laser generation which occurs at low population inversion and discuss about the threshold of laser action occurring in dielectric structures with different filling factors. We also study the spacial distribution of the electric field amplitude in several frequency regions where light strongly amplifies.

Self-Collimation in Square Lattice Two Dimensional Photonic Crystals with Ring-Shaped Holes

We demonstrate self-collimation phenomena based on a new type of photonic crystals made of square lattice with ring shaped holes. The plane wave expansion (PWE) method is used to get the three dimensional band diagram and equi-frequency of the second band which displays the self-collimation phenomena for the structure we proposed in this paper. The collimation angle is mainly depending on the maximum flatness half width (MFHW) of the equi-frequency. The FDTD method is employed to demonstrate the electric field amplitude distributions for the collimation phenomena. Partly, in order to achieve high efficient coupling of the input and output port, we modify both surface structures to modulate the wave-front to obtain desired effect. The parameter of the input surface is modified which will prevent the production of surface modes which takes away the EM power and enhance the transmittance. For a square lattice with the modified parameters at each side of the input surface, the surface modes are suppressed to couple with the continuum of the dielectric waveguide modes. More importantly, they might have potential application in integrated optical circuits.

O princípio de Huyghens, a óptica de Fourier e a propagação de feixes de laser

Neste trabalho é apresentada uma abordagem que foi adotada para o ensino de óptica física e de propagação de feixes de laser para alunos provenientes de um Curso de Bacharelado em Matemática, no programa de Iniciação Científica do Instituto de Estudos Avançados. Inicialmente é feita uma apresentação fenomenológica do Princípio de Huyghens e de como o tratamento matemático adequado deste princípio resulta nos conceitos básicos da optica de Fourier. Posteriormente é postulada a existência de um feixe de laser cuja distribuição de amplitude de campo elétrico é dada por uma função gaussiana e foi calculada a propagação deste feixe usando a óptica de Fourier. Como resultado deste cálculo foram obtidas as propriedades de propagação do feixe de laser gaussiano. Embora os resultados a respeito da propagação de feixes de lasers gaussianos encontrados neste trabalho sejam amplamente divulgados na literatura, a abordagem se mostrou muito útil para iniciar alunos na área de óptica física sem que seja necessário passar pela teoria do eletromagnetismo

Electro-optic measurement of terahertz pulse energy distribution

An accurate and direct measurement of the energy distribution of a low repetition rate terahertz electromagnetic pulse is challenging because of the lack of sensitive detectors in this spectral range. In this paper, we show how the total energy and energy density distribution of a terahertz electromagnetic pulse can be determined by directly measuring the absolute electric field amplitude and beam energy density distribution using electro-optic detection. This method has potential use as a routine method of measuring the energy density of terahertz pulses that could be applied to evaluating future high power terahertz sources, terahertz imaging, and spatially and temporarily resolved pump-probe experiments.

Electrochemical SERS at Periodic Metallic Nanopyramid Arrays

This paper reports a simple and scalable colloidal templating nanofabrication technology for generating periodic metallic nanopyramid arrays as electrodes for electrochemical surface-enhanced Raman spectroscopy (SERS). These periodic arrays of nanopyramids with nanoscale sharp tips and high tip density can enhance the local electromagnetic field in the vicinity of the nanotips, resulting in high SERS enhancement (on the order of 106). The effect of the applied electrode potential and the electrode redox reactions on the SERS enhancement has been investigated. Finite element electromagnetic modeling has also been developed to simulate the electric field amplitude distribution and the corresponding Raman enhancement factors surrounding arrays of nanopyramids.

Optimal pixel profiles for spatially discrete coherent imaging systems

The term “pixel profile” refers to the spatial distribution of electric field amplitude within the aperture of a single pixel of a spatial light modulator. We discuss the use of non-uniform pixel profiles for increasing the energy throughput of spatially discrete coherent optical imaging systems. Prolate spheroidal pixel profiles are shown to be optimal and can produce throughput gains of up to 32% for optical systems with rectangular symmetry and 27% for those with circular symmetry. These throughput gains are shown to impact the capacity of volume holographic data storage systems. A simple M/#-based analysis predicts a 15% capacity increase as compared with the use of uniform pixel profiles.

Computation of electric field components and standing wave coefficient for a waveguide structure with a lowohm semiconductor plate

The boundary problem for Maxwell equations has been solved and computer codes have been written to compute the electrodynamic characteristics of the waveguide structure. The standing wave coefficient's (SWC) dependence on sizes of a n-Si plate, displacement from the waveguide's symmetry plane and electric field amplitude's distribution in these plates have been computed.

On the amplitude of intense Langmuir waves in the terrestrial electron foreshock

Waveforms of large‐amplitude Langmuir oscillations were recorded by the Wind spacecraft in the Earth's upstream electron foreshock region. We present some statistics of the waveforms and discuss them in the context of various saturation mechanisms. In particular, it is found that the value of Epeak/Erms is not large, as previously suggested, and that the largest‐amplitude Langmuir waveforms are generally somewhat sinusoidal and lack structure on small spatial scales. The measured probability distribution of electric field amplitude and dimensionless energy suggest that some stochastic process may play a role in wave generation. The values of dimensionless energy needed to arrest Langmuir wave collapse occur with very small probability and the value of Epeak/Erms for large fields suggests that, statistically, Langmuir wave collapse is not an important process in the terrestrial foreshock.