Input impedance, nanocircuit loading, and radiation tuning of optical nanoantennas

Here we explore the radiation features of optical nanoantennas, analyzing the concepts of optical input impedance, optical radiation resistance, impedance matching, and loading of plasmonic nanodipoles. We discuss how the concept of antenna impedance may be applied to optical frequencies and how its quantity may be properly defined and evaluated. We exploit these concepts in the optimization of nanoantenna loading by optical nanocircuit elements, extending classic concepts of radio-frequency antenna theory to the visible regime for the proper design and matching of plasmonic nanoantennas.     

Plasmonic abilities of gold and silver spherical nanoantennas in terms of size dependent multipolar resonance frequencies and plasmon damping rates

Absorbing and emitting optical properties of a spherical plasmonic nanoantenna are described in terms of the size dependent resonance frequencies and damping rates of the multipolar surface plasmons (SP). We provide the plasmon size characteristics for gold and silver spherical particles up to the large size retardation regime where the plasmon radiative damping is significant. We underline the role of the radiation damping in comparison with the energy dissipation damping in formation of receiving and transmitting properties of a plasmonic particle. The size dependence of both: the multipolar SP resonance frequencies and corresponding damping rates can be a convenient tool in tailoring the characteristics of plasmonic nanoantennas for given application. Such characteristics enable to control an operation frequency of a plasmonic nanoantenna and to change the operation range from the spectrally broad to spectrally narrow and vice versa. It is also possible to switch between particle receiving (enhanced absorption) and emitting (enhanced scattering) abilities. Changing the polarization geometry of observation it is possible to effectively separate the dipole and the quadrupole plasmon radiation from all the non-plasmonic contributions to the scattered light.     

Resonance tuning and broadening of bowtie nanoantennas on graphene

Metallic bowtie antennas are used in nanophotonics applications in order to confine the electromagnetic field into volumes much smaller than that of the incident wavelength. Electrically controllable carrier concentration of graphene opens the door to the use of plasmonic nanoantenna structures with graphene so that the resonant nature of nanoantennas can be tuned. In this study, we demonstrated with the Fourier transform infrared (FTIR) spectroscopy and the Finite Difference Time Domain (FDTD) method that the intensity and resonance peak of bowtie nanoantennas on monolayer graphene can be tuned at mid-infrared (MIR) wavelength regime by applying a gate voltage, since the optical properties of graphene change by changing the carrier concentration.     

Optical binding forces between plasmonic nanocubes：A numerical study based on discrete-dipole approximation

Plasmonic nanocubes are ideal candidates in realizing controllable reflectance surfaces, unidirectional nanoantennas and other plasmon-associated applications. In this work, we perform full-wave calculations of the optical forces in threedimensional gold nanocube dimers. For a fixed center-to-center separation, the rotation of the plasmonic nanocube leads to a slight shift of the plasmonic resonance wavelength and a strong change in the optical binding forces. The effective gap and the near field distribution between the two nanocubes are shown to be crucial to this force variation. We further find that the optical binding force is dominated by the scattering process while the optical forces in the wavevector direction are affected by both scattering and absorption, making the former relatively more sensitive to the rotation of(an effective gap between) the nanocubes. Our results would be useful for building all-optically controllable meta-surfaces.     

Binary teaching-learning-based optimization algorithm is used to investigate the superscattering plasmonic nanodisk

A new efficient binary optimization method based on Teaching-Learning-Based Optimization (TLBO) algorithm is proposed to design an array of plasmonic nanodisks in order to achieve maximum scattering coefficient spectrum. In binary TLBO (BTLBO), a group of learner consists of a matrix with binary entries; control the presence (‘1’) or the absence (‘0’) of nanodisks in the array. Simulation results show that scattering coefficient strongly depends on the localized position of nanoparticles and non-periodic structures have more appropriate response in term of scattering coefficient. This approach can be useful in optical applications such as plasmonic nanoantennas.     

Plasmonic nanoparticle monomers and dimers: from nanoantennas to chiral metamaterials

We review the basic physics behind light interaction with plasmonic nanoparticles. The theoretical foundations of light scattering on one metallic particle (a plasmonic monomer) and two interacting particles (a plasmonic dimer) are systematically investigated. Expressions for the effective particle susceptibility (polarizability) are derived, and applications of these results to plasmonic nanoantennas are outlined. In the long-wavelength limit, the effective macroscopic parameters of an array of plasmonic dimers are calculated. These parameters are attributable to an effective medium corresponding to a dilute arrangement of nanoparticles, i.e., a metamaterial where plasmonic monomers or dimers have the function of “meta-atoms”. It is shown that planar dimers consisting of rod-like particles generally possess elliptical dichroism and function as atoms for planar chiral metamaterials. The fabricational simplicity of the proposed rod-dimer geometry can be used in the design of more cost-effective chiral metamaterials in the optical domain.     

Metamaterials and plasmonics: From nanoparticles to nanoantenna arrays,metasurfaces, and metamaterials

The rise of plasmonic metamaterials in recent years has unveiled the possibility of revolutionizing the entire field of optics and photonics, challenging well-established technological limitations and paving the way to innovations at an unprecedented level To capitalize the disruptive potential of this rising field of science and technology, it is important to be able to combine the richness of optical phenomena enabled by nanoplasmonics in order to realize metamaterial components, devices, and systems of increasing complexity. Here, we review a few recent research directions in the field of plasmonic metamaterials, which may foster further advancements in this research area. We will discuss the anomalous scattering features enabled by plasmonic nanoparticles and nanoclusters, and show how they may represent the fundamental building blocks of complex nanophotonic architectures. Building on these concepts, advanced components can be designed and operated, such as optical nanoantennas and nanoantenna arrays, which, in turn, may be at the basis of metasurface devices and complex systems. Following this path, from basic phenomena to advanced functionalities, the field of plasmonic metamaterials offers the promise of an important scientific and technological impact, with applications spanning from medical diagnostics to clean energy and information processing.     

Near-field plasmonic coupling for enhanced nonlinear absorption by femtosecond pulses in bowtie nanoantenna arrays

Plasmonic bowtie nanoantennas (BNAs) can exhibit a strong enhancement of optical field, leading to large nonlinear effects. We investigated the nonlinear optical absorption of an array of BNA by femtosecond pulses, using the open-aperture Z-scan technique. The BNA array composed of paired gold nanotriangles was fabricated by nanosphere lithography. We experimentally demonstrated that upon decreasing the gap width, nonlinear absorption is enhanced due to both the enhancement of near-field coupling of nanoantennas and the minimum of the spectral detuning between the center wavelength of the laser for excitation and the localized surface plasmon resonances. The role of near-field resonant plasmonic coupling in BNA is analyzed theoretically and confirmed by our simulations.     

Ultra-compact graphene plasmonic filter integrated in a waveguide

Graphene plasmons have become promising candidates for deep-subwavelength nanoscale optical devices due to their strong field confinement and low damping. Among these nanoscale optical devices, band-pass filter for wavelength selection and noise filtering are key devices in an integrated optical circuit. However, plasmonic filters are still oversized because large resonant cavities are needed to perform frequency selection. Here, an ultra-compact filter integrated in a graphene plasmonic waveguide was designed, where a rectangular resonant cavity is inside a graphene nanoribbon waveguide. The properties of the filter were studied using the finite-difference time-domain method and demonstrated using the analytical model. The results demonstrate the band-pass filter has a high quality factor(20.36) and electrically tunable frequency response. The working frequency of the filter could also be tuned by modifying the cavity size. Our work provides a feasible structure for a graphene plasmonic nano-filter for future use in integrated optical circuits.     

Plasmonic enhancing nanoantennas for photodetection

This paper discusses the use of plasmonic nanostructured systems as nanoantennas for photodetection. Even though semiconductors and their heterostructures have many useful properties and widely used in photodetection, their electron density is very small compared to that of metals and, therefore, they have low absorption cross sections. The idea of using metal nanostructured antennas is to combine the high optical responses of metals with the functional electric properties of semiconductors.     

Fabrication of electrically contacted plasmonic Schottky nanoantennas on silicon

We fabricate Schottky contact photodetectors based on electrically contacted Au nanoantennas on p-Si for the plasmonic detection of sub-bandgap photons in the optical communications wavelength range. Based on a physical model for the internal photoemission of hot carriers, photons coupled onto the Au nanoantennas excite resonant plasmons, which decay into energetic "hot" holes emitted over the Schottky barrier at the Au/p-Si interface, resulting in a photocurrent. In our device, the active Schottky area consists of Au/p-Si contact and is very small, whereas the probing pad for external electrical interconnection is larger but consists of Au/Ti/p-Si contact having a comparatively higher Schottky barrier, thus producing negligible photo and dark currents.We describe fabrication that involves an electron-beam lithography step overlaid with photolithography. This highly compact component is very promising for applications in high-density Si photonics.     

Spectral imaging of individual split-ring resonators

We report on spectral imaging within individual silver split-ring resonators (SRRs) operating in the near infrared-visible range. We classified the optical eigenmodes from the measurement of their energies and nanometer scale spatial distributions. They are plasmonic standing waves that show great similarities with that of nanoantennas. We, however, evidenced marked differences in the near-field electric field lines' spatial distribution and the energies' dispersion. We also showed that the subwavelength defect's influence on the SRRs' eigenmodes spatial distribution is small.     

Nanoplasmonic triple-wavelength demultiplexers in two-dimensional metallic waveguides

A novel kind of plasmonic wavelength demultiplexers (WDMs) based on two-dimensional metal–insulator–metal waveguides with side coupled nanocavities (SCNCs) is proposed and numerically investigated. The WDMs contain three waveguide output channels, each of which functions as a dual-stopband plasmonic filter. The demultiplexing wavelengths can be tuned by controlling the lengths and widths of SCNCs. The finite-difference time-domain results can be accurately analyzed by the resonant theory of nanocavity. Our structures have important potential applications for design of WDM systems in highly integrated optical circuits.     

Enhanced light extraction with silicon nanoantenna arrays for white light LED applications

High conversion efficiency and quantum efficiency is essential for the phosphor in an efficient phosphor-based white light LEDs. Here, based on the coherent harmonic and the random independent emitter model, we demonstrate theoretically that the silicon nanoantenna array can dramatically enhance the output power of emitters in a phosphor layer by investigating the far-field radiation enhancement of an electric dipole assisted by silicon nanopillars in a waveguide structure. Compared with the plasmonic silver nanoantenna array, the silicon nanoantenna array can increase the enhancement factor of light extraction efficiency (LEE) over 50% for the dipole source at the wavelength of 620 nm, thus showing potential applications in white light LEDs. The enhanced LEE is ascribed to the low-loss directional light scattering of silicon nanoantennas and the strong guided mode resonances caused by their array. The calculation results also indicate that the far-field radiation can be tailored significantly by changing the aspect ratio of silicon nanopillars while presenting a good directivity. Our research is expected to give more insights into the design and optimization of the solid-state lighting, gaining and lasing systems by integrating silicon-based nanoantennas.     

Resonant plasmonic and vibrational coupling in a tailored nanoantenna for infrared detection

A novel resonant mechanism involving the interference of a broadband plasmon with the narrowband vibration from molecules is presented. With the use of this concept, we demonstrate experimentally the enormous enhancement of the vibrational signals from less than one attomol of molecules on individual gold nanowires, tailored to act as plasmonic nanoantennas in the infrared. By detuning the resonance via a change in the antenna length, a Fano-type behavior of the spectral signal is observed, which is clearly supported by full electrodynamical calculations. This resonant mechanism can be a new paradigm for sensitive infrared identification of molecular groups.     

Spatial polarization sensitivity of single Au bowtie nanostructures

Metallic bowtie nanostructures as plasmonic nanoantennas can create highly enhanced local fields when resonating with the incident light. With Au bowtie nanostructures fabricated by lithography method, we experimentally observed that the photoluminescence (PL) spatial profile from a single Au bowtie nanoantenna was strongly dependent on the excitation light polarization. While varying the incident light polarization, the spatial distribution of the PL intensity in the nanogap of an Au bowtie changed as predicted by the simulation results on the electromagnetic field enhancement distribution. The polarization feature of the PL intensity relative to the polarization direction of incident excitation light was also discussed. The study may find application in the design of polarization sensitive plasmonic sensors.     

基于石墨烯纳米带的齿形表面等离激元滤波器的研究

提出了一种基于石墨烯纳米带的齿形表面等离激元波导滤波器, 并且用时域有限差分法研究了这种结构. 单个齿形的滤波器可以实现带阻滤波, 其滤波特性可以用基于散射矩阵的解析模型解释. 滤波器的透射谱特性可以通过调节齿的长度、宽度以及石墨烯的化学势来改变. 由于石墨烯的化学势可以用门电路来调节, 这种结构的滤波器可以在器件加工完成后灵活地调节其工作波长. 同时研究了多齿滤波器, 这种结构可以实现宽带滤波, 文中对具有不同齿数、周期的滤波器的透射谱进行了细致的研究. 研究结果对实现基于石墨烯的大规模集成光电子器件提供了重要的理论参考.     

Design of diamond-shape photonic crystal fiber polarization filter based on surface plasma resonance effect

A novel plasmonic polarization filter based on the diamond-shape photonic crystal fiber(PCF) is proposed. The resonant coupling characteristics of the PCF polarization filter are investigated by the full-vector finite-element method. By optimizing the geometric parameters of the PCF, when the fiber length is 5 mm, the polarization filter has a bandwidth of 990 nm and an extinction ratio(ER) of lower than -20 dB. Moreover, a single wavelength polarization filter can also be achieved, along with an ER of -279.78 dB at wavelength 1.55 μm. It is believed that the proposed PCF polarization filter will be very useful in laser and optical communication systems.