Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.     

DNA-based self-assembly of chiral plasmonic nanostructures with tailored optical response

Matter structured on a length scale comparable to or smaller than the wavelength of light can exhibit unusual optical properties. Particularly promising components for such materials are metal nanostructures, where structural alterations provide a straightforward means of tailoring their surface plasmon resonances and hence their interaction with light. But the top-down fabrication of plasmonic materials with controlled optical responses in the visible spectral range remains challenging, because lithographic methods are limited in resolution and in their ability to generate genuinely three-dimensional architectures. Molecular self-assembly provides an alternative bottom-up fabrication route not restricted by these limitations, and DNA- and peptide-directed assembly have proved to be viable methods for the controlled arrangement of metal nanoparticles in complex and also chiral geometries. Here we show that DNA origami enables the high-yield production of plasmonic structures that contain nanoparticles arranged in nanometre-scale helices. We find, in agreement with theoretical predictions, that the structures in solution exhibit defined circular dichroism and optical rotatory dispersion effects at visible wavelengths that originate from the collective plasmon-plasmon interactions of the nanoparticles positioned with an accuracy better than two nanometres. Circular dichroism effects in the visible part of the spectrum have been achieved by exploiting the chiral morphology of organic molecules and the plasmonic properties of nanoparticles, or even without precise control over the spatial configuration of the nanoparticles. In contrast, the optical response of our nanoparticle assemblies is rationally designed and tunable in handedness, colour and intensity-in accordance with our theoretical model.     

Synthesis and Largely Enhanced Nonlinear Refraction of Au@Cu_2O Core-Shell Nanorods

Au@Cu_2O core-shell nanorods with tunable thickness of Cu_2O shell were synthesized and their linear and nonlinear optical responses were investigated. Two transverse plasmon resonance peaks were observed when the Au nanorods were coated with Cu_2O shells, which were adjusted by the Cu_2O shell thickness. The nonlinear absorption of the Au@Cu_2O nanorods is enhanced by 5 times at the longitudinal plasmon resonance wavelength compared with that of bare Au nanorods. More intriguingly, largely enhanced nonlinear refraction and suppressed nonlinear absorption at the transverse plasmon resonance wavelength were observed in the Au@Cu_2O nanorods. Our findings indicate the existence of strong local field enhancement at the interface between the Au core and the Cu_2O shell, which would provide a promising strategy in designing plasmonic nonlinear nanodevices with good nonlinear figures of merit.     

Synthesis and Largely Enhanced Nonlinear Refraction of Au@Cu<Subscript>2</Subscript>O Core-Shell Nanorods

Au@Cu₂O core-shell nanorods with tunable thickness of Cu₂O shell were synthesized and their linear and nonlinear optical responses were investigated. Two transverse plasmon resonance peaks were observed when the Au nanorods were coated with Cu₂O shells, which were adjusted by the Cu₂O shell thickness. The nonlinear absorption of the Au@Cu₂O nanorods is enhanced by 5 times at the longitudinal plasmon resonance wavelength compared with that of bare Au nanorods. More intriguingly, largely enhanced nonlinear refraction and suppressed nonlinear absorption at the transverse plasmon resonance wavelength were observed in the Au@Cu₂O nanorods. Our findings indicate the existence of strong local field enhancement at the interface between the Au core and the Cu₂O shell, which would provide a promising strategy in designing plasmonic nonlinear nanodevices with good nonlinear figures of merit.     

Solving surface plasmon resonances and near field in metallic nanostructures: Green’s matrix method and its applications

With the development of nanotechnology, many new optical phenomena in nanoscale have been demonstrated. Through the coupling of optical waves and collective oscillations of free electrons in metallic nanostructures, surface plasmon polaritons can be excited accompanying a strong near field enhancement that decays in a subwavelength scale, which have potential applications in the surface-enhanced Raman scattering, biosensor, optical communication, solar cells, and nonlinear optical frequency mixing. In the present article, we review the Green’s matrix method for solving the surface plasmon resonances and near field in arbitrarily shaped nanostructures and in binary metallic nanostructures. Using this method, we design the plasmonic nanostructures whose resonances are tunable from the visible to near-infrared, study the interplay of plasmon resonances, and propose a new way to control plasmonic resonances in binary metallic nanostructures.     

可调谐石墨烯光电子器件

凭借着良好的光电、力学和热学性能,石墨烯是目前凝聚态物理领域一个重要的研究热点.而且由于石墨烯的色散关系在狄拉克点附近呈现线性特性,石墨烯的光电特性可以通过外加电场、磁场和温度来加以调节.因此,石墨烯是研究可调谐器件的良好平台.基于石墨烯的电导率随费米能级有明显改变的特点,在分析石墨烯器件的调制机制的基础上,对石墨烯可调谐器件在太赫兹、中红外和近红外的应用发展进行了综述研究.     

Quantum plasmon resonances of individual metallic nanoparticles

The plasmon resonances of metallic nanoparticles have received considerable attention for their applications in nanophotonics, biology, sensing, spectroscopy and solar energy harvesting. Although thoroughly characterized for spheres larger than ten nanometres in diameter, the plasmonic properties of particles in the quantum size regime have been historically difficult to describe owing to weak optical scattering, metal-ligand interactions, and inhomogeneity in ensemble measurements. Such difficulties have precluded probing and controlling the plasmonic properties of quantum-sized particles in many natural and engineered processes, notably catalysis. Here we investigate the plasmon resonances of individual ligand-free silver nanoparticles using aberration-corrected transmission electron microscope (TEM) imaging and monochromated scanning TEM electron energy-loss spectroscopy (EELS). This technique allows direct correlation between a particle's geometry and its plasmon resonance. As the nanoparticle diameter decreases from 20 nanometres to less than two nanometres, the plasmon resonance shifts to higher energy by 0.5 electronvolts, a substantial deviation from classical predictions. We present an analytical quantum mechanical model that describes this shift due to a change in particle permittivity. Our results highlight the quantum plasmonic properties of small metallic nanospheres, with direct application to understanding and exploiting catalytically active and biologically relevant nanoparticles.     

局域表面等离子体研究进展

阐述了局域表面等离子体特性,金属纳米粒子的常用制备方法,以及不同形状、尺寸等因素对局域表面等离子体光谱和灵敏度的影响,分析了表面增强拉曼散射的增强因子与金属纳米粒子的等离子共振波长和拉曼激发波长之间的关系,介绍了局域表面等离子体在生物传感方面的应用.     

低维量子系统等离激元介电函数表象变换理论

推导低维量子系统中等离激元介电函数的矩阵表达式，建立了矩阵表达式、张量表达式和傅立叶表达式之间的变换关系.根据有限子带模型,在不同表象下计算量子阱结构和半导体超晶格中等离激元模式的色散关系,并通过数值计算证实了在这3种表象下等离激元介电函数表达式是彼此等价的.     

基于表面等离激元的量子相干效应的理论研究

起源于金属中自由电子集体振荡的表面等离激元,具有超小的光学模式体积和亚波长局域的近场增益,为纳米尺度上研究光和物质相互作用带来新的机遇．共振的纳米金属结构的近场区域,具有各向异性的珀塞尔系数,并且可以为量子体系提供近场激发．我们理论上演示了基于表面等离激元结构的单分子共振荧光、原子布居数的本征量子拍频及其在表面等离激元结构中的纳米尺度上的实现、表面等离激元诱导的各向异性珀塞尔系数导致的亚波长尺度自发辐射谱线的变化．这些结果在超紧凑的有源量子器件中有潜在应用．     

表面等离激元亚波长光波导滤波器研究进展

综述了本研究小组有关单齿、多齿、侧耦合腔、迂回型等结构的表面等离激元波导滤波器的研究进展.在单齿状金属-电介质-金属波导结构的研究基础上,发展出对称和非对称多齿结构以及侧耦合腔型反射式滤波器、低通波长和高通波长滤波器等其他结构.时域有限差法模拟揭示了它们均可具有一定的波长选择特性,对称多齿结构展现一个宽的陡峭禁带,不对称多齿结构却能实现窄带滤波功能.分析和模拟研究均表明谷或峰的中心波长与齿的深度或宽度有关;截止型结构的截止波长则可通过改变结构的相应长度和宽度等参数来选择.这些滤波器结构有可能应用于未来的纳米集成光学回路.     

Plasmonic Bloch oscillations and resonant Zener tunneling in subwavelength metal-dielectric-air waveguide superlattices

In this paper,we study the propagation properties of surface plasmon polaritons in plasmonic single-cavity superlattices and two-cavity superlattices which are composed of two kinds of alternately stacked subwavelength metal-dielectric-air waveguides with large dispersion.Theoretical predictions of plasmonic time-resolved Bloch oscillations existing in single-cavity superlat-tices and resonant Zener tunneling(ZT) occurring in two-cavity superlattices by the transfer matrix method(TMM) are well dem-onstrated by the numerical simulations on the propagation of SPPs pulse in the two kinds of superlattices by the finite-difference time-domain(FDTD) method.The two proposed superlattices can be conveniently fabricated by present nanotechnology,and the study may promote the realization of plasmonic BO and resonant ZT in nanoscale devices experimentally.     

基于石墨烯光电特性的超宽带可调超材料吸波体设计

利用石墨烯的电导率可调特性设计了一种超宽带可调超材料吸波体。模拟计算了不同石墨烯费米能级时吸波体的吸收率,结果表明,当石墨烯费米能级为0.7 eV时,吸波体在1.74 GHz ~10.44 GHz 的吸收率保持在90%以上,实现了电磁波的超宽带吸收;当改变外加电压使石墨烯的费米能级从0.7 eV逐渐减少到0 eV时,吸波体在1.74 GHz~10.44 GHz的吸收率逐渐下降,其调制深度可达53.8%,实现了吸收率可调的功能;通过对表面电流分布进行仿真与分析,阐述了其电磁波宽带吸收及吸收率可调的机理;模拟分析了石墨烯费米能级为0.7 eV时,入射波极化状态和入射角度对吸波体吸收特性的影响,结果表明,由于结构单元的旋转对称性,吸波体的吸收特性具有极化不敏感的特点;随着电磁波入射角度的增大,其吸收率逐渐降低。     

表面等离子体激元增强荧光辐射的机理研究

荧光光谱技术在研究物质分子结构及物质相互作用动力学过程中具有重要应用,基于荧光的化学或生物传感器得到人们广泛研究,如何提高基于荧光传感器的灵敏度是一个重要研究课题,表面等离子体激元可有效增强荧光分子的发光强度,因而从物理机制上研究表面等离子体激元增强荧光的内函具有重要意义。本文在讨论荧光辐射基本原理的基础上,论述了基于金属的表面等离子体激元增强荧光的物理机制,主要有荧光共振能量转移、表面等离子体激元共振增强荧光辐射理论和RP模型等三种理论模型。     

Atom-by-atom spectroscopy at graphene edge

The properties of many nanoscale devices are sensitive to local atomic configurations, and so elemental identification and electronic state analysis at the scale of individual atoms is becoming increasingly important. For example, graphene is regarded as a promising candidate for future devices, and the electronic properties of nanodevices constructed from this material are in large part governed by the edge structures. The atomic configurations at graphene boundaries have been investigated by transmission electron microscopy and scanning tunnelling microscopy, but the electronic properties of these edge states have not yet been determined with atomic resolution. Whereas simple elemental analysis at the level of single atoms can now be achieved by means of annular dark field imaging or electron energy-loss spectroscopy, obtaining fine-structure spectroscopic information about individual light atoms such as those of carbon has been hampered by a combination of extremely weak signals and specimen damage by the electron beam. Here we overcome these difficulties to demonstrate site-specific single-atom spectroscopy at a graphene boundary, enabling direct investigation of the electronic and bonding structures of the edge atoms-in particular, discrimination of single-, double- and triple-coordinated carbon atoms is achieved with atomic resolution. By demonstrating how rich chemical information can be obtained from single atoms through energy-loss near-edge fine-structure analysis, our results should open the way to exploring the local electronic structures of various nanodevices and individual molecules.     

Manipulation of light in MIM plasmonic waveguide systems

Plasmonic waveguides that allow deeply subwavelength confinement of light provide an effective platform for the design of ultracompact photonic devices.As an important plasmonic waveguide,metal-insulator-metal(MIM)structure supports the propagation of light in the nanoscale regime at the visible and near-infrared ranges.Here,we focus on our work in MIM plasmonic waveguide devices for manipulating light,and review some of the recent development of this topic.We introduce MIM plasmonic wavelength filtering and demultiplexing devices,and present the electromagnetic induced transparency(EIT)-like and Fano resonance effects in MIM waveguide systems.The slow-light and rainbow trapping effects are demonstrated theoretically.These results pave a way toward dynamic control of the special and useful optical responses,which actualize some new plasmonic waveguide-integrated devices such as nanoscale filters,demultiplexers,sensors,slow light waveguides,and buffers.     

Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.     

Tunable nanowire nonlinear optical probe

One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion, structured illumination, and photoactivated localization microscopy, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.     

基于石墨烯复合薄膜的等离激元传感研究进展

 做为由单层碳原子紧密堆积而成的六边形蜂窝状二维晶体,石墨烯具有高载流子迁移率、良好的生物兼容性和优异的化学稳定性。本文简要综述了石墨烯-金属纳米粒子复合薄膜在表面增强拉曼散射研究进展,以及石墨烯等离激元的激发方式和传感性能。在可见光波段,石墨烯和金属纳米粒子之间的耦合使复合薄膜具有强的光学吸收和局域电场增强,从而使复合薄膜可以作为高灵敏的表面增强拉曼基底。在中红外波段,除可以利用石墨烯微纳结构激发等离激元,还可以对介电基底进行微纳加工利用波导模式激发,使得石墨烯等离激元可能用于折射率传感。讨论了石墨烯基复合薄膜研究过程中面临的机遇和挑战,展望了其在表面增强拉曼和传感方面的应用前景。     

基于金属-绝缘体-金属光栅结构的功分器及分波器设计

设计了一种基于仿表面等离激元（Spoof surface plasmon polaritons,Spoof SPPs）的金属-绝缘体-金属（metal/insulator/metal,MIM）光栅结构的Y型功分器和分波器。MIM光栅结构由两块内侧加工有周期性凹槽的有限厚度金属板构成,工作于微波波段,采用单极子作为激励来激发光栅结构上的仿表面等离激元。其中功分器的两个输出分支采用凹槽深度相同的光栅结构,可以将从输入端来的电磁波平均分成两个部分,然后分别朝着两个输出分支传播;而分波器的两个输出分支则采用凹槽深度不同的光栅结构,可以使不同频率的电磁波朝着不同的分支传播。利用三维电磁仿真软件CST微波工作室（CSTmicro-wave studio）对该功分器和分波器进行仿真分析,加工出分波器实物并进行了实验测试,测试结果与仿真分析基本吻合,验证了方案的可行性。