Near-field Plasmon Research Articles

Near-Field Mid-Infrared Plasmonics in Complex Nanostructures with Monochromated Electron Energy Loss Spectroscopy

Near-Field Mid-Infrared Plasmonics in Complex Nanostructures with Monochromated Electron Energy Loss Spectroscopy

Patterned Plasmonic Surfaces-Theory, Fabrication, and Applications in Biosensing.

Low-profile patterned plasmonic surfaces are synergized with a broad class of silicon microstructures to greatly enhance near-field nanoscale imaging, sensing, and energy harvesting coupled with far-field free-space detection. This concept has a clear impact on several key areas of interest for the MEMS community, including but not limited to ultra-compact microsystems for sensitive detection of small number of target molecules, and "surface" devices for optical data storage, micro-imaging and displaying. In this paper, we review the current state-of-the-art in plasmonic theory as well as derive design guidance for plasmonic integration with microsystems, fabrication techniques, and selected applications in biosensing, including refractive-index based label-free biosensing, plasmonic integrated lab-on-chip systems, plasmonic near-field scanning optical microscopy and plasmonics on-chip systems for cellular imaging. This paradigm enables low-profile conformal surfaces on microdevices, rather than bulk material or coatings, which provide clear advantages for physical, chemical and biological-related sensing, imaging, and light harvesting, in addition to easier realization, enhanced flexibility, and tunability.

Nanofibre optic force transducers with sub-piconewton resolution via near-field plasmon-dielectric interactions.

Ultrasensitive nanomechanical instruments, including the atomic force microscope (AFM)1-4 and optical and magnetic tweezers5-8, have helped shed new light on the complex mechanical environments of biological processes. However, it is difficult to scale down the size of these instruments due to their feedback mechanisms9, which, if overcome, would enable high-density nanomechanical probing inside materials. A variety of molecular force probes including mechanophores10, quantum dots11, fluorescent pairs12,13 and molecular rotors14-16 have been designed to measure intracellular stresses; however, fluorescence-based techniques can have short operating times due to photo-instability and it is still challenging to quantify the forces with high spatial and mechanical resolution. Here, we develop a compact nanofibre optic force transducer (NOFT) that utilizes strong near-field plasmon-dielectric interactions to measure local forces with a sensitivity of <200 fN. The NOFT system is tested by monitoring bacterial motion and heart-cell beating as well as detecting infrasound power in solution.

Sensitive plasmonic-photonic nanosensor as a morphologic mask

In this study, a new nanosensor is assembled in the form of a phantom model to optically scan the breast for early cancer detection based on the plasmonic and plasmonic–photonic interaction phenomena. Sensing is carried out through a user-friendly method by improving imaging through the traditional optical tomography method. The novelty of the designed sensor is attributed to the coupling of the nanoparticle plasmonic near-field intensity to the far-field region (photonic mode interaction with the near-field plasmon resonance). It is shown that the plasmonic–photonic interaction has a dramatic influence on the gradient image and therefore, the edge detection and segmentation of the image are effectively altered. This is due to the fact that the plasmonic fields of the nanoparticles in the near- and far-field manipulate the field gradient, which leads to a modification of the intensity discontinuities at different interfaces. In fact, it is well-known that the fundamental idea behind edge detection is utilized to detect parts of the image where the intensity varies rapidly. Based on this knowledge, interestingly, it is shown that the segmentation and edge detection of the image are improved by the manipulating optical properties of the mask.

Enhancement of spatial resolution of terahertz imaging systems based on terajet generation by dielectric cube

The terahertz (THz, 0.1–10 THz) region has been attracting tremendous research interest owing to its potential in practical applications such as biomedical, material inspection, and nondestructive imaging. Those applications require enhancing the spatial resolution at a specific frequency of interest. A variety of resolution-enhancement techniques have been proposed, such as near-field scanning probes, surface plasmons, and aspheric lenses. Here, we demonstrate for the first time that a mesoscale dielectric cube can be exploited as a novel resolution enhancer by simply placing it at the focused imaging point of a continuous wave THz imaging system. The operating principle of this enhancer is based on the generation—by the dielectric cuboid—of the so-called terajet, a photonic jet in the THz region. A subwavelength hotspot is obtained by placing a Teflon cube, with a 1.46 refractive index, at the imaging point of the imaging system, regardless of the numerical aperture (NA). The generated terajet at 125 GHz is experimentally characterized, using our unique THz-wave visualization system. The full width at half maximum (FWHM) of the hotspot obtained by placing the enhancer at the focal point of a mirror with a measured NA of 0.55 is approximately 0.55λ, which is even better than the FWHM obtained by a conventional focusing device with the ideal maximum numerical aperture (NA = 1) in air. Nondestructive subwavelength-resolution imaging demonstrations of a Suica integrated circuit card, which is used as a common fare card for trains in Japan, and an aluminum plate with 0.63λ trenches are presented. The amplitude and phase images obtained with the enhancer at 125 GHz can clearly resolve both the air-trenches on the aluminum plate and the card’s inner electronic circuitry, whereas the images obtained without the enhancer are blurred because of insufficient resolution. An increase of the image contrast by a factor of 4.4 was also obtained using the enhancer.

Toroidal Dipolar Excitation in Metamaterials Consisting of Metal nanodisks and a Dielectrc Spacer on Metal Substrate

We have investigated numerically toroidal dipolar excitation at optical frequency in metamaterials whose unit cell consists of three identical Ag nanodisks and a SiO2 spacer on Ag substrate. The near-field plasmon hybridization between individual Ag nanodisks and substrate forms three magnetic dipolar resonances, at normal incidence of plane electromagnetic waves. The strong coupling among three magnetic dipolar resonances leads to the toroidal dipolar excitation, when space-inversion symmetry is broke along the polarization direction of incident light. The influences of some geometrical parameters on the resonance frequency and the excitation strength of toroidal dipolar mode are studied in detail. The radiated power from toroidal dipole is also compared with that from conventional electric and magnetic multipoles.

Near-field surface plasmons on quasicrystal metasurfaces.

Excitation and manipulation of surface plasmons (SPs) are essential in developing cutting-edge plasmonic devices for medical diagnostics, biochemical spectroscopy and communications. The most common approach involves designing an array of periodic slits or grating apertures that enables coupling of the incident light to the SP modes. In recent years, plasmonic resonances, including extraordinary optical transmission through periodic arrays, quasicrystals and random aperture arrays, have been investigated in the free space. However, most of the studies have been limited to the far field detection of the transmission resonance. Here, we perform near-field measurements of the SPs on quasicrystal metasurfaces. We discover that the reciprocal vector determines the propagation modes of the SPs in the quasicrystal lattice which can be well explained by the quasi-momentum conservation rule. Our findings demonstrate vast potential in developing plasmonic metasurfaces with unique device functionalities that are controlled by the propagation modes of the SPs in quasicrystals.

Far-Field Super-resolution Detection of Plasmonic Near-Fields.

We demonstrate a far-field single molecule super-resolution method that maps plasmonic near-fields. The method is largely invariant to fluorescence quenching (arising from probe proximity to a metal), has reduced point-spread-function distortion compared to fluorescent dyes (arising from strong coupling to nanoscopic metallic features), and has a large dynamic range (of 2 orders of magnitude) allowing mapping of plasmonic field-enhancements regions. The method takes advantage of the sensitivity of quantum dot (QD) stochastic blinking to plasmonic near-fields. The modulation of the blinking characteristics thus provides an indirect measure of the local field strength. Since QD blinking can be monitored in the far-field, the method can measure localized plasmonic near-fields at high throughput using a simple far-field optical setup. Using this method, propagation lengths and penetration depths were mapped-out for silver nanowires of different diameters and for different dielectric environments, with a spatial accuracy of ∼15 nm. We initially use sparse sampling to ensure single molecule localization for accurate characterization of the plasmonic near-field with plans to increase density of emitters in further studies. The measured propagation lengths and penetration depths values agree well with Maxwell finite-difference time-domain calculations and with published literature values. This method offers advantages such as low cost, high throughput, and superresolved mapping of localized plasmonic fields at high sensitivity and fidelity.

Creation of Graphene Plasmons Vortex via Cross Shape Nanoantennas Under Linearly Polarized Incidence

In this paper, we propose a method of creating plasmons vortex on graphene through cross shape nanoantennas under linearly polarized incidence. For each cross shape antenna, the linearly polarized incidence can be coupled to a near-field plasmons vortex on graphene through antenna resonances. When multiple antennas are arranged into a closed circular array, the sign of topological charge of plasmons vortex can be controlled by the linearly polarized direction of incident light with the distribution of absolute field component exhibiting a non-ideal donut profile. The creating of plasmons vortex on graphene may provide new possibilities in various applications, such as the nanoparticles trapping.

Ultrafast Plasmonic Control of Second Harmonic Generation

Efficient frequency conversion techniques are crucial to the development of plasmonic metasurfaces for information processing and signal modulation. In principle, nanoscale electric-field confinement in nonlinear materials enables higher harmonic conversion efficiencies per unit volume than those attainable in bulk materials. Here we demonstrate efficient second-harmonic generation (SHG) in a serrated nanogap plasmonic geometry that generates steep electric field gradients on a dielectric metasurface. An ultrafast control pulse is used to control plasmon-induced electric fields in a thin-film material with inversion symmetry that, without plasmonic enhancement, does not exhibit an even-order nonlinear optical response. The temporal evolution of the plasmonic near-field is characterized with ∼100 as resolution using a novel nonlinear interferometric technique. The serrated nanogap is a unique platform in which to investigate optically controlled, plasmonically enhanced harmonic generation in dielectric materials on an ultrafast time scale. This metamaterial geometry can also be readily extended to all-optical control of other nonlinear phenomena, such as four-wave mixing and sum- and difference-frequency generation, in a wide variety of dielectric materials.

Toroidal Dipolar Response in Metamaterials Composed of Metal–Dielectric–Metal Sandwich Magnetic Resonators

Toroidal metamaterials have been drawing increasing interest recently because of their unusual electromagnetic properties and a variety of potential applications. In this work, we have investigated numerically toroidal dipolar response at optical frequency in metamaterials whose unit cell includes three magnetic resonators. The magnetic resonators are metal–dielectric–metal sandwich nanostructures, which are composed of two Ag rods and a SiO 2 spacer. They have the same shape and dimension, but they are placed at different positions to break the space-inversion symmetry. The near-field plasmon coupling between magnetic resonators leads to the excitation of a toroidal dipolar mode, which is characterized by a head-to-tail distribution of magnetic dipoles within magnetic resonators. In our designed toroidal metamaterials, space-inversion symmetry breaking is needed only in the polarization direction of incident light, and light can be normally incident on the toroidal metamaterials.

Linearly dichroic plasmonic lens and hetero-chiral structures.

We present an experimental study of Hetero-Chiral (HC) plasmonic lenses, comprised of constituents with opposite chirality, demonstrating linearly dichroic focusing. The lenses focus only light with a specific linear polarization and result in a dark focal spot for the orthogonal polarization state. We introduce the design concepts and quantitatively compare several members of the HC family, deriving necessary conditions for linear dichroism and several comparative engineering parameters. The HC lenses were experimentally investigated using aperture-less near field scanning microscope collecting the amplitude of the plasmonic near-field. Our results exhibit an excellent match to the simulation predictions. The demonstrated ability for linearly dichroic functional focusing could lead to novel sensing applications.

Integrating plasmonic Au nanorods with dendritic like α-Bi2O3/Bi2O2CO3 heterostructures for superior visible-light-driven photocatalysis

To explore the relationship between semiconductor structure and plasmonic noble metal nanoparticles (NPs) property is crucial for developing highly efficient visible light driven photocatalyst. Here, dendritic α-Bi2O3/Bi2O2CO3 biphasic heterostructures were first synthesized by a facile and low-cost phase transformation method. Then, plasmonic Au NPs (including Au nanospheres (NSs, ∼30nm)) and Au nanorods (NRs, ∼20, ∼30, and ∼35nm) were loaded onto the α-Bi2O3/Bi2O2CO3 heterostructure. The results revealed that these α-Bi2O3/Bi2O2CO3 heterostructures exhibited much higher visible-light photocatalytic activities than α-Bi2O3 for dye degradation. More importantly, compared to plain α-Bi2O3/Bi2O2CO3 heterostructures, loading of Au NSs brought ∼4 times increase in activity and Au NRs 5–11 times depending on nanorods size. The significant boosting of activity is attributed to the large enhancement of charge separation by the formation of α-Bi2O3/Bi2O2CO3 interface and more production of OH radicals by Au NSs or Au NRs. The surface plasmon resonance (SPR) absorption of these gold NPs on the α-Bi2O3/Bi2O2CO3 heterostructures could also have significant contribution to the activities due to their strong plasmonic near-fields. This work demonstrates that tailoring the semiconductor substrate structure and the plasmonic noble metal NPs properties should constitute a promising strategy for the design efficient solar energy driven photocatalytic materials.

Maximizing the Plasmonic Near-Field Transducer Efficiency to Its Limit for HAMR

Plasmonic near-field transducer (NFT) facilitates optical power transfer into a nano-spot in the recording media for heat-assisted magnetic recording. Typically, impedance matching is a condition for maximum power transfer between a given source and load. Even for waveguide-NFT-media stack system, we verify that matching the impedances of waveguide and media-stack at NFT resonance maximizes power transfer. However, it is not a sufficient condition. Since, NFT guides the light into the media-stack as symmetric and asymmetric optical modes, higher conversion of dielectric mode into a mode that exhibits maximum coupling into the media is the second condition for maximizing the power transfer into the media. In this study, operating wavelength range, material refractive indices, and NFT geometrical parameters are considered in the design space for maximizing the efficiency of an exemplary taper-based NFT structure. We first estimate the maximum efficiency and compare with the benchmark efficiency of “ideal” version of the considered NFT to confirm that the obtained efficiency is maximum. At maximum efficiency point, we show that the impedance matching between waveguide and high-index media stack is enhanced and mode-conversion efficiency into asymmetric plasmon mode is also enhanced because of high-index contrast feeding waveguide and suitable operating wavelength range.

Direct On-Chip Optical Plasmon Detection with an Atomically Thin Semiconductor.

The determination to develop fast, efficient devices has led to vast studies on photonic circuits but it is difficult to shrink these circuits below the diffraction limit of light. However, the coupling between surface plasmon polaritons and nanostructures in the near-field shows promise in developing next-generation integrated circuitry. In this work, we demonstrate the potential for integrating nanoplasmonic-based light guides with atomically thin materials for on-chip near-field plasmon detection. Specifically, we show near-field electrical detection of silver nanowire plasmons with the atomically thin semiconductor molybdenum disulfide. Unlike graphene, atomically thin semiconductors such as molybdenum disulfide exhibit a bandgap that lends itself for the excitation and detection of plasmons. Our fully integrated plasmon detector exhibits plasmon responsivities of ∼255 mA/W that corresponds to highly efficient plasmon detection (∼0.5 electrons per plasmon).

Generation of ultrashort extreme-ultraviolet pulses by enhanced plasmonic near-fields in metallic nanoparticles

We investigate high-order harmonic generation in a metallic-nanoparticle–quantum-dot hybrid system. Driven by a moderate electric field intensity Guassian-shape few-cycle pulse which is chirped by a hyperbolic-tangent-form carrier envelope phase, the hybrid system can generate extreme-ultraviolet super-continuum harmonics (13.3 eV maximal photon energy) and isolated ultrashort pulses ( FWHM). Numerical simulation results reveal that the plasmonic field enhancement could extend the cutoff order and continuum region in harmonic spectra. The extension effect by plasmonic enhancement could be well understood theoretically by the semi-classical two-level atom model.

Plasmon enhanced Raman scattering effect for an atom near a carbon nanotube.

Quantum electrodynamics theory of the resonance Raman scattering is developed for an atom in a close proximity to a carbon nanotube. The theory predicts a dramatic enhancement of the Raman intensity in the strong atomic coupling regime to nanotube plasmon near-fields. This resonance scattering is a manifestation of the general electromagnetic surface enhanced Raman scattering effect, and can be used in designing efficient nanotube based optical sensing substrates for single atom detection, precision spontaneous emission control, and manipulation.

Near-field plasmon effects in extraordinary optical transmission through periodic triangular hole arrays

We present a theoretical investigation of the transmission properties of light through a metallic film perforated with different arrays of compound triangular holes. The extraordinary optical transmission (EOT) in the optical region is obtained by employing the finite-difference time-domain method. The excitation of localized surface plasmon resonances (LSPRs) at the top corners and surface plasmon polaritons (SPPs) on the metal surface, plasmon coupling effects between adjacent apertures, and the waveguide modes for delivering light mainly contribute to the EOT in such structures. The optical characteristics can be effectively tailored by changing the arrangement of triangular holes and the structural parameters. This study may be helpful for plasmonic nanostrucutres based on EOT, and has potential applications in optoelectronic devices.

Plasmonic near-field coupling induced absorption enhancement and photoluminescence of silver nanorod arrays

Silver nanorod (Ag NR) arrays with average diameter about 18 nm and period about 50 nm were electrodeposited in anodic aluminum oxide (AAO) templates. The filling factor of Ag NRs in the AAO nanopores could be adjusted by the deposition temperature. The filling factor increased to ∼98% when the deposition temperature decreased to 1 °C. As the filling factor increased, the plasmon absorption intensity of Ag NR arrays was greatly enhanced, and the longitudinal surface plasmon resonance increased faster. Meanwhile, the AgNR:AAO exhibit strong photoluminescence at ∼570 nm, obviously different with that of the bare AAO at ∼495 nm. Our observation revealed the strong near-field plasmon coupling among the Ag NR arrays.

Hybrids of carbon Nanotube Forests and Gold Nanoparticles for Improved Surface Plasmon Manipulation

We report the fabrication and characterization of hybrids of vertically-aligned carbon nanotube forests and gold nanoparticles for improved manipulation of their plasmonic properties. Raman spectroscopy of nanotube forests performed at the separation area of nanotube-nanoparticles shows a scattering enhancement factor of the order of 1 × 10(6). The enhancement is related to the plasmonic coupling of the nanoparticles and is potentially applicable in high-resolution scanning near-field optical microscopy, plasmonics, and photovoltaics.