Broadband LWIR and MWIR absorber by trapezoid multilayered grating and SiO<Subscript>2</Subscript> hybrid structures

A broadband metamaterial absorber with high absorption simultaneously in mid-wave infrared (MWIR) and long-wave infrared (LWIR) was proposed. In the MWIR, the absorption higher than 0.8 is from 4 to 6.3 µm, while the absorption in the LWIR is from 8.7 and 9.6 µm. The absorber is insensitive to the incident angle. The broadband absorption in the MWIR is due to the slow-light effect of the trapezoid multilayered grating structure. And the broadband absorption in the LWIR is due to the phonon polariton resonant of trapezoid SiO₂ layer. In the broadband high absorption region, the atmosphere is transparent, which may greatly promote the practical application of the absorber in double-color IR imaging, detecting, infrared stealth and thermal emitting.     

介电常数近零模式与表面等离激元模式耦合实现宽带光吸收

介电常数为零或近零模式在微纳结构中提供了一个新的方式调控光与物质的相互作用.本文首先利用金属圆盘阵列结构激发了表面等离激元共振,在共振频率处实现了光的局域效果;然后通过在金属-绝缘体-金属超表面微纳结构中加入掺杂半导体材料,利用上层金属圆盘阵列激发的表面等离激元共振诱导介电常数近零模式的产生,从而使得介电常数近零模式与表面等离激元模式发生耦合,在中红外波段实现了一个470 nm的宽带吸收效果;数值模拟结果显示,在宽带吸收处存在光场的强局域效果.与窄带吸收相比,宽带吸收有更广泛的应用,比如吸收器、传感器、滤波器、微测辐射热计、光电探测器、相干热发射器、太阳能电池、指纹识别和能量收集装置等.     

Broadband absorption enhancement with ultrathin MoS_2 film in the visible regime

The broadband absorption enhancement effect in ultrathin molybdenum disulfide(Mo S2)films is investigated.It is achieved by inserting the Mo S2 film between a dielectric film and a one-dimensional silver grating backed with a silver mirror.The broadband absorption enhancement in the visible region is achieved,which exhibits large integrated absorption and short-circuit current density for solar energy under normal incidence.The optical properties of the proposed absorber are found to be superior to those of a reference planar structure,which makes the proposed structure advantageous for practical photovoltaic application.Moreover,the integrated absorption and short-circuit current density can be maintained high for a wide range of incident angles.A qualitative understanding of such broadband absorption enhancement effect is examined by illustrating the electromagnetic field distribution at some selected wavelengths.The results pave the way for developing high-performance optoelectronic devices,such as solar cells,photodetectors,and modulators.     

High-absorbing gradient multilayer coatings with silver nanoparticles

We suggest an efficient way to enhance broadband visible light absorption in multilayer nanostructured systems formed by silver nanoparticle arrays. This approach is based on the explicit use of a gradient change in the parameters of the arrays (such as nanoparticle size or concentration). We analytically derive general conditions of the spectral characteristics of individual arrays which must be satisfied for the realization of highly-absorptive gradient multilayer nanostructured coatings. Two specific types of gradient coatings, (i) with the gradient of the size of the silver nanoparticles in the arrays and (ii) with the gradient of their surface concentration, are numerically studied in detail. The obvious advantages of the size-gradient coatings are revealed, in particular their total thickness of less than a wavelength. Multilayer coatings with a concurrent gradient of both concentration and mean particle size are fabricated from silver island films and their high and broadband absorption is demonstrated experimentally. PACS 78.67.Pt; 78.67.-n; 73.20.Mf; 42.25.Bs     

On the theory of Thomson self-oscillatory systems

A nonlinear mechanism of the effect of broadband fluctuations on oscillations in a Thomson self-oscillator has been revealed. The correlation function for oscillations, which takes into account all appreciable effects in the second approximation, has been obtained for the first time. A new method has been developed for analyzing steady-state synchronous oscillations in a generator forming a part of the phase-synchronization system; linearized equations of steady-state synchronous oscillations, which take into account the effect of broadband noise, have been developed for the first time. An example of calculation of a complex phase-synchronization system is considered, in which the possibility of synthesizing highly effective systems with the help of the proposed methods of analysis is illustrated. The diffusion coefficients of the phase are estimated.     

Broadband light absorption using a multilayered gap surface plasmon resonator

A broadband visible light absorber composed of multiple metal-dielectric-metal (MMDM) layers is proposed and numerically investigated. Dielectric layers of different thicknesses in the MMDM structure lead to multiple plasmon resonances at different wavelengths; as a result, efficient broadband absorption can be achieved under optimized conditions. We found that an average simulated absorption of 93 % was obtained over the entire visible spectrum of 400–700 nm by controlling the geometric parameters. Furthermore, the origin of the broadband absorption was studied, and the effects of the diameter and pitch of the pattern on the absorption were investigated. Our proposed structure with a periodic array of circular patterns represents a novel candidate for future applications in photovoltaic cells and thermal emitters.     

Metallic subwavelength structures for a broadband infrared absorption control

We present a method to control the absorption of a resonator by using a subwavelength structure consisting of thin metallic plates that behaves as a metamaterial film. We demonstrate the ability to tailor the conductivity of such a metallic subwavelength structure to achieve a resonator with the desired impedance matching for the mid-infrared range. This approach provides for broadband, as well as broad-angle, enhanced absorption. Theoretical analyses, as well as experimental results of the optical properties of a metallic NiCr structure at 8-12 microm spectral range are introduced.     

基于超材料的偏振不敏感太赫兹宽带吸波体设计

本文设计了一种基于超材料的偏振不敏感太赫兹宽带吸波体.吸波体包含两层金属和一层中间介质,表面金属层每一个周期单元由五种尺寸接近的金属块按照相邻不同的规律排列成5×5的方形阵列.各种尺寸金属块分别产生单峰谐振吸收,五个谐振吸收峰相互靠近从而产生宽带吸收.通过研究吸波体表面电流和电场z分量分布情况可知,入射太赫兹能量的吸收主要是由y方向上电场引起的电偶极子振荡和z方向上磁场引起的磁极化产生,而且金属层的欧姆损耗起主要作用.仿真结果表明,吸波体吸收率在80%以上的带宽约为1.2 THz,最高吸收率可达98.7%,半峰全宽(FWHM)为1.6 THz,该宽带吸波体的厚度约为中心波长的二十分之一,对偏振方向不敏感,且能实现大角度吸收,在太赫兹频段的电磁隐身、测辐射热探测器以及宽带通信等领域有潜在的应用价值.     

Electromagnetic modeling of near-field phase-shifting contact lithography with broadband ultraviolet illumination

Near-field phase-shifting contact lithography is modeled to characterize electromagnetic absorption in a photoresist layer with one face in contact with a quartz binary phase-shift mask. The broadband ultraviolet illumination is represented as a frequency-spectrum of normally incident plane waves. A rigorous coupled-wave analysis is carried out to determine the absorption spectrum of the photoresist layer. The specific absorption rate in the photoresist layer is calculated and examined in relation to the geometric parameters. Columnar features in the photoresist layer are of higher quality on broadband illumination in contrast to monochromatic illumination, in conformity with some recent experimental results. Feature resolution and profile are noticeably affected by the depth of the grooves in the phase-shift mask. Ideally, the feature linewidth can be less than about 100 nm for broadband illumination in the transverse-magnetic mode. These conclusions are subject to modification by the photochemistry-wavelength characteristics of the photoresist.     

Acoustic Absorption Characteristics of New Underwater Omnidirectional Absorber

We investigate a new underwater omnidirectional absorber with acoustic black hole effect to realize a broadband omnidirectional acoustic wave absorption. Based on multiple scattering theory, a two-dimensional axisymmetric model of underwater omnidirectional absorber comprised of an acoustic gradient refractive index structure and a hollow core is developed, and the mechanisms of omnidirectional absorption and dissipation of acoustic waves are studied. The numerical results indicate that the omnidirectional absorber developed here can achieve the omnidirectional absorption of incident acoustic waves in a broadband frequency and can effectively reduce the backscattering of acoustic waves. It potentially provides a new notion for underwater acoustic coating design.     

一种新型双共轭链分子非线性光学性质的理论研究

对实验室合成的双共轭链分子1,4-二(4-二乙胺基苯乙烯基)-2-［4-(N-甲基-N-羟乙基)氨基-4′-硝基偶氮苯］-5-己烷氧基苯(BSBAB)及合成它的单共轭链分子的单光子和双光子吸收特性在从头计算的基础上利用密度泛函理论进行了研究.分子BSBAB的优化结构显示，组成该分子的横链和纵链除了保持各自的共轭面外，几近相互垂直.因此，分子BSBAB较好地继承了两个单共轭链分子的光学特性.计算结果表明，在低能量范围内，分子BSBAB具有三个双光子吸收峰，分别来自于两个单共轭链分子以及两者的耦合作用.从理论上证明了双共轭链分子BSBAB是一种具有宽带强双光子吸收的分子材料.理论结果和实验结果符合得较好.在HF水平上的响应函数方法进一步证实了有限态求和方法计算结果的可靠性.还给出了电荷转移态的电荷迁移过程. 关键词： 双光子吸收 双共轭链有机分子 非线性光学     

Broadband visible metamaterial absorber based on a three-dimensional structure

In this paper, a broadband metamaterial absorber is successfully designed by a three-dimensional structure. And the three-dimensional absorber is just obtained by a two-dimensional structure which rotates 90°along x-axis. The simulated results show that the absorption of the three-dimensional metamaterial absorber is much better than the two-dimensional absorber. Moreover, the absorber is polarization-sensitive for the incident electromagnetic waves due to the asymmetry of the structure. Compared with the Y-polarization wave, the proposed absorber can realize broadband absorption with greater than 90% from 355.6 to 737.7 THz for X-polarized wave. Finally, based on the analysis of the electric field and surface current distributions, it can demonstrate that the localized surface plasmons and dipoles resonances will play an important role in the broadband absorption. And we believe that the metamaterial absorber will have many potential applications in emitter and energy harvesting.     

基于第一性原理计算的掺杂柔性黑硅吸收光谱特性分析

针对宽波段微型光谱仪缺乏宽波段柔性探测器这一难题,提出了一种在紫外-可见-近红外波段内具有高吸收效率的掺杂柔性黑硅作为探测器吸光材料。首先,基于第一性原理计算了S和F元素掺杂后柔性黑硅的电子结构、能带结构和紫外-可见-近红外波段的光学吸收特性,得到了不同元素及浓度掺杂时,柔性黑硅的光学吸收系数。其次,将第一性原理计算结果与时域有限差分算法相结合,建立了柔性黑硅的吸收光谱模型。结果表明,掺入S和F元素后柔性黑硅的能带带隙均减小,吸收截止波长发生红移,且掺杂浓度越高,光学吸收系数越大。在1 500 nm波长处,50%浓度的S元素掺杂黑硅的吸光系数是1.5%浓度的S元素掺杂黑硅的吸光系数的8.3倍,50%浓度的F元素掺杂黑硅的吸光系数是1.5%浓度的F元素掺杂黑硅的吸光系数的3倍。在相同掺杂条件下,表面具有小尺寸微结构的柔性黑硅在近红外波段具有最高的吸收效率。最后,测试了制作的柔性黑硅样品,其吸光效率在紫外-可见波段高于95%,在近红外波段为70%~80%。     

Gain without population inversion in a two-level atom damped by a broadband squeezed vacuum

We study the absorption spectrum of a two-level atom driven by a resonant laser field and damped by a broadband squeezed vacuum. We show that for some values of the Rabi frequency of the driving field the central component of the absorption spectrum can switch from absorption to amplification. We find that the presence of the squeezed vacuum field is essential for the gain at the central component. Moreover, the gain is not attributed to any population inversion in both the atomic bare states and in the dressed atomic states. We show that the gain originates from the coherent population oscillations, which are significantly enhanced when the atom is damped by a broadband squeezed vacuum.     

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.     

Enhanced and broadband absorber with surface pattern design for X-Band

A broadband and thin-layer microwave absorber is designed based on surface pattern design made by carbonyl iron and rubber composite. The bandwidth with reflection less than −10 dB covers the full X-band owing to two absorption peaks appeared simultaneously in both the simulation results and experimental results. In this work, the power loss and power flow diagram were present by CST simulation, which clearly explain the broadband absorption caused by double λ/4 matching absorption and interfacial scattering synergistic effect. A facile splicing method was provided to extend the absorption bandwidth for the magnetic absorbing materials.     

Broadband low-frequency acoustic absorber based on metaporous composite

Broadband absorption of low-frequency sound waves via a deep subwavelength structure is of great and ongoing interest in research and engineering. Here, we numerically and experimentally present a design of a broadband low-frequency absorber based on an acoustic metaporous composite (AMC). The AMC absorber is constructed by embedding a single metamaterial resonator into a porous layer. The finite element simulations show that a high absorption (absorptance A>0.8) can be achieved within a broad frequency range (from 290 Hz to 1074 Hz), while the thickness of AMC is 1/13 of the corresponding wavelength at 290 Hz. The broadband and high-efficiency performances of the absorber are attributed to the coupling between the two resonant absorptions and the trapped mode. The numerical simulations and experimental results are obtained to be in good agreement with each other. Moreover, the high broadband absorption can be maintained under random incident acoustic waves. The proposed absorber provides potential applications in low-frequency noise reduction especially when limited space is demanded.     

Using a thin actuator as secondary source for hybrid passive/active absorption in an impedance tube

This paper describes the practical implementation of a piezoelectric actuator as secondary source for hybrid passive/active broadband sound absorption in a standing-wave tube. This actuator consists of a thin circular aluminium plate driven by a piezoelectric patch and glued to a flexible rubber support. The resulting device has been mounted in a thin metallic ring that fits perfectly to the tube diameter. Passive absorption is afforded by either a porous layer or a microperforated panel, backed by an air gap. Active absorption is accomplished by releasing the sound pressure at a microphone behind the material, using either a loudspeaker or the actuator as secondary source. Results of broadband sound absorption reveal the feasibility of the piezoactuator. Compared to the loudspeaker, this actuator allows to greatly reduce the whole thickness of the hybrid absorber.     

Design of a type of broadband metamaterial absorber based on metal and graphene

In this paper, we demonstrate a kind of broadband metamaterial perfect absorber using both graphene and metal resonator elements. Through step by step design and simulation, wider absorption band from about 4.22 THz to 7.48 THz with average absorption rate up to 98.21% is achieved in the absorption spectrum. In addition, the absorber has characteristics of polarization insensitivity and wide incident angle due to its inherent rotational symmetry. Moreover, the absorption band can be adjusted by changing the chemical potential of the graphene. The superiorities of broadband, high absorption rate, polarization independent and wide-angle characteristics make it have potential application prospects in electromagnetic wave absorbing, signal sensing and detection, and other optoelectronic devices.