The transportable cesium fountain clock NIM5: its construction and performance

The second laser cooling cesium fountain clock NIM5 at the National Institute of Metrology (NIM) China adopts the (1,1,1) direct optical molasses ( OM) configuration. NIM5 has been running with a stability of 3×10⁻¹⁵/d and an operation ratio of 99% since 2007. Preliminary evaluations of NIM5 in 2008 showed a typical combined uncertainty of 3×10⁻¹⁵. The NIM5 clock is operating in parallel with NIM’s first fountain clock NIM4. NIM4 and NIM5 are used to steer the frequency of the calculated NIM atomic time TA-c(NIM) and the first set of results are promising. We are now at the stage of comparing the frequency of NIM5 with UTC to support the independent frequency shift evaluations of NIM5 and contribute to the international atomic time in the near future.      

含有负折射率介质层的布喇格镜的光学特性研究

从电磁场理论推导了光波在负折射率介质薄层中的传输矩阵,对由正、负折射率介质构成的全介质布喇格反射镜的反射带宽和角度特性进行了分析和讨论,与普通的均由正折射率介质构成的布喇格反射镜的相应特性做了对比,并根据薄膜理论对研究结果作了解释.结果表明含有负折射率介质的布喇格全反射镜的反射带宽和角度特性均优于普通由正折射率介质构成的布喇格镜.     

飞秒光梳和碘稳频532nm Nd:YAG激光频率的测量

报道了中国计量科学研究院研制的基于掺钛蓝宝石(Ti:Sapphire)锁模飞秒脉冲激光器的飞秒光学频率梳装置，并利用此装置测量了碘稳频532nm(¹²⁷I₂R(56)32-10) Nd∶YAG固体激光器的频率，结果为 563260223512991±20Hz,相对不确定度为3.6×10^-14.这一数值是直接溯源到铯原子微波频率基准的光学频率测量结果.     

Theoretical model for electromagnetic wave propagation in negative index material induced by cubic-quintic nonlinearities and third-order dispersion effects

We analyze the problem of characterization of electromagnetic (EM) wave that propagates in a negative index material (NIM) and compare to the behavior that observes in a positive index material (PIM), from the conventional technique of characterization of the pulse intensity profile. Firstly, we establish the nonlinear Schrödinger equation (NLSE) and use the Drude model to obtained the NLSE terms. Secondly, we apply the collective variables technique at each frequency range and show that the stability and the light pulse intensity are significantly depending on specific normalized frequencies. We determine the specific frequency range allowing a fear characterization of the intensity profile of light pulse when the frequency increases. We illustrate the effectiveness of this EM wave stabilization by applying the indicated frequency range. This conventional technique in NIM and PIM systems where the second-order dispersion, third-order dispersion, self phase modulation (SPM) and quintic effects originated from SPM called quintic phase modulation act to provoke strong perturbations.     

Metallic negative index nanostructures at optical frequencies: losses and effect of gain medium

One of the most important modern problems in modern electromagnetics is a design and study of negative index metamaterials that may enable sub-wavelength imaging at optical frequencies. Plasmonic periodic metal-dielectric nanostructures present one interesting possibility for both 2D and 3D negative index medium (NIM) systems. The displacement current near plasmonic resonance excitations may produce both negative permittivity and negative permeability (most difficult) in a NIM with e.g. “fishnet” metal-dielectric composite type structure. One obvious problem with a metallic NIM is that the response is strongly dispersive and lossy. Both of these effects are detrimental to sub-wavelength imaging. One way of mitigating losses is to use a gain medium. We address the question of sub-wavelength resolution in the fishnet NIM with and without gain medium. PACS 42.79.-e; 42.79.Dj; 42.81.Dp; 74.78.Fk; 78.66.Bz     

Saturation of the magnetic response of split-ring resonators at optical frequencies

We investigate numerically the limits of the resonant magnetic response with a negative effective permeability mu(eff) for single-ring multicut split-ring resonator (SRR) designs up to optical frequencies. We find the breakdown of linear scaling due to the free electron kinetic energy for frequencies above approximately 100 THz. Above the linear scaling regime, the resonance frequency saturates, while the amplitude of the resonant permeability decreases, ultimately ceasing to reach negative value. The highest resonance frequency at which mu(eff) < 0 increases with the number of cuts in the SRR. A LC circuit model provides explanation of the numerical data.     

太赫兹频段开口环谐振器的可调谐振模式转换

在硅衬底上设计了一种单开口环谐振器，对其太赫兹频段内的透射性质进行了研究。假定通过光注入方式改变衬底硅的电导率，实现了谐振环的双谐振透射率可调。将砷化镓材料生长于该谐振环的开口处，通过光注入方式改变砷化镓材料的电导率，可以实现谐振环的双频LC共振和偶极子共振模式与单频闭合环共振模式之间的转换。这种通过光注入改变半导体材料电导率的方法，可以在不破坏原来谐振器件物理结构的前提下，实现谐振环谐振模式的可逆转换。     

负折射率介质层中光波的相位和传输特性研究

针对在理解负折射率材料中光波的传播特性时相位的特殊性质 ,详细分析推证了负折射率介质中折射率、波矢k的大小和方向、空间坐标系以及波相相位互之间的关系。分析结果表明负折射率介质层中的波矢k的反常取向是由于其标量值为负 ;在统一的坐标系下 ,由该标量值参与数值计算和分析 ,k则由标量值和单位矢量k0 (k0 >0 )共同决定 ,而不必再考虑左、右手系的区别。在此基础上依据电磁场理论推导了光波在负折射率介质层中的传输矩阵 ,用以分析含有负折射率介质层的复杂薄膜系统的光学性质。初步计算结果显示含有负折射率介质层的膜系对TE和TM波的光谱特性都有显著改变     

超材料微波磁导率色散行为的电可调控性研究

研究在开口金属谐振环(SRR)结构嵌入一个电容二极管后,通过电压调控二极管的电容使得SRR结构的等效电容值发生改变,能实现对SRR的谐振频率和等效磁导率的调控,从而提出了一种智能的微波磁性超材料结构.采用时域差分有限元法(Finite-Difference Time-Domain)和恢复算法模拟了变容二极管的工作电压变化下,SRR结构谐振频率和磁导率的变化规律.研究结果表明工作电压增大使变容二极管的电容值减小时,将导致SRR结构的谐振频率向低频段移动,其磁导率的共振频率也将向低频移动.最后指出了SRR结构与常规磁性材料(如铁氧体)磁导率色散行为的不同之处. 关键词： 超材料 微波磁导率     

新型八边形谐振环金属线复合周期结构左手材料奇异性质研究

通过实验及仿真研究了基于C环的新型八边形开口谐振环金属线复合周期结构左手材料.以金属铜八边形谐振环(SRRs)为基本单元的周期结构负磁导率材料,与闭口环(CSRRs)结果对比发现八边形谐振环能产生很好的谐振效果即能产生负磁导率;复合结构仿真结果显示,八边形谐振环金属线复合结构实现负折射具有可行性.设计、制作并实验和仿真研究了两种尺寸的八边形谐振环金属线复合周期结构左手材料,实验结果显示,分别在9.8—15GHz和9.5—15GHz出现良好负折射效应,表明小尺寸材料负折射频段较宽但整体能量透过率较小.通过与尺寸相近的传统C环样品实验对比发现八边形样品损耗较大,但其负折射区域能量分布比例较大,具有一定的优越性.该研究对新型周期结构左手材料的研究、设计和研制具有重要的科学意义,在国防、通信等领域也具有广阔的应用前景.     

大气激光通信非对称限幅光正交频分复用技术

将一种新的正交频分复用调制技术运用到大气激光通信系统中,分析了采用这种非对称限幅光正交频分复用调制技术的大气激光通信系统在大气湍流信道下的性能.在此基础上,把非对称限幅光正交频分复用调制方案与传统的开关键控及直流偏置光正交频分复用调制方案进行了比较,并在大气湍流信道中进行了仿真.仿真结果表明:非对称限幅光正交频分复用调...     

谐振频率可调的环状开口谐振器结构及其效应

理论和实验研究表明，开口谐振环(SRRs)中可以激励磁谐振从而实现负磁导率.通过在SRRs结构中引入与其开口边平行的金属短杆设计并制备了新的磁谐振单元，采用波导法系统研究了短杆对SRRs和左手材料的微波透射特性以及左手效应的影响.实验和数值模拟表明：金属短杆和SRRs开口边形成附加电容，导致SRRs开口电容增大从而引起谐振频率降低.随短杆长度l和短杆与SRRs间距d的增大，SRRs谐振频率也随之减小和增加.短杆的加入不影响SRRs的负磁导率特性，改变短杆与SRRs间距d< 关键词： 开口谐振环 金属短杆 调控 左手材料     

基于石墨烯超材料表面等离子体激元共振的增强太赫兹调制

基于表面等离子体共振原理,采用石墨烯超材料设计了开口环结构,用于调制太赫兹波.增加石墨烯的费米能级,改变开口环的开口距离,叠加多层石墨烯以增强石墨烯超材料的共振强度,进而增强太赫兹波调制,调制频率范围包括低频段和高频段.由于石墨烯费米能级的可调谐性,单层结构在高低两个频段的调制深度分别为81%和68%,多层结构在高低两个频段的调制深度分别增加到93%和95%,为动态调制提供了可能.该设计为调制器、吸收体等太赫兹器件的设计提供了指导和借鉴.     

一维负磁导率材料中的缺陷效应

理论和实验已经表明，金属开口谐振环（split ring resonators,简称SRR）可以实现材料的有效磁导率μeff在某一频率范围内为负.采用波导法实验研究了不同几何尺寸的六边形SRR在X波段微波作用下的近邻相互作用.实验发现，相同尺寸的六边形SRR周期性排列而成的一维负磁导率材料存在一个谐振频率；当缺陷谐振环引入一维负磁导率材料时，主谐振频率和缺陷环谐振频率同时发生移动；主谐振频率和缺陷环谐振频率的移动量随缺陷与双环谐振频率之差的增加而减小，当缺陷环谐振频率与双环谐振频率接近时，环间的相互作用增强，频率的移动量增大. 关键词： 负磁导率 缺陷效应 开口谐振环     

Investigation of negative index properties of planar metamaterials based on split-ring pairs

A low losses broadband planar negative refractive index metamaterial based on split-ring resonator (SRR) pairs is proposed and investigated experimentally and numerically at microwave frequency range. The transmission spectra of the single-layer SRR pairs were measured, and exhibited left-handed (LH) transmission passband clearly. The metal-dielectric-metal structure exhibits strong magnetic and electric responses simultaneously, and leads to negative permeability and negative permittivity. To further verify the LH properties of planer structure metamaterials, effect media parameters were retrieved, a refraction phenomenon based on a wedge-shaped model and negative phase advance between consecutive numbers of layers were also demonstrated.     

射电天文用太赫兹三通带频率选择表面设计

本文设计一种基于单屏改进型开口谐振环(SRR)的太赫兹频率选择表面(FSS). 改进型 SRR 谐振单元由具有开口缝的金属贴片组成, 开口缝的物理尺寸会影响其阶跃特征阻抗特性. 本文通过对改进型 SRR 单元结构建立LC等效电路模型, 提取等效电路模型参数, 并结合传输线理论, 得到 FSS 的基频计算公式和谐波关系式. 相比于传统均匀 SRR, 本文所提出的改进型 SRR 多频带传输的控制更为灵活. 基于此特点, 设计了一款中心频率依次为 0.46 THz, 0.86 THz和 1.03 THz, 可应用于射电天文的三通带太赫兹 FSS. 采用电磁仿真软件对影响该 FSS 传输特性的关键参数、周期间隔、小型化程度以及入射角敏感性等重要指标进行分析研究. 结果表明, 改进型 SRR 三通带 FSS 在三个通带内的反射系数分别为 -37.6 dB, -13 dB和 -19.6 dB, 在0°–60° 范围内均具有稳定的频率响应特性, 且具有小型化程度高、损耗低等特点. 这种三通带 FSS 在太赫兹频段射电天文方面有潜在的应用价值.     

Experimental analysis of true left-handed behaviour and transmission properties of composite metamaterials

We report the true left-handed transmission of a composite metamaterial (CMM) consisting of periodically stacked split-ring resonator (SRR) and wire elements. The negative permeability (μ < 0) gap is demonstrated explicitly by comparing SRR and closed-ring resonator structures. We confirm experimentally that the plasma cut-off frequency of the CMM is determined by the combined dielectric response of SRR and wire elements, and it is much lower than that of the wire-only medium. This is crucial to identify the left-handed transmission bands of the CMM. We further investigate the effect of intralayer and interlayer disorder on the transmission spectrum of CMM arising from misaligned fabrication and stacking of the SRR layers. We found that the intralayer disorder affects the μ < 0 gap of SRRs and the left-handed transmission band of CMM significantly, whereas the SRR transmission is rather immune to interlayer disorder.     

基于开环振荡器阵列磁振荡效应的全光开关

We demonstrate an all-optical switching of the magnetic resonance properties associated with a metallic Split Ring Resonator（SRR） array. The periodically spaced elements are fabricated on a high-resistivity silicon wafer and probed by using conventional Terahertz （THz） time-domain spectroscopy. We use a continuous-wave laser diode to generate carriers in the gaps of the SRR elements. Using a sufficient power, this opti- cal excitation can create an effective short gap, which would switch the resonant properties of the metamaterial from that of an SRR array to that of a closed ring resonator array and leads to dramatic changes in the THz transmission. In the present experiment, the optically induced switching is associated with the magnetic reso- nance. However, with appropriate changes in the device structure, this approach can be extended to switch a medium with a negative real index of refraction to a medium with a positive real index of refraction. This opens the way to creat a broad new range of active devices.     

注入半导体激光器混沌调制性能与内部相位键控编码方法研究

研究注入半导体激光混沌调制解调及其性能特性，提出外部光注入半导体激光器混沌内部相位编码方法. 小信号调制分析了系统混沌调制响应函数和解调响应函数以及解调因子，系统的调制解调响应函数以及解调因子在60MHz有一个峰值，系统在高频调制时，响应函数和解调因子明显下降. 理论推导出混沌调制解调方程，给出了小信号调制时的信息信号的解，数值分析出系统在低频有一个调制解调峰值，高频调制解调效果下降，但系统仍可以进行10⁹Hz频率调制解调. 数值模拟并证明了系统在较大的参数范围内都能实现同步，模拟了混沌调制速率200Mb/s和调制频率1.4GHz的保密通信应用. 还通过键控调制注入光的相位相移对激光混沌态进行了编码，通过同步与非同步进行混沌解码，数值模拟了具有调制速率10Mb/s的混沌相位键控编码通信应用. 关键词： 混沌 同步 激光器 相位