Polarization-Independent Broadband Optical Regime Metamaterial Absorber for Solar Harvesting: A Numerical Approach

In this article, a broadband metamaterial (MTM) absorber is proposed that exhibits near-unity absorption in the terahertz regime. The proposed metamaterial absorber was initiated on a quartz (fused) substrate, whereas the resonator and backplane are constructed with tungsten. The resonator is designed with a square ring loaded with a face-to-face E structure at the center. It also consists of diagonally extended arrow-like shapes loaded from the corners and a concave-shaped structure extended from the middle of the square ring. Near-perfect absorption is observed at the frequencies of 465.2 THz, 585.2 THz, 648.8 THz, and 762.8 THz with absorption peaks of 99.8%, 99.9%, 99.92%, and 99.92%, respectively. Moreover, it exhibits broadband absorption properties above 90% absorption with bandwidths 20.4 THz, 80.8 THz, 41.6 THz, and 90 THz, respectively, at these resonance frequencies. Due to its symmetrical structure, it shows polarization-insensitivity behavior up to 90° with maximum absorption greater than 90% both in transverse (TE) and transverse magnetic (TM) modes. It also exhibits insensitivity to changes of incident angle from 0°–45°. Metamaterial properties of the proposed absorber are also analyzed, showing single negative behavior. Absorber property has been examined through surface current and equivalent circuit electric and magnetic field analysis. The effect of the cross-polarization is negligible and is verified through simulation. Due to its large bandwidth, polarization-insensitive behavior, and low PCR, the proposed MTM absorber can be incorporated into photovoltaic devices as a solar-energy harvester.     

Enhanced absorption process in the thin active region of GaAs based p–i–n structure

The optical absorption is the most important macroscopic process to characterize the microscopic optical transition in the semiconductor materials. Recently, great enhancement has been observed in the absorption of the active region within a p–n junction. In this paper, Ga As based p–i–n samples with the active region varied from 100 nm to 3 μm were fabricated and it was observed that the external quantum efficiencies are higher than the typical results, indicating a new mechanism beyond the established theories. We proposed a theoretical model about the abnormal optical absorption process in the active region within a strong electric field, which might provide new theories for the design of the solar cells,photodetectors, and other photoelectric devices.     

An ultra-broadband metamaterial absorber based on the hybrid materials in the visible region

In this paper, an ultra-broadband metamaterial absorber is successfully designed in the visible region. The structure of the absorber is just obtained by the two-dimensional plane structure which rotate 90° along x-axis. Furthermore, the formation of the structure for the hybrid materials is based on the four U-shaped structure of the metal titanium is embedded in the semiconductor (indium antimonide). The simulated results show that the proposed metamaterial absorber can achieve an ultra-broadband absorption with greater than 90% from 252.2 to 822.3 THz, and the relative absorption bandwidth gets to 106.1%. Finally, the simulated electric field, surface current and power loss density distributions further illustrate the absorption mechanism of the metamaterial absorber. And we believe the metamaterial absorber will have many potential applications in energy harvesting and stealth devices.     

Tunable dual-band terahertz graphene absorber with guided mode resonances

A tunable dual-band terahertz absorber is designed and investigated. The unit cell of the proposed absorber consists of a graphene monolayer on a guided-mode resonant filter. The graphene absorber presents > 40% absorption at two resonance frequencies, which is attributed to the guided mode resonances with different mode numbers. The electric field intensity distribution is analyzed to disclose the physical mechanism of such a dual-band absorption effect. Furthermore,the influence of optical properties of graphene, including Fermi level and relaxation time, on the absorption spectra are investigated. Finally, the influence of geometric parameters on the absorption spectrum is studied, which will provide useful guidance for the fabrication of this absorber. We believe that the results may be useful for developing the next-generation graphene-based optoelectronic devices.     

圆环结构人工电磁吸波材料的仿真与实验研究

提出了两种圆环型微波电谐振结构,这两种谐振结构分别与金属线进行适当的组合形成电磁耦合单元,对正入射的电磁波产生谐振响应并具有强烈的吸收作用.通过对两种耦合单元的电磁仿真和优化计算使其各自获得最佳的吸波性能.在微波段8—12 GHz利用自由空间法对这两种耦合结构的电磁特性(即反射系数S₁₁和透射系数S₂₁)进行实验测量,在10.7 GHz和10.07 GHz吸收率分别达到95%和98%以上. 关键词： 耦合单元 吸收率 阻抗匹配     

超薄柔性透射型超构材料吸收器

设计并加工了一种超薄柔性透射型吸收器,总体厚度为0.288 mm,可实现柔性弯曲,容易做到与曲面目标共形.该吸收器由三层结构组成,底层是金属光栅,中间为介质层,表面单元由两条平行放置的尺寸不同的金属线组成.仿真和实验结果表明,对横电波在5和7 GHz的吸收分别达到97.5%和96.0%,对横磁波在3.0—6.5 GHz都能保持90%以上的透射率.两个吸收频点可分别独立调节,增加了设计的灵活性.另外,当入射角增大到60°时,该吸收器的性能基本不受影响,表现出良好的广角特性.     

极化控制的双波段宽带红外吸收器研究

红外吸收器在红外隐身、辐射制冷、红外探测、传感器等方面有重要的应用前景.一维光栅型吸收器由于其结构简单、易于加工的优势备受关注,然而其不足之处是频带很窄,且只对一种极化有效.本文提出了一种基于简单一维周期结构的双波段宽带吸收器,对横磁波和横电波都有效,且吸波频段随入射波的极化方式而改变.该结构的基本单元由八个梯度排列的子单元构成,每个子单元由两层金属-介质双层膜垂直层叠组成.全波仿真结果表明,在1.68—2μm波段,该结构对横磁波吸收超过90%,而对横电波吸收很小(小于6%);在3.8-3.9μm波段,该结构对横电波吸收超过90%,而对横磁波吸收很小(小于5%).另外,该结构具有宽角度吸收特性,当入射角增大到60°时仍然能够保持较高的吸收率和较宽的吸收频带.     

A novel coupled quantum well structure and its excellent electro-optical properties

A novel InGaAs/InAlAs coupled quantum well structure is proposed for large field-induced refractive index change with low absorption loss. In the case of low applied electric field of 15 kV/cm and low absorption loss (α≤ 100 cm-1), a large field-induced refractive index change (for transverse electric (TE) mode, △n = 0.012; for transverse magnetic (TM) mode, △n = 0.0126) is obtained in the structure at the operation wavelength of 1.55 μm. The value is larger by over one order of magnitude than that in a rectangular quantum well. The result is very attractive for semiconductor optical switching devices.     

Optical, electrical, and photophysical properties of films of polycomplexes of azobenzene derivatives with cobalt

The absorption spectra, photoconductivity, and electrical conductivity of films of new polycomplexes of 4-methacyloyloxy-(4′-carboxy-3′-oxy)azobenzene and 4-methacryloyloxy-2-nitro-(4′-carboxy-3′-oxy)azobenzene with cobalt were investigated with switched-off and switched-on external electric fields. The effect of an external electric field on the transmission of linearly polarized light through films near the long-wavelength absorption edge is revealed. It is shown that the change in the light transmission under the action of an external electric field is due to the spatial reorientation of the dipole moments of azobenzene groups photoinduced by polarized light in an external electric field. The effect of an electric field on the light transmission is explained by the formation of forces acting on the dipole moments of azobenzene groups and on metal ions. The increase in the dipole moments of azobenzene groups as a result of introduction of nitro groups into their composition decreases the effect of cobalt ions on the electro-optical properties of polycomplex films.     

基于GaAs/AlxGa1-xAs多量子阱光子晶体的窄带滤波器

讨论了利用GaAs/AlxGa1-xAs多量子阱构成的一维光子晶体实现窄带滤波的原理.研究了外加电场下GaAs/AlxGa1-xAs多量子阱的激子吸收所导致的反常色散行为.研究发现,在反常色散材料的共振频率附近,存在一个较强色散且吸收很小的区域,该区域内的色散可以在PCs的带隙中产生一个窄的通频带,其对应的最大透过率约为0.73,全固态滤光器的带宽约为4.9 nm,并可利用外加电场进行调谐,这将为设计一种全新的固态滤波器提供指导.     

Simulated and experimental studies of a multi-band symmetric metamaterial absorber with polarization independence for radar applications

We develop a simple new design for a multi-band metamaterial absorber (MTMA) for radar applications. Computer Simulation Technology (CST) Studio Suite 2018 was used for the numerical analysis and absorption study. The simulated results show four high peaks at 5.6 GHz, 7.6 GHz, 10.98 GHz, and 11.29 GHz corresponding to absorption characteristics of 100%, 100%, 99%, and 99%, respectively. Furthermore, two different structures were designed and compared with the proposed MTMA. The proposed structure remained insensitive for any incident angle and polarization angle from 0° to 60°. Moreover, negative constitutive parameters were retrieved numerically. To support the simulated results, the proposed design was fabricated by using a computer numerical control-based printed circuit board prototyping machine and tested experimentally in a microwave laboratory. The absorption mechanism of the proposed MTMA is presented through the surface current and electric field distributions. The novelties of the proposed structure are a simple and new design, ease of fabrication, low cost, durability, suitability for real-time applications and long-term stability given the fabrication technique and non-destructive measurement method and very high absorption. The proposed structure has potential applications in C and X band frequency ranges.     

氮化铌纳米线光学特性

氮化铌(NbN)纳米线是超导纳米线单光子探测器(SNSPD)常用的光敏材料,其光学性质是影响SNSPD性能的关键因素.本文结合实验数据和仿真结果,系统研究了多种NbN超导纳米线探测器器件结构的光学特性,表征了以下四种器件结构下的反射光谱以及透射光谱:1)双面热氧化硅衬底背面对光结构;2)双面SiN硅衬底背面对光结构;3)硅衬底上以金层+SiN缓冲层为反射镜的正面对光结构;4)以分布式布拉格反射镜(DBR)为衬底的正面对光结构.并在上述四种器件结构基础上,生长了不同厚度的NbN薄膜,观察不同厚度NbN薄膜的吸收效率.经分析,发现在不同器件结构下的最佳NbN厚度与光吸收率的关系如下:双面热氧化硅衬底上的NbN层在1606 nm处最大吸收率为91.7%,其余结构在最佳NbN厚度条件下吸收率都能达到99%以上.其中双面SiN的硅衬底结构中最大吸收率为99.3%, Au+SiN为99.8%, DBR为99.9%.最后,将DBR器件实测结果与仿真结果进行了差异性分析.这些结果对高效率SNSPD设计与研制具有指导意义.     

Light transmission by a three-level emitter embedded in a waveguide-coupled two-mode photonic crystal nanocavity

We investigate light transmission obtained from a three-level V-type emitter embedded in a waveguide-coupled two-mode photonic crystal nanocavity operating in the weak-coupling regime. It is shown that the composite system exhibits double electromagnetically-induced-transparency-like characteristics and a π phase shift while not suffering from absorption in the experimentally available parameter range. The double-frequency transparency of the input light expands the frequency range of EIT and may improve the controllability of EIT. The proposed scheme also provides a way to achieve integrated photonic devices on a chip for applications requiring multiple EIT effect.     

Giant Broadband One Way Transmission Based on Directional Mie Scattering and Asymmetric Grating Diffraction Effects

High performance optical diode-like devices are highly desired in future practical nano-photonic devices with strong directional selectivity. We demonstrate a kind of giant broadband reciprocity optical diode-like devices by simultaneously using the directional Mie scattering effect and the asymmetric grating diffraction effect. The maximum asymmetric subtraction and the asymmetric transmission ratio can reach nearly 100% and 40 d B at specified wavelength, respectively. In a wide waveband from 500 nm to 800 nm, the asymmetric subtraction and the ratio keep larger than 80% and 3.5 d B, respectively, even under oblique incidence. To the best of our knowledge, this is the best one-way-transmission effect observed in the reciprocity optical diode-like devices. In addition, we further demonstrate that this one-way-transmission effect can bring an effective absorption enhancement on gold films. The giant, broadband and angle-insensitive one-way-transmission effect demonstrated here is far beyond the well-known anti-reflection effect in the light-trapping devices and will bring new design philosophy for nano-photonic devices.     

Numerical investigation on the enhanced carrier collection efficiency of Ga-face GaN/InGaN p-i-n solar cells with polarization compensation interlayers

The impact of the polarization compensation InGaN interlayer between the heterolayers of Ga-face GaN/InGaN?p-i-n solar cells is investigated numerically. Because of the enhancement of carrier collection efficiency, the conversion efficiency is improved markedly, which can be ascribed to both the reduction of the polarization-induced electric field in the InGaN absorption layer and the mitigation of potential barriers at heterojunctions. This beneficial effect is more remarkable in situations with higher polarization, such as devices with a lower degree of relaxation or devices with a higher indium composition in the InGaN absorption layer.     

Conduction-electron spin resonance in two-dimensional structures

The influence of the conduction-electron spin magnetization density, induced in a two-dimensional electron layer by a microwave electromagnetic field, on the reflection and transmission of the field is considered. Because of the induced magnetization and electric current, both the electric and magnetic components of the field should have jumps on the layer. A way to match the waves on two sides of the layer, valid when the quasi-two-dimensional electron gas is in the one-mode state, is proposed. By following this way, the amplitudes of transmitted and reflected waves as well as the absorption coefficient are evaluated.     

Terahertz absorption window and high transmission in graphene bilayer

A detailed theoretical study of terahertz (THz) optical absorption and transmission in graphene bilayer is presented. Considering an air/graphene/dielectric-wafer system, it is found that there is an absorption window in the range 3-30 THz and the optical transmission is up to 96%. Such an absorption window is induced by different transition energies required for inter- and intra-band optical absorption in the presence of the Pauli blockade effect. As a result, the position and width of this THz absorption window depend sensitively on temperature and carrier density of the system. These results are pertinent to the applications of recently developed graphene systems as novel optoelectronic devices such as THz photodetectors.     

Giant Asymmetric Transmission and Circular Dichroism with Angular Tunability in Chiral Terahertz Metamaterials

Inspired by the chiral structure from mode interaction, a chiral terahertz metamaterial and its complementary structure with the coexistence of asymmetric transmission and circular dichroism response are designed. The asymmetric transmission happens in the presence of electric and magnetic modes perpendicular to each other, resulting from intrinsic chirality under normal incidence. Circular dichroism exists in the presence of electric and magnetic modes parallel to each other under oblique incidence because of extrinsic chirality. Both asymmetric transmission and circular dichroism are enhanced by complementary structures, benefitting from the coupling between adjacent units. The maximum of chiral parameter κ can achieve 450, which is one hundred times higher than previously reported. The chiral response can be tuned by the incident angle and is sensitive to the environmental refractive index. The results highlight the potential applications of these metamaterials in chiral sensing and polarization transformation devices.     

Electrooptical effect in polymeric composites containing heterometallic Cu(II)/Mn(II) complexes

We studied the effect of an electric field on the transmission of linearly polarized light through films of composites based on polyvinylbutyral doped with particles of Cu(II)/Mn(II) complexes with “skeleton” and “planar” structures. The action of an electrostatic field causes light absorption to become anisotropic. The effect increases with increasing distance between the cation and anion. Its sign reverses if the spatial structure of the complex is changed. A phenomenological model is proposed according to which the electrooptical properties of the composites are due to a change in the mutual orientation of complex building blocks on exposure to an external electric field. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 481–484, July–August, 2007.     

Design of periodic metal-insulator-metal waveguide back structures for the enhancement of light absorption in thin-film solar cells

To increase the absorption in a thin layer of absorbing material (amorphous silicon, a-Si), a light trapping design is presented. The designed structure incorporates periodic metal-insulator-metal waveguides to enhance the optical path length of light within the solar cells. The new design can result in broadband optical absorption enhancement not only for transverse magnetic (TM)-polarized light, but also for transverse electric (TE)-polarized light. No plasmonic modes can be excited in TE-polarization, but because of the coupling into the a-Si planar waveguide guiding modes and the diffraction of light by the bottom periodic structures into higher diffraction orders, the total absorption in the active region is also increased. The results from rigorous coupled wave analysis show that the overall optical absorption in the active layer can be greatly enhanced by up to 40%. The designed structures presented in this paper can be integrated with back contact technology to potentially produce high-efficiency thin-film solar cell devices.