Numerical calculation of the reflection, absorption, andtransmission of microwaves by a nonuniform plasma slab

The reflection, absorption, and transmission of microwaves by a magnetized, steady-state, two-dimensional, nonuniform plasma slab is studied. A discussion on the effect of various plasma parameters on the reflected power, absorbed power, and transmitted power is presented. The nonuniform plasma slab is modeled by a series of subslabs. Even though it is assumed that the number density is constant in each subslab, the overall number density profile across the whole slab follows a parabolic function. The partial reflection coefficient at each subslab boundary is computed along with the absorption at each subslab. The total reflected, absorbed, and transmitted powers are then deduced and their functional dependence on the number density, collision frequency, and angle of propagation is studied     

Large negative Goos-Hänchen shift from a weakly absorbing dielectric slab

It is theoretically shown that the negative Goos-H?nchen shifts near resonance, Re[k(z)d] = m pi, can be an order of magnitude larger than the wavelength for both TE- and TM-polarized beams reflected from a weakly absorbing dielectric slab if the absorption of the slab is sufficiently weak, which is different from the case for a lossless dielectric slab [Phys. Rev. Lett. 91, 133903 (2003)].     

Giant positive and negative lateral shifts from the Kretschmann-Raether configuration with a weakly absorbing left-handed slab

The lateral shift of a TE-polarized beam reflected from the Kretschmann-Raether structure with a weakly absorbing left-handed slab is studied theoretically. It is shown that the lateral shift can be very large negative as well as positive near the resonant condition. These large negative and positive shifts can be one order of magnitude much larger than the shift from the corresponding nonabsorbing slab. As the absorption factor increases, the incident angle of producing largest lateral shift increases when the thickness is kept unchanged but the thickness of the slab of producing largest lateral shift decreases when the incident angle is kept unchanged.     

Interaction of electromagnetic waves with a magnetized nonuniform plasma slab

The absorption, reflection, and transmission of electromagnetic waves by a nonuniform plasma slab immersed in an ambient uniform magnetic field of various strengths are studied in this paper. The effects of the plasma parameters and magnetic field strength on the absorbed, reflected, and transmitted power are discussed. The magnetized nonuniform plasma slab is modeled by a series of magnetized uniform plasma subslabs. The calculation results show that the effects of the magnetic field strength and density gradient on the absorbed power, as well as the frequency band of resonant absorption, are significant. A complete analysis utilizing the scattering matrix method is also used to compare the above calculation results which neglect multiple reflections between subslab interfaces. Broadband absorption of electromagnetic waves can be achieved by changing the magnetic field strength and plasma density. More than 90% of the electromagnetic wave power can be absorbed in a magnetized nonuniform plasma slab with width of 12 cm and the absorption bandwidth can range from 1 to 20 GHz with different plasma parameters and external magnetic field strengths.     

均匀磁化等离子体与雷达波相互作用的数值分析

采用平板几何对不同磁场强度下等离子体对电磁波的吸收、反射和透射特性进行了数值分析.结果表明,不同磁场可以显著改变等离子体对不同频率的雷达波的吸收和反射特性.当雷达波频率接近等离子体高混杂频率时,磁化等离子体将对该波产生强的共振吸收,带宽在2GHz左右,吸收比可达90%以上.因此,通过适当调整磁场强度、等离子体密度和碰撞频率,可实现较宽雷达波段的等离子体隐身 关键词： 电磁波 磁化等离子体 共振吸收 等离子体隐身     

Numerical analysis of the reflection and absorption of electromagnetic wave in nonuniform magnetized plasma slab

In this paper, a model for calculating the reflection and absorption powers of electromagnetic wave (EM wave) in nonuniform magnetized plasma slab is given out based on layer propagation theory. The effects of various plasma parameters and different values of magnetic field intensity on the reflected and absorbed powers are discussed. The results illustrate that the thickness of plasma seldom affects the reflection of radar wave, but it can broaden or reduce the absorption width. Meanwhile, the background magnetic field intensity has an influence upon the results, and it could change the resonance spectrum of magnetized plasma. We also find out that, with appropriate plasma density, collision frequency and magnetic field intensity, more than 90% of radar wave power can be absorbed and the resonant absorption band is about 2 GHz.     

Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

弱双折射光纤布喇格光栅反射偏振对温度响应特性的研究

理论分析了切趾弱双折射光纤布喇格光栅反射偏振相关特性与温度之间的关系.数值模拟了切趾弱双折射光纤光栅的反射谱、偏振相关损耗和差分群时延随波长变化曲线.实验测出了不同温度下反射谱、偏振相关损耗和差分群时延随波长变化曲线.根据实验结果对偏振相关损耗和差分群时延的变化情况作出了分析.反射偏振相关损耗呈现两个峰值,随温度增加两峰漂移程度相同,表明偏振相关损耗无明显差异.差分群时延最大值随温度增加成线性向长波方向漂移,证明了光纤光栅正交模损耗变化的等同性.综合理论分析与实验结果表明:切趾弱双折射光纤布喇格光栅的偏振特性随温度产生明显的变化,其正交模变化呈现等比例特性.     

Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes

We study theoretically the mechanisms by which optical absorption is enhanced in an optical nanostructure consisting of a slab waveguide, made of a-Si:H, sandwiched between a periodic array of metallic nanowires and a substrate, both made of Au. We demonstrate that for the TM polarization the optical absorption in the slab waveguide can be enhanced by almost an order of magnitude by the excitation of plasmon modes, whereas for both the TM and TE polarizations the grating-induced excitation of slab waveguide modes leads to more than twofold enhancement of the optical absorption.     

Backing effects on the underwater acoustic absorption of a viscoelastic slab with locally resonant scatterers

Backing effects on the underwater acoustic absorption of a viscoelastic polymer slab embedded with locally resonant scatterers are reported. The polymer slab is embedded with two layers of locally resonant scatterers, i.e. Al spheres coated by soft silicon rubber. Theoretical absorption coefficients of the polymer slab under different backings using a layer multiple scattering method show good agreement with the experimental results, which supports unambiguously the experimental observation. Then relations between the resonance modes and the low-frequency absorption peaks of the composite slab are clarified to address the absorption mechanisms. It shows that the mass of the steel backing affects evidently the low-frequency absorption, the absorption peak shifts to lower frequency range while increasing the backing mass.     

Radiative heat transfer in a plane-layer mixture of non-gray particulates and molecular gases

The interaction between particulate clouds and molecular gases with respect to radiative heat transfer in a one-dimensional, plane-parallel geometry is investigated. The particulates are assumed to have a non-gray absorption coefficient with a power-law dependence on wavenumber. The absorption coefficient of the molecular gases is described by the popular exponential wide-band model. Heat transfer rates are determined through the evaluation of internal and external slab emissivities of the particulate cloud and through evaluation of slab band absorptances for the molecular gases, modified to account for the interaction with the particles. Simple, closed-form expressions for the slab emissivities and slab absorptances are also introduced, resulting in good accuracy.     

Analysis of absorption performances of anechoic layers with steel plate backing

Rubber layers with air-filled cavities or local resonance scatters can be used as anechoic coatings. A lot of researches have focused on the absorption mechanism of the anechoic coatings. As the anechoic coatings are bonded to the hull of submarine, the vibration of the hull should not be neglected when the analysis of the absorption characters is carried out. Therefore, it is more reasonable to treat the anechoic coating and the backing as a whole when the acoustic performance is analyzed. Considering the effects of the steel plate backing, the sound absorption performances on different models of anechoic coatings are investigated in this paper. The Finite Element Method is used to illustrate the vibrational behaviors of the anechoic coatings under the steel backings by which the displacement contours is obtained for analysis. The theoretical results show that an absorption peak is induced by the resonance of the steel slab and rubber layer. At the frequency of this absorption peak, the steel plate and the coating vibrates longitudinally like a mass-spring system in which the steel slab serves for mass and the coating layer is the spring. To illuminate the effects of the steel slab backing on the acoustic absorption, the thicknesses of the steel slab and the anechoic layer are discussed. Finally, an experiment is performed and the results show a good agreement with the theoretical analysis.     

Optimization of locally resonant acoustic metamaterials on underwater sound absorption characteristics

The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500 Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings.     

Ultrasonic reflection from randomly distributed cylindrical cavities

The propagation of steady-state time-harmonic waves in an elastic solid containing cylindrical cavities that are randomly distributed in a slab region is investigated. When an antiplane wave is normally incident on the cavities, the amplitudes of the coherent reflected and transmitted waves outside the slab are evaluated and plotted as functions of frequency and porosity.     

Transmission and group delay in a double microring resonator reflector

We present a microring resonator scheme for an optical reflector in which two microrings are coupled to two bus waveguides, to produce reflected signal at the input port. In this system, a single narrow reflected peak with maximum reflectivity near 100% in a wide spectral range due to the Vernier effect are achieved. We also investigate versatile reflection, transmission spectra and group delay characteristic. These effects are useful for applications in ring resonator-based photonic devices such as all-optical single wavelength tunable laser end mirrors, reflective wavelength switching, reflective filter and dual-wavelength delay line.     

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method. Based on the plane-wave angular spectrum expansion and Fresnel approximation, the analytical expressions of the reflected field, internal field as well as transmitted field in each region are obtained. Through numerical simulations, the intensity distributions of the incident beam, reflected beam, internal beam as well as transmitted beam are presented at oblique incidence. Besides, we also compare the intensity distributions of the geometrical-optics beam field, the first order beam mode field and the actual beam field, which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab. Meanwhile, the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated. Finally, the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail. The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.     

Average energy flow of optical pulses in dispersive media

The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points is shown to consist of two parts: a spectral superposition of group delays (inverse of group velocity) and a delay due to spectral reshaping via absorption or amplification. The result provides a context wherein group velocity is always meaningful even for broad band pulses and when the group velocity is superluminal or negative. The result imposes luminality on sharply defined pulses.     

磁化碰撞等离子体对雷达波的共振吸收

使用平板几何对雷达工作频段的电磁波在磁化碰撞等离子体中的传播作了数值计算,计算结果表明,在均匀等离子体中,当等离子体碰撞频率f∞=0.1,0.5,1GHz及电磁波频率接近高混杂频率时,电磁波衰减和被吸收功率出现最大峰值,即出现共振吸收;当fen=1,10GHz时,电磁波衰减、被吸收功率和透射功率曲线变得很平坦;衰减和吸收功率随等离子体密度的增大而增大,在n=10^11cm^-3时,衰减可达100dB,吸收比可达80％。在非均匀等离子体中,电磁波的反射功率峰值比在均匀等离子体中大。因此,磁化均匀等郭了体更有利于等离子体隐身。     

Field enhancement of gold optical nanoantennas mounted on a dielectric waveguide

In this paper, we investigate the localized field enhancement of optical nanoantennas consisting of different types of coupled gold nanoparticles and fed by a silicon nitrite slab waveguide. The optical nanoantenna is mounted on the surface of the waveguide, and the incident blue-violet light is guided in the slab waveguide and then reflected to the nanoatenna at the end of the waveguide. We also study the performance of the slab waveguide with different optimized parameters and also the field enhancement achieved by different shapes of nanoantennas.