二维各向同性异向介质负折射特性仿真验证

提出了一种工作在C波段的二维各向同性异向介质，相比于以往的二维异向介质，它除了具有二维各向同性的优点外，还具有带宽更宽，单元结构的尺寸更小，构造也更简单的特点.通过分别仿真验证异向介质的双负特性和负折射特性最终肯定上述异向介质的存在. 关键词： 各向同性异向介质 宽频带 小单元 双负特性     

Designing a broad locally-resonant bandgap in a phononic crystals

Producing a broad locally-resonant bandgap for low frequencies in a phononic crystals is a challenging task, using conventional methods. In this paper, we describe the design of a broad, locally-resonant, bandgap in a phononic crystals using a numerical simulation. The structure consists of periodic double-sided novel composite sonators, deposited on a 2D locally-resonant phononic-crystals plate made of a rubber-filler array, which is embedded in an epoxy plate. Using the finite element method, we calculate the dispersion relations, the power-transmission spectra, and the displacement fields for the eigenmodes. Our results confirm that, the new structure facilitates a significant increase in absolute bandwidth (by a factor of 4.2) compared to conventional phononic crystals. It also broadens the range of elastic wave attenuation. The formation mechanisms that generates the broad locally-resonant bandgap is explored numerically. The simulation indicates that the formation of this bandgap is possible due to coupling between the entire vibration mode of the novel composite-resonator and the Lamb-wave mode of the 2D locally-resonant phononic-crystals plate. The bandwidth of the locally-resonant bandgap is determined by the resonator mode. This study opens new possibilities to broaden locally-resonant bandgaps of phononic crystals for low frequencies. The results can potentially be used to reduce vibration and noise in many applications.     

Wide bandwidth acoustic transmission via coiled-up metamaterial with impedance matching layers

Many acoustic metamaterials suffer from a narrow bandwidth transmission because of the impedance mismatch at the airmetamaterial interface. In this paper, a two-dimensional impedance-matched metamaterial with broadband transmission performance is investigated. The impedance matching layer is introduced for a gradient variation of effective impedance from the inlet of the unit to the outlet. The effective medium theory and corresponding effective model are used to explain the underlying mechanism. The improved energy transmission of our designs is demonstrated by experiment and numerical simulation within a broad frequency bandwidth over 6 kHz. Our impedance-matched design can be used to enhance sound absorption, which is expected to present improved acoustic performance in the applications of acoustic damper and muffler.     

一种宽频带超材料吸波体的设计及其特性

为了获得吸收率高、吸波带宽宽的超材料,设计了一种谐振超材料吸波体.该吸波体由多个开口圆环组成,采用商业软件CST Studio Suite 2009频域求解器计算了其在25~35 GHz波段内的S参量,并计算了其吸波率A(ω),在28.4 GHz处吸收率达到86%,带宽达到3.5 GHz.利用不同吸波频段的叠加效应,设计了一种谐振超材料吸波组合体,计算了在25~35 GHz波段的S参量,在29.7 GHz处吸波率达99.9%,吸波带宽达到3.1 GHz,吸收率明显增加.将GHz波段的结构缩小1 000倍,在THz波段同样可以达到高吸收,说明超材料吸波体可以通过对结构尺寸调节改变吸收波段.同时,对其阵列进行仿真计算,发现不同的排列方式仿真结果不同.由于各个谐振环之间的相互作用对吸收效果影响较大,吸收率减小.该吸波材料由金属组成,能灵活地对介电常量和磁导率进行调节,从而实现高吸收.     

Antireflection coating on germanium for dual channel (3–5 and 7.5–10.6 μm) thermal imagers

The dual channel thermal imager, operating in the 3–5 and 7.5–10.6 μm wavelength bands, is one of the latest achievements in instrumentation for target recognition and acquisition. While the 3–5 μm band is utilised for detecting hot objects such as engine exhausts of vehicles and fighter planes, the 7.5–10.6 μm band is employed for human bodies and objects at ambient temperatures. Many substrates are available which transmit in both these wavelength bands and their transmission can be enhanced by providing a suitable antireflection coating. In this paper, a broad band antireflection coating on germanium substrate is reported. The design approach involves achieving a continuously varying refractive index from that of the incident medium to the substrate. The continuously varying refractive index profile may be generated by using a sequence of thin layers of high and low refractive index materials. In this design a continuous refractive index profile is approximated by using a 13-layer stack of thorium fluoride and germanium as low and high index coating materials respectively. This coating conforms to environmental stability standards and shows an average transmission of 91% in 3–5 μm band and 94.5% in 7.5–10.6 μm band with a peak of 97% at 9 μm on 10 mm thick germanium substrate. Polycrystalline germanium has 2.5% absorption for a 10 mm thick substrate.     

基于表面等离子体共振增强的硅基锗金属-半导体-金属光电探测器的设计研究

金属-半导体-金属光电探测器的光栅结构可激发表面等离子体, 有效增强探测器的吸收. 为深入研究器件结构对于表面等离子体的激发及共振增强的影响, 本文提出了一种具有超薄有源层的硅基锗金属-半导体-金属光电探测器的设计方法. 采用时域有限差分的方法详细分析了光栅周期、光栅厚度、 光栅间距及有源层厚度对于表面等离子体共振增强器件性能的影响, 通过仿真模拟获得了器件的最佳结构, 详细地分析了各个界面激发的表面等离子体及其共振模式对于光谱吸收增强的机理. 仿真结果表明, 有源层锗的厚度为400nm的超薄器件在通信波段具有较高的吸收, 尤其在1550nm波长处器件的归一化的光谱吸收率可以高达53.77%, 增强因子达7.22倍. 利用共振效应能够极大地提高高速器件的光电响应, 为解决光电探测器响应度与响应速度之间的相互制约关系提供了有效途径. 关键词： 表面等离子体 锗探测器 时域有限差分仿真     

Simplified Approach for Optimizing Optical Asymmetric Curved Waveguides of Broadband Directional Couplers

ABSTRACT

Optical broadband directional couplers (BDCs) are indispensable components for providing wavelength-insensitive and flexible optical splitting in the construction of functional photonic integrated circuits (PICs). The existing BDC device structures are usually required to determine specific design parameters for different waveguide structures and operating wavelength bands. To circumvent this dilemma, here we present a novel optimization procedure to realize a compact BDC by using the asymmetric curved waveguide structure. The versatile particle swarm optimization (PSO) technique is adopted to determine the optimal device parameters of the compact and broadband asymmetric curved directional couplers (ACDCs) for different coupling ratios. In order to reduce the computational complexity in the optimization, the 3D ACDC is first converted to an equivalent 2D structure by using the modified effective index method (MEIM). The device parameters of the equivalent 2D ACDC are optimized by the PSO with the objective function of a wavelength flattened coupling ratio. Afterward, the optimized 2D structure is converted to the 3D one by including the waveguide thickness. To cope with the approximation error by the MEIM, the 3D ACDC is further fine-tuned by sweeping one of the device parameters with the full 3D simulation but keeping all of the other optimal parameters obtained from the PSO intact. As a result, a DC with broad bandwidth of 100 nm is obtained over the wavelength range from 1.50 µm to 1.60 µm with a very small coupling length of 6 µm. The semi-optimized ACDC is used to construct an unbalanced Mach-Zehnder interferometer (MZI) and a Sagnac loop mirror (SLM), both of which show high extinction ratios of >25 dB over a broad wavelength range with low excess loss.     

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.     

Awakening the Dumb Site to Realize Ultra-Broadband NIR Phosphor

Near-infrared (NIR) light sources have gained immense popularity in recent years due to their wide range of applications in various fields, including spectroscopy and biomedical imaging. However, the limited emission bandwidth of NIR phosphors is a significant bottleneck in their development. Here, a novel strategy is reported to broaden the emission bandwidth of NIR phosphors by awakening the dumb site. Na₄M₃Ta(PO₄)₆: Cr³⁺ (M = Al³⁺, Ga³⁺, In³⁺) phosphor is synthesized, which exhibits a greatly broadened bandwidth from 134 to 232 nm. Structural and spectral analysis reveals that the NaO₆ octahedral site has a severe t_2g-type distortion, making it a dumb site for Cr³⁺. By introducing larger In³⁺ at the M site, the angular distortion at the NaO₆ site decreases to the normal range, enabling the luminescence of Cr³⁺ again. Along with the broadened bandwidth, the emission peak also redshifts from 802 to 977 nm, giving advantages in NIR spectroscopy applications. Interestingly, the awakened luminescence at the NaO₆ site shows even better luminescence properties than the original M/TaO₆ octahedral site. These findings reveal a novel insight into the luminescence of Cr³⁺ at the octahedral site, which could potentially revolutionize the design of NIR phosphors.     

掺Yb硅酸盐玻璃的热漂白性能研究

采用传统高温熔融法制备了掺Yb硅酸盐玻璃, 玻璃组成为 60SiO₂-12Al₂O₃-28CaO-1.0 mol%Yb₂O₃. 将玻璃分成两组, 一组经总剂量3 kGy的Co⁶⁰γ辐射源辐射, 另一组做空白对照. 然后测试了玻璃未辐射、辐射后及热漂白后的吸收谱和近红外发光谱. 实验结果表明: 一定的热处理会使得玻璃在辐射过程中产生的色心缺陷发生分解, 即辐射后的玻璃在300–900 nm波段的吸收系数显著地降低了, 在400 nm处未经辐射、经3 kGy辐射以及热漂白后的玻璃吸收系数分别是0.93, 2.9, 1.89 cm^-1. 另外, 玻璃的近红外发光强度明显增强, 在1028 nm处未经辐射、经3 kGy辐射以及热漂白后的玻璃近红外发光相对强度分别是588, 261, 436, 从而极大地改善了玻璃的光学性能. 所以一定的热处理可以使辐射后的玻璃产生热漂白的现象. 研究结果为进一步发展抗辐射玻璃材料提供了新的实验依据.     

基于超材料与电阻型频率选择表面的薄型宽频带吸波体的设计

设计了三种类型吸波体, 分别为基于正方形金属贴片(square metal patch, SMP) 结构超材料吸波体、 电阻型频率选择表面(Resistance Frequency Selective Surface, RFSS) 吸波体和SMP与RFSS的复合结构吸波体. 采用FDTD算法分别对这三种类型吸波体的电磁波吸收特性进行数值模拟分析. 模拟得到的结果表明: 在整个2-30 GHz频率范围内, SMP吸波体, 通过几何参数的设计可以实现多频窄带强吸收; RFSS吸波体, 通过方块电阻的设计可以实现高频宽带强吸收, 但强吸收的带宽有限; SMP与RFSS的复合结构吸波体, 在3-25 GHz之间吸收率大于90%以上, 且宽频范围内与自由空间具有较好的阻抗匹配特性.     

Broadband and ultra-low reflection metamaterial absorber embedded with magnetic materials

In this study, we design, prepare and characterize a broadband, ultra-low reflectivity and incidence angle-insensitive metamaterial absorber. The design of this absorber not only provides a novel idea for the design of broadband absorbers, but also enhances the application prospects of metamaterial absorbers. By introducing FeSiAlp/epoxy magnetic composite and optimizing the structural parameters, the absorption performance of the metamaterial absorber has been significantly improved. The effective absorption bandwidth (bandwidth with reflectivity less than −10dB) is increased by 3.4 times from 2.19 GHz to 7.49 GHz, and the RL_min (minimum reflection loss) value reaches −38.31 dB at 17.83 GHz, that is the absorption rate reaches 99.99%. Meanwhile, the experimental results also verify the simulation design results. Therefore, the absorber not only plays the characteristics of strong absorption of metamaterial, but also absorbs the advantages of broadband of magnetic material.     

High-speed waveguide-integrated Ge/Si avalanche photodetector

Waveguide-integrated Ge/Si heterostructure avalanche photodetectors(APDs) were designed and fabricated using a CMOS-compatible process on 8-inch SOI substrate. The structure of the APD was designed as separate-absorption-chargemultiplication(SACM) using germanium and silicon as absorption region and multiplication region, respectively. The breakdown voltage(V_b) of such a device is 19 V at reverse bias and dark current appears to be 0.71 μA at 90% of the V_b. The device with a 10-μm length and 7-μm width of Ge layer shows a maximum 3-dB bandwidth of 17.8 GHz at the wavelength of 1550 nm. For the device with a 30-μm-length Ge region, gain-bandwidth product achieves 325 GHz.     

窄带有机光电探测器的优化设计

针对基于无机材料的光电探测器需要借助滤光器或棱镜耦合实现窄带响应,提出了一种通过有机材料制备窄带光电探测器并提高吸收峰值和降低半高全宽的方法和结构。该器件由分布布拉格反射器和有机光电二极管构成。有机光电二极管的顶电极和底电极之间构成光学微腔。采用传输矩阵法,详细分析了分布布拉格反射器的中心波长、有机光电二极管透明顶电极和光敏感层的厚度对有机光电探测器吸收性能的影响。研究结果表明,Tamm等离激元共振波长接近光敏感层的光学带隙时,可获得半高全宽小于20 nm的窄带响应,并且吸收峰值在70%以上。基于PTB7∶PC~(71)BM和PTB7-Th∶IEICO-4F的有机光电探测器分别可用于探测红光和近红外光。该研究从基本物理机制出发,结合材料和器件结构可将有机光电探测器的响应窗口从可见光拓展至近红外光。     

Enhanced Cross-Phase Modulation via Phase Control in a Quantum dot Nanostructure

A four-level quantum dot (QD) nanostructure interacting with four fields (two weak near-infrared (NIR) pulses and two control fields) forms the well-known double-cascade configuration. We investigate the cross-phase modulation (XPM) between the two NIR pulses. The results show, in such a closed-loop scheme, that the XPM can be greatly enhanced, while the linear absorption and two-photon absorption (gain) can be efficiently depressed by tuning the relative phase among the applied fields. This protocol may have potential applications in NIR all-optical switch design and quantum information processing with the solid-state materials.     

Enhancement of the 808 nm Photothermal Effect of Gold Nanorods by Thiol‐Induced Self‐Assembly

Gold (Au) nanomaterials are promising photothermal agents for the selective treatment of tumor cells owing to the strong capability to convert near‐infrared (NIR) irradiation into heat energy. One basic issue for practical photothermal therapy is the enhancement of photothermal effect in NIR region. Here, various low‐molecular‐weight thiols are applied to induce one‐dimensional (1D) self‐assembly of Au nanorods (NRs), which leads to the redshift of absorption peak towards NIR region. As a result, the 1D assembled Au NRs exhibit improved photothermal effect at 808 nm in comparison to unassembled Au NRs.     

两注THz折叠波导行波结构的增益与传输特性

为了提高太赫兹行波管的输出功率,提出了多个传输信号进行功率合成的方法。首先,对D波段多注慢波结构进行设计及功分器的优化;然后,通过微铣削工艺完成了两注THz折叠波导结构的加工,加工精度满足实际设计要求;最后,利用CST软件对该结构的冷测与互作用特性进行了分析。仿真结果表明:该结构的S11值小于-20 dB,实际测试值在-20 dB左右,两者较吻合。冷测分析表明该结构具有22 GHz（16%）的冷带宽,3 dB增益带宽为12.5 GHz。在各电子注的电压、电流、峰值输入功率大小分别为15.79 kV, 12 mA, 10 mW时,单注结构获得了1.58 W的输出功率及22 dB的增益;而两路信号在互作用完成后,获得了2.91 W的合成功率输出,合成效率不低于90%。通过合成的方法可以有效提高THz行波管的输出功率。.     

2(3)-四-(2-异丙基-5-甲基苯氧基)钛氧酞菁的合成及其近红外发光特性

通过分步合成的方法,在钛氧酞菁的外围引入了四个苯氧基团,合成了一种2(3)-四-(2-异丙基-5-甲基苯氧基)钛氧酞菁.通过质谱、核磁、元素分析、紫外可见吸收光谱、近红外光荧光谱和循环伏安法对其结构和性质进行了表征,并利用旋涂加真空热蒸发的方法制得了该材料的电致发光器件(ITO/PVK:TiOPc/BCP/LiF/Al).结果表明,四取代的钛氧酞菁固态的近红外光荧光峰值在1080nm左右;其近红外电致发光的峰值波长落在1050nm左右.在PVK掺杂的器件中,钛氧酞菁的质量分数为30%时有较强的发光强度.     

Broad Coverage Precoding for 3D Massive MIMO with Huge Uniform Planar Arrays

In this paper, we propose a novel broad coverage precoder design for three-dimensional (3D) massive multi-input multi-output (MIMO) equipped with huge uniform planar arrays (UPAs). The desired two-dimensional (2D) angle power spectrum is assumed to be separable. We use the per-antenna constant power constraint and the semi-unitary constraint which are widely used in the literature. For normal broad coverage precoder design, the dimension of the optimization space is the product of the number of antennas at the base station (BS) and the number of transmit streams. With the proposed method, the design of the high-dimensional precoding matrices is reduced to that of a set of low-dimensional orthonormal vectors, and of a pair of low-dimensional vectors. The dimensions of the vectors in the set and the pair are the number of antennas per column and per row of the UPA, respectively. We then use optimization methods to generate the set of orthonormal vectors and the pair of vectors, respectively. Finally, simulation results show that the proposed broad coverage precoding matrices achieve nearly the same performance as the normal broad coverage precoder with much lower computational complexity.     

Numerical study of metamaterial absorber and extending absorbance bandwidth based on multi-square patches

A simple design of metamaterial absorber (MA) was proposed based on a periodic array of metal patch at microwave frequencies. Our design could exhibit absorption of 99.9% confirmed by numerical simulation. Such high narrowband absorption which mainly based on strong electric and magnetic resonances overlapping in a certain frequency range and perfect impedance-matched (z = 1) to the free space. Numerical simulations demonstrate that the MA could achieve very high absorptivity at wide angles of incidence for both transverse electric (TE) wave and transverse magnetic (TM) wave. The absorption band of our device is effectively extended by patterning multi-square patches of different dimension elements with appropriate geometrical parameters in a co-planar. Finally, the composite MA is only 0.4 mm thick, with a maximum absorption of 99.8% at 15.8 GHz, and a full width at half maximum (FWHM) bandwidth of 2 GHz by numerical simulation, which may have potential applications in the detection of explosives and stealth.