金属-介质-金属渔网超表面结构调控的荧光辐射:空间选择性激发和双增强（英文）

增强荧光辐射在生物成像、高灵敏探测、集成光源等方面都具有重要的应用价值.金属纳米颗粒的周围或者金属纳米结构的间隙都可以产生强的电磁场,相应的,这些结构附近的局域态密度也被极大地增强.虽然增强荧光辐射已经在多种金属纳米颗粒和颗粒对中被证明,但是利用金属纳米结构对荧光分子的吸收和辐射过程同时进行调制仍然是一个有挑战的问题.本文研究了金属-介质-金属超表面对荧光辐射的调控,其中局域表面等离激元(LSP)和磁等离激元(MPP)分別与于分子的吸收和辐射过程发中耦合相互作用.对于吸收过程,LSP的耦合作用使得可以通过旋转泵浦激光的偏振态来实现荧光分子的空间选择激发.此外,MPP模式的偏振依赖特性使得矩形渔网结构中的荧光分子的辐射波长和偏振态也受到调控.实验观测结果经过了时域有限差分模拟的验证.本文报道的纳米结构在光辐射器件和纳米尺度集成光源等方面都具有潜在的应用价值.     

金属-介质-金属纳米天线阵列的模式特性及荧光发射调控

设计一种支持多模式的金属-介质-金属纳米天线阵列结构,分析结构中的模式特性及其调控的发光过程。利用时域有限差分的方法模拟该结构的透射谱和电场分布,分析结构中局域表面等离激元模式和磁等离激元共振模式的特性以及激发光偏振调控的模式变化。将偶极子光源放在介质层中,模拟该天线阵列结构调控荧光分子的发光过程。结果表明:荧光分子的辐射和非辐射衰减速率增强因子、量子效率以及偏振特性受到了所提结构模式的有效调控;在一定波长范围内改变激发光的偏振方向可以对发光谱进行调谐。     

阴极荧光在表面等离激元研究领域的应用

表面等离激元以其独特的光学性质广泛应用于纳米尺度的局域电磁场增强、超高分辨成像及微弱光电探测.阴极荧光是电子与物质相互作用而产生的光学响应,利用电子束激发金属纳米结构能够实现局域等离激元共振,并在亚波长尺度实现对共振模式的调控,具有超高空间分辨的成像特点.阴极荧光探测通常结合扫描电子显微镜或透射电子显微镜而实现,目前己被应用于表面等离激元的探测及共振模式的分析.本文从阴极荧光物理机理出发,综述了单一金属纳米结构和金属耦合结构的等离激元共振模式阴极荧光研究进展,并总结了阴极荧光与角分辨、时间分辨以及电子能量损失谱等关键技术相结合的应用,进一步分析了其面临的关键问题,最后展望了阴极荧光等离激元研究方向.     

单个等离激元纳米颗粒和纳米间隙结构与量子发光体的强耦合

在腔量子电动力学中,如果量子发光体与腔模式的耦合强度超过二者的平均损耗,就进入了强耦合区域,此时会形成部分光部分物质的新量子态—极化激元态.强耦合在室温玻色-爱因斯坦凝聚、极化激元激光、单光子非线性、量子信息等领域有重要的应用价值.基于单个金属纳米颗粒的结构可以支持局域表面等离激元共振,拥有极小的模式体积,非常有利于强耦合现象的发生.本文主要介绍了强耦合的理论背景、单个金属纳米颗粒和纳米间隙结构与量子发光体的强耦合、以及强耦合的动态调控,并展望了该领域的研究前景.     

银纳米颗粒阵列的表面增强拉曼散射效应研究

利用具有高密度拉曼热点的金属纳米结构作为表面增强拉曼散射(SERS)基底,可以显著增强吸附分子的拉曼信号.本文通过阳极氧化铝模板辅助电化学法沉积制备了高密度银(Ag)纳米颗粒阵列;利用扫描电子显微镜和反射谱表征了样品的结构形貌和表面等离激元特性;用1, 4-苯二硫醇(1, 4-BDT)为拉曼探针分子,研究了Ag纳米颗粒阵列的SERS效应.通过优化沉积时间,制备出高SERS探测灵敏度的Ag纳米颗粒阵列,检测极限可达10~(-13)mol/L;时域有限差分法模拟结果证实了纳米颗粒间存在强的等离激元耦合作用,且发现纳米颗粒底端的局域场增强更大.研究结果表明Ag纳米颗粒阵列可作为高效的SERS基底.     

光激发作用下分子与多金属纳米粒子间的电荷转移研究

金属纳米粒子在光激发作用下的增强作用是纳米科学领域的一个研究热点. 针对分子和多个不同位形下的金属纳米粒子在光激发下的相互作用展开了理论研究. 应用密度矩阵理论描述分子和金属纳米粒子同时激发产生表面等离激元后的电荷输运过程. 研究发现, 表面等离激元增强效应与分子和各个金属纳米粒子的相对位置有密切关系. 详细分析了金属纳米粒子间的耦合强度、分子和金属纳米粒子间的耦合强度、表面等离激元能级杂化、分子激发能和外场频率对表面等离激元增强效应的影响.     

局域表面等离激元

局域表面等离激元使得贵金属纳米颗粒具有丰富的光学性质,其应用涵盖能源、生物医学、安全、信息、超材料等诸多领域。文章简要介绍局域表面等离激元的基本性质、贵金属纳米结构中的局域表面等离激元共振耦合以及具有局域表面等离激元特性的贵金属纳米结构的一些重要应用。     

局域态密度对表面等离激元特性影响的研究

表面等离激元是一种在金属与介质界面上激发并耦合电荷密度起伏的电磁振荡, 具有近场增强和短波长等特性, 在纳米光子学的研究中扮演重要角色. 将表面等离激元的效应用于单光子源的制备, 不但可以有效减小器件的体积, 而且可以有效提高单光子的辐射和收集效率. 本文根据表面等离激元的珀赛尔系数与光子态密度的关系, 采用局域态密度计算的方法, 分析了不同金属材料的局域态密度及珀赛尔系数的特性. 通过计算比较, 选择银为最佳金属材料, 并在此基础上讨论了探测距离和电介质材料对局域态密度和珀赛尔系数的影响, 为基于表面等离子激元的单光子源制备提供重要参数.     

金局域表面等离激元增强砷化镓发光特性

研究了金纳米颗粒局域表面等离激元共振耦合效应,并实现了砷化镓薄膜的近场发光增强.通过理论计算金纳米颗粒的吸收光谱及电场分布,分析金属纳米颗粒形貌尺寸的改变对等离激元共振频率调控及局域场增强效果的影响,模拟半径为50nm的金颗粒并实现了35倍近场增强效果.通过对双球型的模拟,分析了一种金纳米颗粒增强GaAs的积极方式,即密集颗粒之间的近场耦合形成的"hotspots".此外,研究了不同溅射时间及快速退火对金纳米颗粒吸收特性的影响,发现金纳米颗粒吸收峰位主要位于560～680nm波段,而且随着溅射时间的增加发生红移现象.经过快速退火处理后,金纳米颗粒吸收峰位蓝移到510～550nm波段,形成与532nm激发波长相匹配的共振吸收峰.最后,实现砷化镓薄膜9.6倍的光致发光增强.     

表面等离激元——机理、应用与展望

等离激元光子学(plasmonics)的研究内容是金属纳米结构独特的光学性质及其应用.随着纳米科技的进步,等离激元光子学已经迅速发展成为一门新兴学科,在生物、化学、能源、信息等领域具有重要的应用前景.文章主要介绍表面等离激元(surface plasmons,SPs)的一些基本物理性质,包括局域的表面等离激元(localized surface plas-mon,LSP)和传导的表面等离激元(propagating surface plasmon polariton,SPP),文章还介绍了表面等离激元的几个重要的应用方向,例如生物/化学传感器、表面等离激元激光、光开关器件以及表面等离激元光逻辑运算,等等.     

Enlarging the negative-index bandwidth of optical metamaterials by hybridized plasmon resonances

By exploiting the concept of internal surface plasmon polariton (I-SPP) resonances, which appear at nonsingle metallic film stacks, we have designed a metamaterial showing a negative effective index within a large frequency bandwidth. The designed structure consists of an arrangement of several fishnet layers. By properly adjusting the lattice and the thickness of the dielectric slab of the fishnet, an I-SPP mode can be excited at a certain frequency, giving rise to a negative permeability. Thus, when combining several fishnet layers, each one configured to excite an I-SPP at a slightly different frequency, the coupling among the fishnet layers will cause a plasmon hybridization effect that enables us to extend the negative-index bandwidth.     

纳米银六角阵列在掺氧氮化硅中的局域表面等离激元共振特性仿真

通过COMSOL Multiphysics 和 Lumerical FDTD solution对不同尺寸纳米银六角阵列在非晶态掺氧氮化硅(a-SiN_x:O)介质中的局域表面等离激元共振(LSPR)特性进行仿真, 计算结果表明半径为25 nm的纳米银六角阵列形成的局域表面等离激元(LSP)与厚度为70 nm的a-SiN_x:O的蓝光发射(460 nm)的共振效果最为显著, 随着纳米银颗粒尺寸的增大其消光共振峰红移. 在460 nm波长激发下半径为25 nm的纳米银阵列在a-SiN_x:O中的极化强度和表面极化电荷的分布模拟证明了该阵列在460 nm激发下形成的LSP为偶极子极化模式, 通过对该尺寸的纳米银阵列的LSP 在a-SiN_x:O中的最强垂直辐射空间计算, 获得了银颗粒上方a-SiN_x:O的最佳厚度为30 nm, 仿真结果对硅基蓝光发射器件(450–460 nm)的设计提供了重要的理论参考.     

The effect of gain and absorption on surface plasmons in metal nanoparticles

The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption. PACS 61.46.Df; 73.20.Mf; 78.67.Bf     

芯帽纳米颗粒的光热性质

贵金属纳米结构中的光热效应在肿瘤光热治疗、光热成像、纳米药物等领域具有重要的应用价值.各向异性的芯帽纳米结构以其丰富的可调结构参数和对激发光偏振态敏感的特性,可灵活地在近红外波段获得理想的光学吸收性质,从而可以实现温度的高效调节.本文基于有限元方法研究了颗粒物纳米结构参数对其光热效果的作用规律,数值结果表明:通过对结构参数的微量改变(包括金壳厚度、芯壳比、芯径、金属表面覆盖率等)可实现温度的显著调整;在偏振态的旋转范围(30?—70?)内可快速地产生大温变光热的准线性调整.其不弱于纳米芯壳和纳米棒结构的光热性能可为纳米光热生医研究提供一种新的选择.     

Enhanced photoluminescence of conjugated polymer thin films on nanostructured silver

Photoluminescence of a conjugated polymer in the presence of surface plasmons on metallic nanoparticles is studied. A layered device structure was constructed that enabled control over nanoparticle diameter and separation between the polymer and nanoparticles layers. The dependence of the surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at a 40 nm distance separation between the fluorophore and the surface plasmon. A spectrum of surface plasmon resonances ranging from the emission to the absorption energies of the conjugated polymer revealed largest enhancements when the resonance was tuned to the conjugated polymer emission energy. At peak photoluminescence the maximum photoluminescent enhancement was found to be 5.6 times of the photoluminescence of the control structure and the total integrated enhancement was 5.9 times.     

Spontaneous emission in micro- and nano-structures

Spontaneous emission of emitters governing the performance of optoelectronic devices is a fundamental phenomenon, and it has strong environment-dependent characteristics. In this article, we mainly review the experimental and theoretical progresses in the control of spontaneous emission by manipulating optical modes with photonic crystals, optical microcavities and metallic nanostructures. The spontaneous emission from emitters in photonic crystals can be modified by the local density of states, and by employing photonic crystals, the devices’ efficiency is enhanced, the angular radiation pattern can be engineered, and highly efficient optoelectronic devices are achieved through decreasing the radiative lifetime. In quantum optical devices, microcavities would alter the lifetime of an excited state through tuning the resonance in the frequency and positioning between the emitters and cavity field, and inducing the emitters to emit spontaneous photons in a desired direction. The emerging enhanced electromagnetic field near metallic nanostructures can help to control and manipulate the spontaneous emission of an emitter. The use of micro- and nano-structures to manipulate spontaneous emission will open unprecedented opportunities for realizing functional photonic devices.     

Quantum optics with surface plasmons

We describe a technique that enables strong, coherent coupling between individual optical emitters and guided plasmon excitations in conducting nanostructures at optical frequencies. We show that under realistic conditions optical emission can be almost entirely directed into the plasmon modes. As an example, we describe an application of this technique involving efficient generation of single photons on demand, in which the plasmon is efficiently outcoupled to a dielectric waveguide.     

Spontaneous emission in micro- and nano-structures

Spontaneous emission of emitters governing the performance of optoelectronic devices is a fundamental phenomenon, and it has strong environment-dependent characteristics. In this article, we mainly review the experimental and theoretical progresses in the control of spontaneous emission by manipulating optical modes with photonic crystals, optical microcavities and metallic nanostructures. The spontaneous emission from emitters in photonic crystals can be modified by the local density of states, and by employing photonic crystals, the devices’ efficiency is enhanced, the angular radiation pattern can be engineered, and highly efficient optoelectronic devices are achieved through decreasing the radiative lifetime. In quantum optical devices, microcavities would alter the lifetime of an excited state through tuning the resonance in the frequency and positioning between the emitters and cavity field, and inducing the emitters to emit spontaneous photons in a desired direction. The emerging enhanced electromagnetic field near metallic nanostructures can help to control and manipulate the spontaneous emission of an emitter. The use of micro- and nano-structures to manipulate spontaneous emission will open unprecedented opportunities for realizing functional photonic devices.     

Perfect coupling of light to surface plasmons by coherent absorption

We show theoretically that coherent light can be completely absorbed and transferred to surface plasmons in a two- or three-dimensional metallic nanostructure by exciting it with the time-reversed mode of the corresponding surface plasmon laser ("spaser"). The narrow-band perfect absorption is a generalization and application of the concept of critical coupling to a nanocavity with surface plasmon resonances. Perfect coupling of light to nanostructures has potential applications to nanoscale probing as well as background-free spectroscopy and ultrasensitive detection or sensing.     

Numerical synthesis of metallic nanostructures for enhancing the emission of a dipole through surface plasmon coupling

In this study, we numerically synthesize a two-dimensional metallic nanostructure consisting of a Au half-space and two separate Ag elliptical cylinders by the simulated annealing (SA) method. The simulated nanostructure is so designed that the surface plasmon polariton (SPP) and the localized surface plasmon (LSP) are simultaneously excited at their common resonant wavelength (535 nm), leading to the enhancement of emission of a nearby dipole source. This enhancement effect is more significant than that of the case where only one of the SPP and LSP is excited. In numerically synthesizing a metallic nanostructure, we try to maximize both the downward emission (in the direction away from the metallic structure) and the emission efficiency. A cost function is defined as some combination of the downward emission and the emission efficiency. We adjust the simulated structure by SA to minimize the cost function at a designated resonant wavelength, and calculate and analyze the spectra of downward emission and emission efficiency for the optimal structure. Other structures are also investigated for comparison. From numerical simulations, it is demonstrated that the enhancement of dipole emission is better for optimization at wavelength 535 nm than at other wavelengths. Note that the downward emission and the emission efficiency can reach maxima almost simultaneously when the SPP and the LSP couple effectively at a common resonant wavelength. This implies that the lighting efficiency of green light-emitting diodes (LEDs) can be increased by the coupling effect at a common resonant wavelength of SPP and LSP.