Comparison of subwavelength focusing properties of diffraction and Fresnel zone plate plasmonic planar lenses

The subwavelength focusing properties of diffractive plasmonic planar lenses (DPLs) and Fresnel zone plate plasmonic planar lenses (FZPs) have been compared in this paper. To that end, we use the same lens material, incident wavelength, thickness and focal length for comparison. Both DPLs and FZPs consist of central circular slits surrounded by transparent and opaque zones and can get clear focusing performances. By using the rigorous electromagnetic numerical method, the fields in the focal region are analyzed in detail, and our results demonstrate that FZP can generate higher transmission efficiency, and higher peak field intensity at the focal plane. Focusing polarization properties of the lens illuminated by linearly polarized state, are calculated and analyzed also. The numerical results show that both the DPL and the FZP produce asymmetrical focal spot distributions with a low f-number. To the FZP, the full-width at half-maximum (FWHM) varies from 256 nm (along y-axis) to 516 nm(along x-axis) and to the DPL, the FWHM varies from 256 nm(along y-axis) to 580 nm(along x-axis), respectively. But for a high f-number, the asymmetrical performance of the focal spot will be reduced due to complicated electromagnetical field interferences and micro waveguide effect. Otherwise, the DPL can get a higher depolarization effect than the FZP does.     

表面等离子体平面金属透镜及其应用

由于衍射极限的存在,传统光学透镜成像分辨率理论上只能达到入射光波长的一半。通过恢复和增强携带物体细部特征信息的高频倏逝波,基于表面等离子体的平面金属透镜有望突破这种光学衍射极限,实现超分辨成像。本文对平面薄膜式与纳米结构式两类平面金属透镜进行了综述,详细介绍了若干典型平面金属透镜的结构设计、工作机理及其聚焦性能,并对其特点与存在的问题进行了分析与讨论。由于光波在金属中传播时存在一定损耗,如何更高效地增强高频倏逝波信号并转换成传播波,使其参与成像,以更好地实现远场超分辨成像,以及如何进一步增大近场超高分辨率聚焦光斑焦深以及减小远场聚焦光斑尺寸,是表面等离子体平面金属透镜进一步研究的重点。     

基于石墨烯和二氧化钒的太赫兹宽带可调谐超材料吸收器

太赫兹超材料吸收器作为一种重要的太赫兹功能器件,被广泛应用于生物医学传感、电磁隐身、军用雷达等多个领域.但这种传统的超材料吸收器结构具有可调谐性差、功能单一、性能指标不足等缺点,已经无法满足复杂多变的电磁环境的要求,因此可调谐超材料吸收器逐渐成为了太赫兹功能器件领域的研究热点.为实现超材料吸收器吸收特性的调谐,通常从调...     

Plasmonic electromagnetically-induced transparency in metamaterial based on second-order plasmonic resonance

Using only two gold strips, we propose a scheme for generation of the plasmonic analogue of electromagnetically-induced transparency (EIT) in stacked optical metamaterials by utilizing the second-order plasmon resonance. In addition, we show that the plasmonic EIT can be achieved with asymmetric structure, since the asymmetric structure allows the excitation of the dark mode.     

Terahertz power splitter based on ferrite photonic crystal

In this paper, we design a novel Y-junction power splitter which is optimized according to coupled mode theory. Two rods are added in the junction. One rod is a silicon rod and another one is a ferrite rod, the refractive index of which can be varied by adjusting the external magnetic field. So, it not only can split the power, but also controls the output energy of the two branches automatically. The transmission spectrum is calculated by finite difference time domain (FDTD) method, which shows that the output energy of the two branches is closely related to the frequency of the incident terahertz (THz) wave and the refractive index of the ferrite rod. The device is a good candidate for integrated optical circuit, the tuning rate of which is about a few microseconds.     

Terahertz wave narrow bandpass filter based on photonic crystal

We designed a narrow bandpass terahertz wave filter using photonic crystals with a line defect. An inserted linear defect in one-dimensional photonic crystal structures for a channeled filtering in the terahertz range are studied and designed theoretically. By using transfer matrix method, we examined the transmittance spectra for the proposed terahertz wave filter has a 3 dB transmission loss bandwidth of 20 MHz ranging from 0.29998 THz to 0.30001 THz. The simulated results show that a very narrow transmission band and high transmission (higher than 99.99%) centered at λ₀, and very sharp edges can be achieved.     

基于Z-FDTD的THz波与等离子体相互作用

利用Z变换时域有限差分法(Z-FDTD)计算等离子体鞘层中太赫兹(THz)波的传输特性,得出太赫兹波的功率反射和透射系数在等离子鞘层中随电磁波频率的变化曲线图,分析了太赫兹波的传输特性与等离子体结构参数(鞘层厚度、碰撞频率以及电子密度)的关系,对利用太赫兹波缓解等离子体鞘层通信中出现的“黑障”现象做了探讨。结果表明：太赫兹波能改善等离子体鞘层通信,为解决“黑障”问题提供了有效途径。     

基于深度神经网络的镜片佩戴舒适度研究

为了客观评价镜片佩戴舒适度,研究设计眼动实验,利用眼动仪采集并分析眼动数据,提出"眼睑间距变动百分比"作为镜片佩戴舒适度评价指标之一,再结合其他传统眼动指标,设计了一种基于深度神经网络的镜片佩戴舒适度评价模型.结果 表明提出的眼睑间距变动百分比指标用于评价镜片佩戴舒适度具有良好的效果,最终构建的深度神经网络模型预测准确...     

Large‐acceptance diamond planar refractive lenses manufactured by laser cutting

For the first time, single‐crystal diamond planar refractive lenses have been fabricated by laser micromachining in 300 µm‐thick diamond plates which were grown by chemical vapour deposition. Linear lenses with apertures up to 1 mm and parabola apex radii up to 500 µm were manufactured and tested at the ESRF ID06 beamline. The large acceptance of these lenses allows them to be used as beam‐conditioning elements. Owing to the unsurpassed thermal properties of single‐crystal diamond, these lenses should be suitable to withstand the extreme flux densities expected at the planned fourth‐generation X‐ray sources.     

Focusing surface plasmons on Er3+ ions through gold planar plasmonic lenses

Gold plasmonic lenses consisting of a planar concentric rings-groove with different periods were milled with a focused gallium ion beam on a gold thin film deposited onto an Er³⁺-doped tellurite glass. The plasmonic lenses were vertically illuminated with an argon ion laser highly focused by means of a 50× objective lens. The focusing mechanism of the plasmonic lenses is explained using a coherent interference model of surface plasmon-polariton (SPP) generation on the circular grating due to the incident field. As a result, phase modulation can be accomplished by the groove gap, similar to a nanoslit array with different widths. This focusing allows a high confinement of SPPs that can excite the Er³⁺ ions of the glass. The Er³⁺ luminescence spectra were measured in the far-field (500?C750?nm wavelength range), where we could verify the excitation yield via the plasmonic lens on the Er³⁺ ions. We analyze the influence of the geometrical parameters on the luminescence spectra. The variation of these parameters results in considerable changes of the luminescence spectra.     

Stored light in a plasmonic nanocavity based on extremely-small-energy-velocity modes

We propose a novel scheme of the plasmonic cavity that is based on the insulator–metal–insulator (IMI) waveguide of finite length. Surface plasmon polaritons (SPPs) guided by the IMI structure can exhibit extremely small energy velocity if the metal film is thin enough. In that case, the losses are determined mainly by the intrinsic properties of the metal rather than the geometry of the cavity. Accordingly, we should care only about the purity of the metal and need not accurately control the length of the cavity that simplifies the fabrication process. All dimensions of the cavity can be less than the wavelength of light, therefore it is a really nanoscale device. In this paper, we characterize the cavity analytically and demonstrate the stored light using our finite difference time domain (FDTD) code.     

Optical properties of hybrid plasmonic nanofluid based on core/shell nanoparticles

The plasmonic effect is used in nanofluid to help capture and absorb sunlight. The optical absorption is significantly enhanced as plasmonic effect excited. To obtain an enhanced absorption in a broad band, the hybrid plasmonic nanofluid is developed. It is composed of core/shell nanoparticles of different sizes. The overall absorption of hybrid nanofluid is examined. Compared to the nanofluid of single particle size, the hybrid nanofluid exhibits a broadband absorption. As particle size increases, the plasmon resonance peak is shifted to longer wavelength. The variation in the sizes of core/shell nanoparticles can broaden the absorption spectrum. In the near-infrared region, the proportion of different size particles has an obvious influence. With the increase of proportion of larger particles, the absorption band is broadened. Since the suspended nanoparticles have different sizes, the particle distribution in base fluid also has an effect on absorption of light. The large particle in upper has a broadband absorption, however, less energy can be transmitted to lower after the absorption of upper particles. The contribution from the particles in lower is relatively weak.     

Terahertz wave omnidirectional dielectric mirror

We design an omnidirectional dielectric mirror that operates over a wide terahertz wavelength range and is based on a periodic bilayer (tellurium and polystyrene) structure. The structure is characterized by using transfer matrix calculation. Results of simulation show that the presented dielectric mirror is highly reflecting for all incidence angles and TE as well as TM polarization in the frequency band from 0.451 to 0.821 THz (i.e. wavelength ranges from 365.5 to 665.5 μm), when the incidence angle is less than 45°. Tolerance analysis reveals a large tolerance to fabrication errors.     

Terahertz investigations on photoisomerisable compounds

We performed computational terahertz studies on organometallic photoisomerisable compounds, employing both gas phase and solid phase calculations. The calculations demonstrate the potential of employing terahertz techniques on photoisomerisable compounds. In particular, the trans-ligand, counterion and crystal effects are evaluated via the density functional theory calculations. In order to fully understand the terahertz responses of the photoisomerisable compounds, their experimental terahertz spectra were obtained and compared to the calculations. The calculated spectra generally predict the experimentally observed absorption peaks, while combined gas phase and solid phase calculations offer better agreement with the experiments. The first principles calculations also reveal the sensitivity of terahertz signal on the photoisomerisation processes, suggesting a photo-terahertz set-up that could be built in the future to fast screen and fully understand the photoisomerisable compounds, for related applications such as photo-transducer and photo-switch that require photoinduced geometrical changes.     

Terahertz planar antennas for future wireless communication: A technical review

With the monotonic increasing demand of the higher bandwidth for the next generation wireless communication system, the extension of the operating frequency of the communication system to the millimeter/Terahertz wave regime of the electromagnetic spectrum where several low-attenuation windows exist is inevitable. However, before the commercial implementation of the wireless communication in these low-attenuation windows, there are various obstacles which need to be addressed by the scientists and researchers. The atmospheric path loss is the main obstacle to the full-fledged implementation of the terahertz wireless communication. The remedy to this problem is the use of high-power sources, efficient detectors and high gain antenna systems. This paper reviews these technical issues with the special attention to the planar antennas which might contribute to the compact, inexpensive, and low profile future terahertz wireless communication system design.     

Terahertz polarization splitter based on an asymmetric dual-core photonic crystal fiber

By employing the liquid crystal refractive index changes induced by applied electric field, a novel terahertz polarization splitter with tunable the operating frequency and bandwidths has been proposed and theoretically analyzed. It possesses an extinction ratio as high as the polarization splitters based on the two-dimensional photonic crystal waveguides. These distinguished features ensure its important applications in the integrated optical systems.     

Terahertz wave switch based on photonic crystal ring resonators

In this paper, we have presented a compact and integrated terahertz wave switch design based on photonic crystal ring resonators. The photonic crystal structure with square lattice is investigated and applied for design of ring resonators. The switching mechanism of this novel switch is based on the variation of the resonant frequency of the ring resonator inserted between two parallel waveguides. The refractive index of the holes of the structure filled with polyaniline electrorheological fluids are varied by applied external electric field, the result of which is the variation of the ring resonant frequency. The proposed device is analyzed by using finite difference time domain method. Numerical simulation results show that this switch has high extinction ratio, small size, low voltage and advantages of selectivity of coupling THz wave to different output ports.     

太赫兹技术在农产品检测中的应用研究进展

我国是农业大国,保障粮食安全是国家发展的战略需要。农产品检测技术的应用和发展对监控质量,预防由农产品品质问题引发的安全事故至关重要。太赫兹(Terahertz, THz)波位于电磁频谱空隙,频率高于微波而低于红外线,具备光子能量低、穿透性好、能表征分子结构等优点。基于太赫兹波的光谱检测技术受到研究人员广泛关注,在生物医学、安全检查等方面得到应用,被证明是一种可靠的检测手段。在农产品应用领域,太赫兹波特有的非接触、无标记检测能力为农产品成分分析、质量控制提供了技术手段,其良好的穿透性和无损害性,可以用来在不破坏农产品表面及外包装的前提下,检测内部成分变化。与其他光谱(超声、 X射线、红外等)检测手段相比,太赫兹波频率范围宽、表征能力强,可实现对目标物质的快速无损检测。近几年,随着太赫兹发射源、探测器等设备以及光谱和成像技术的发展,其在农产品领域的应用有了新的进展。通过收集整理近期的文献资料,综述了太赫兹技术在农产品检测方面的应用拓展和研究成果,总结了目前存在的应用局限。在此基础上,对未来太赫兹光谱和图像检测的研究方向进行了展望,提出提高检测灵敏度和检测速度是农产品领域太赫兹技术产业化应用...