Plasmonic Oligomers Research Articles

Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constitutes a promising platform to study magnetic light-matter interactions at the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometric configuration, giving rise to out-of-plane magnetic dipole (MD) moments that cannot couple to the far-field light efficiently. Herein, we propose a new geometric configuration of plasmonic nanoantennas to realize in-plane MD responses. This is achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer, yet with in-plane MD moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant MD characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement of two orders of magnitude while simultaneously featuring a high emission directionality (collection efficiency up to ∼ 70 % evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.

Charge-driven arrested phase-separation of polyelectrolyte-gold nanoparticle assemblies leading to plasmonic oligomers

Aggregates of charged metal particles obtained by electrostatic coupling with a compound of opposite charge in the vicinity of the net zero charge ratio are of interest in the field of plasmonics because the inter-particle distance is minimal, which favours plasmonic coupling. However, these structures present a low colloidal stability limiting the development of applications. In this article we show that globally neutral aggregates formed by electrostatic complexation of citrate-stabilized gold particles and a quaternized chitosan (i.e., polycation) around the net zero charge ratio could be stabilized at a nanometric size by the subsequent addition of polyelectrolyte chains. Furthermore, the sign of the charge carried by the stabilizing chains determines the sign of the global charge carried by the stabilized complexes. The stabilization is demonstrated in saline environment on a broad pH range as well as in a cell culture media over periods of several days. Contrarily to stabilization by charged particles, our stabilized complexes are found to retain their initial characteristics (i.e. shape, size, internal structure and optical properties) after stabilization. Hence, the plasmonic coupling allows to maximize the optical absorption around the 800nm wavelength at which the lasers used for thermoplasmonic and surface enhanced Raman scattering analysis operate.

Theoretical Analysis of Hybrid Metal–Dielectric Nanoantennas with Plasmonic Fano Resonance for Optical Sensing

A nanoantenna with Fano response is designed with plasmonic oligomers as a refractive index sensor to enhance surface-enhanced Raman scattering (SERS) in the visible light spectrum. The scattered radiation and field-enhanced interactions of the outer gallium phosphide (GaP) nanoring assembled with an inner heptamer of silver with Fano response are investigated systematically using the finite element method. The characteristics of Fano resonance are found to depend on the size, shape and nature of the materials in the hybrid nanoantenna. The confined electromagnetic field produces a single-point electromagnetic hotspot with up to 159.59 V/m. The sensitivity obtained from the wavelength shift and variation in the scattering cross-section (SCS) shows a maximum value of 550 nm/RIU. The results validate the design concept and demonstrate near-field enhancement, enabling the design of high-performance nanoantennas with enhanced optical sensing and SERS properties.

Plasmon-coupling-induced photon scattering torque

Bio-compatible Au nanoparticles exhibit great advantages in the application of biomedical research, such as bio-sensing, medical diagnosis, and cancer therapy. Bio-molecules can even be manipulated by laser tweezers with the optically trapped Au nanoparticles as handles. In this paper, optical scattering torque arising from coupled Au nanoparticles driven by circularly polarized light is theoretically presented. The coupled plasmon resonance modes boost the angular momentum transfer from photons to Au nanoparticle dimers and trimers through light absorption and scattering. The scattering process does not bring any optical-heating side effect that always accompanies light absorption. The generated optical torques on the nanostructures highly depend on the plasmon coupling in the structures. Each scattered incident circularly polarized photon can transfer as large as 2 ℏ angular momentum to the nanostructures. The results suggest that coupled plasmonic nanoparticle oligomers are promising candidates to construct optically driven rotary nanomotors that can be applied in biomedical applications.

Mass-determining role in the electrophoretic separation of colloidal plasmonic nanoparticle oligomers.

Gel electrophoresis techniques have been commonly applied in sieving plasmonic nanoparticle oligomers, while the intrinsic role in determining their phoresis velocity differences through the gel remains debatable. In this work, we explore the components and yield in each gel band after bundling two rationally designed types of nanoparticles in a system for electrophoretic separation. All results indicate that the mass property of plasmonic oligomers plays an essential role in determining their phoresis velocity divergences during separation. Further theoretical simulations reveal that the grounds for the mass-determining role stemmed from the random inelastic collisions among the oligomers and the gel-network microchannel. Moreover, under the guidance of such a mass-determining role, it is easy to achieve the direct electrophoretic separation of hetero-structured plasmonic dimers with high purity and high yield. This work will not only facilitate the precise nano-engineering of complex plasmonic oligomers with unique optical properties, but also might remove the obstacles toward their industrial manufacture with high purity.

Lattice Kerker Effect with Plasmonic Oligomers

Lattice Kerker Effect with Plasmonic Oligomers

Anapole-assisted giant electric field enhancement for surface-enhanced coherent anti-Stokes Raman spectroscopy

The coherent anti-Stokes Raman spectroscopy (CARS) techniques are recognized for their ability to detect and identify vibrational coherent processes down to the single-molecular levels. Plasmonic oligomers supporting full-range Fano-like line profiles in their scattering spectrum are one of the most promising class of substrates in the context of surface-enhanced (SE) CARS application. In this work, an engineered assembly of metallic disk-shaped nanoparticles providing two Fano-like resonance modes is presented as a highly-efficient design of SECARS substrate. We show that the scattering dips corresponding to the double-Fano spectral line shapes are originated from the mutual interaction of electric and toroidal dipole moments, leading to the so-called non-trivial first- and second-order anapole states. The anapole modes, especially the higher-order ones, can result in huge near-field enhancement due to their light-trapping capability into the so-called “hot spots”. In addition, independent spectral tunability of the second Fano line shape is exhibited by modulating the gap distance of the corner particles. This feature is closely related to the electric current loop associated with the corner particles in the second-order anapole state and provides a simple design procedure of an optimum SECARS substrate, where the electric field hot spots corresponding to three involved wavelengths, i.e., anti-Stokes, pump, and Stokes, are localized at the same spatial position. These findings yield valuable insight into the plasmonic substrate design for SECARS applications as well as for other nonlinear optical processes, such as four-wave mixing and multi-photon surface spectroscopy.

Dual polarized engineering the extinction cross-section of a dielectric wire using graphene-based oligomers

In this paper, graphene-coated spherical nanoparticles are arranged around an infinite length dielectric cylinder to enhance its extinction cross-section. Initially, a single longitudinal one-dimensional periodic array is considered in different loci concerning the transverse electric (TE) incident plane wave. It is observed that regardless of the position of the particles, the extinction cross-section of the dielectric cylinder is considerably enhanced with respect to the bare one. Later, by increasing the number of longitudinal plasmonic arrays around the cylinder, each residing in a different azimuthal direction, the extinction cross-section is further manipulated to observe double pronounced Fano resonances. The origin of the Fano resonances is described by considering their planar counterparts constructed by the periodic assembly of plasmonic oligomers. Finally, the hexamer configuration is considered as the prototype, and the effect of various optical, geometrical, and material parameters on the optical response is investigated in detail. Interestingly, due to the spherical symmetry of the cells, the extinction cross-section is also enhanced for the transverse magnetic (TM) incident wave, which is unattainable using a continuous plasmonic cover made of metal or graphene. The potential application of our proposed structure is in the design of reconfigurable conformal optical absorbers and sensors.

High spatial and energy resolution electron energy loss spectroscopy of the magnetic and electric excitations in plasmonic nanorod oligomers.

We leverage the high spatial and energy resolution of monochromated aberration-corrected scanning transmission electron microscopy to study the hybridization of cyclic assemblies of plasmonic gold nanorods. Detailed experiments and simulations elucidate the hybridization of the coupled long-axis dipole modes into collective magnetic and electric dipole plasmon resonances. We resolve the magnetic dipole mode in these closed loop oligomers with electron energy loss spectroscopy and confirm the mode assignment with its characteristic spectrum image. The energy splitting of the magnetic mode and antibonding modes increases with the number of polygon edges (n). For the n=3-6 oligomers studied, optical simulations using normal incidence and s-polarized oblique incidence show the respective electric and magnetic modes' extinction efficiencies are maximized in the n=4 arrangement.

Spatial localization of hotspots in Fano-resonant plasmonic oligomers for surface-enhanced coherent anti-Stokes Raman scattering

Realization of Fano resonance in plasmonic oligomers is often exploited to design efficient plasmonic substrates for surface-enhanced coherent anti-Stokes Raman scattering. Disk-type Fano-resonant plasmonic oligomers are widely used to enhance the Raman signal of the probe material. Generally, hot spots are generated in those oligomers at different spatial locations at different wavelengths and only a few spatially overlapping hot spots at multiple wavelengths can be achieved with oblique incidence of excitation light. In this work, we proposed hexagonal gold nanoparticle based Fano-resonant plasmonic oligomers that can yield higher number of spatially overlapped hot spots compared to the disk type oligomers even with the normal incidence of excitation light. The oligomers were numerically modelled and optimized for surface-enhanced coherent anti-Stokes Raman scattering with 780 nm pumping and 500–1800 cm− 1 Raman signature region. The Fano lineshape was engineered to ensure near-field energy coupling at pump while enhancing the coherent anti-Stokes Raman signal at the far field. Our computational studies explored the purely electric origin of Fano resonance in those oligomers and provided maximum Raman enhancements of 1012–1013 from them to enable single-molecular level applications. Our findings provide a way to realize fabrication-friendly nanostructures with higher number of spatially localized hotspots for improving the Raman detection sensitivity.

Plasmonic Oligomers with Tunable Conductive Nanojunctions.

Engineering plasmonic hot spots is essential for applications of plasmonic nanoparticles. A particularly appealing route is to weld plasmonic nanoparticles together to form more complex structures sustaining plasmons with symmetries targeted to given applications. However, control of the welding and subsequent hot spot characteristics is still challenging. Herein, we demonstrate an original method that connects gold particles to their neighbors by another metal of choice. We first assemble gold bipyramids in a tip-to-tip configuration, yielding short chains of variable length, and grow metallic junctions in a second step. We follow the chain formation and the deposition of the second metal (i.e., silver or palladium) via UV/vis spectroscopy, and we map the plasmonic properties using electron energy loss spectroscopy. The formation of silver bridges leads to a huge red shift of the longitudinal plasmon modes into the mid-infrared region, while the addition of palladium results in a red shift accompanied by significant plasmon damping.

Influence of Fano resonance on SERS enhancement in Fano-plasmonic oligomers.

Plasmonic oligomers can provide profound Fano resonance in their scattering responses. The sub-radiant mode of Fano resonance can result in significant near-field enhancement due to its light trapping capability into the so-called hotspots. Appearance of these highly localized hotspots at the excitation and/or Stokes wavelengths of the analytes makes such oligomers promising SERS active substrates. In this work, we numerically and experimentally investigate optical properties of two disk-type gold oligomers, which have different strength and origin of Fano resonance. Raman analysis of rhodamine 6G and adenine with the presence of the fabricated oligomers clearly indicates that an increment in the strength of Fano resonance can improve the Raman enhancement of an oligomer significantly. Therefore, by suitable engineering of Fano lineshape, one can achieve efficient SERS active substrates with spatially localized hotspots.

Colorimetric assay for ultrasensitive detection of Ag(I) ions based on the formation of gold nanoparticle oligomers.

Here, we report an ultrasensitive colorimetric method for determination of Ag+ based on gold nanoparticle (AuNP) oligomers with dark-field microscopy. In a proof-of-concept trial, two cytosine-rich (C-rich) aptamers as the sensing elements and AuNPs as the transducer to produce the detectable signal are designed for Ag+ determination. Obviously, the C bases in the two aptamers play a crucial role in the coordination of highly specific C-Ag+-C. Through a specific C-Ag+-C coordination, one aptamer immobilized on the surface of AuNPs sitting on a glass slide hybridizes with the other aptamer in the solution, which triggers the formation of AuNP oligomers. The formation of plasmonic oligomers results in an obvious increase in the number of yellow and red spots in dark-field images, further leading to a significantly enhanced scattering intensity. By measuring the intensity change of AuNP oligomers in the dark-field images, the concentration of Ag+ can be readily determined. The limits of detection (LODs) of 0.246pM and 0.388pM in solution and river water sample were obtained, respectively. This ultrasensitive colorimetric approach could be extended to probe Hg2+ via T-Hg2+-T coordination in environmental samples.

A quantum mechanical study of optical excitations in nanodisk plasmonic oligomers.

Using state-of-the-art quantum-mechanical calculations, we investigate the optical excitations in plasmonic nanodisk oligomers. We show that different tuning parameters such as the constitution (size and intrinsic optical properties of individual nanodisks), configuration (number of satellite nanodisks), and the spatial arrangement of satellite nanodisks critically influence the optical features of nanodisk oligomers. Our theoretical results demonstrate significantly enhanced absorption in nanodisk oligomers in comparison to individual nanodisks and, additionally, vivid optical modulations due to the creation of nanodisk oligomer homotops. Transition density maps corresponding to the main peaks in the absorption spectra provided new insight into the nature of the optical excitations. The insights gained from this theoretical study will be useful in the design of novel plasmonic nanodisk oligomers.

Hydrophobic assembly of gold nanoparticles into plasmonic oligomers with Langmuir–Blodgett film

Hydrophobic effect was utilized to self-assemble gold nanoparticles into oligomer structures, or plasmonic dimers, trimers, and so on. Langmuir–Blodgett film of n-octadecanamine was prepared on a glass substrate. Citrate-stabilized gold nanoparticles were adsorbed onto the film, resulting hydrophobic point decoration. The gold nanoparticles with the point decoration were removed from the substrate and self-assembled into plasmonic dimers in ethanol. The products were observed by an atomic force microscope, where not only dimers but also trimers and other aggregates were found. The yield of the plasmonic dimer was ∼13% without any purifications. We measured the scattering spectrum of the single plasmonic dimer, exhibiting the optical anisotropy.

Mueller matrix spectroscopy of fano resonance in plasmonic oligomers

Fano resonance in plasmonic oligomers originating from the interference of a spectrally broad superradiant mode and a discrete subradiant mode is under intensive recent investigations due to numerous potential applications. In this regard, development of experimental means to understand and control the complex Fano interference process and to modulate the resulting asymmetric Fano spectral line shape is highly sought after. Here we present a polarization Mueller matrix measurement and inverse analysis approach for quantitative understanding and interpretation of the complex interference process that lead to Fano resonance in symmetry broken plasmonic oligomers. The spectral Mueller matrices of the plasmonic oligomers were recorded using a custom designed dark-field Mueller matrix spectroscopy system. These were subsequently analyzed using differential Mueller matrix decomposition technique to yield the quantitative sample polarimetry characteristics, namely, polarization diattenuation (d) and linear retardance ({\delta}) parameters. The unique signature of the interference of the superradiant dipolar plasmon mode and the subradiant quadrupolar mode of the symmetry broken plasmonic oligomers manifested as rapid spectral variation of the diattenuation and the linear retardance parameters across the Fano spectral dip. The polarization information contained in the Mueller matrix was further utilized to desirably control the Fano spectral line shape. The experimental Mueller matrix analysis was complemented with finite element based numerical simulations, which enabled quantitative understanding of the interference of the superradiant and the subradiant plasmon modes and its link with the polarization diattenuation and retardance parameters.

Plasmonic refractive index sensing using strongly coupled metal nanoantennas: nonlocal limitations

Localized surface plasmon resonance based on coupled metallic nanoparticles has been extensively studied in the refractive index sensing and the detection of molecules. The amount of resonance peak-shift depends on the refractive index of surrounding medium and the geometry/symmetry of plasmonic oligomers. It has recently been found that as the feature size or the gap distance of plasmonic nanostructures approaches several nanometers, quantum effects can change the plasmon coupling in nanoparticles. However, most of the research on plasmonic sensing has been done based on classical local calculations even for the interparticle gap below ~3 nm, in which the nonlocal screening plays an important role. Here, we theoretically investigate the nonlocal effect on the evolution of various plasmon resonance modes in strongly coupled nanoparticle dimer and trimer antennas with the gap down to 1 nm. Then, the refractive index sensing in these nonlocal systems is evaluated and compared with the results in classical calculations. We find that in the nonlocal regime, both refractive index sensibility factor and figure of merit are actually smaller than their classical counterparts mainly due to the saturation of plasmon shifts. These results would be beneficial for the understanding of interaction between light and nonlocal plasmonic nanostructures and the development of plasmonic devices such as nanosensors and nanoantennas.

Single particle dark-field spectroscopy of spherical dimers with down to sub-10 nm gaps fabricated by the annealing of nano-pillars

Abstract In this paper we present a method for the fabrication of plasmonic spherical dimers and oligomers with narrow gaps and tunable distances. High-aspect-ratio nano-pillars are created by the two-step evaporation of gold on structured substrates. By using electron beam lithography, it is possible to control the close spacing of the pillars. Rapid thermal annealing causes the pillars to adopt a spherical shape. Since by melting the spheres gain in diameter compared to the initial nano-pillars, the distance between two adjacent spheres can be reduced to below 10 nm. Dimers with different distances were fabricated and optically characterized by single particle dark-field spectroscopy. The characteristic red-shift of the longitudinal mode due to stronger coupling for smaller distances could be clearly observed and follows a general scaling behavior.

Single Plasmonic Oligomer Chiral Spectroscopy

AbstractChirality plays a crucial role in our everyday lives. Many biomolecules are handed and the associated biological processes rely on their handedness. Thus, chirality is investigated intensely, also because of the fundamental and inherent interest in the concept of chirality. However, virtually all studies are performed on large ensembles of chiral objects, which obstructs the contribution of the individual particle. Hence, it is highly desirable to study individual chiral objects. Here, it is shown how to measure chiroptical spectra in the visible spectral range of left‐ and right‐handed C4 symmetric single plasmonic nanoantenna ensembles, which are termed oligomers. The two‐layered oligomers are fabricated by top‐down electron beam lithography to offer a high degree of structural control. Dark‐field scattering spectroscopy as well as correlative scanning electron microscopy are utilized to study the influence of structural arrangement and individual shape on the optical properties of a single plasmonic oligomer. Even minute structural differences, which cannot be resolved in scanning electron microscopy, manifest themselves in clear differences in the chiroptical response. The results play an important role for further investigations and optimizations in design, fabrication, and measurements of single plasmonic enantiomers but also on the way toward single chiral object sensing.

Collective Effects in Second-Harmonic Generation from Plasmonic Oligomers.

We investigate collective effects in plasmonic oligomers of different symmetries using second-harmonic generation (SHG) microscopy with cylindrical vector beams (CVBs). The oligomers consist of gold nanorods that have a longitudinal plasmon resonance close to the fundamental wavelength that is used for SHG excitation and whose long axes are arranged locally such that they follow the distribution of the transverse component of the electric field of radially or azimuthally polarized CVBs in the focal plane. We observe that SHG from such rotationally symmetric oligomers is strongly modified by the interplay between the polarization properties of the CVB and interparticle coupling. We find that the oligomers with radially oriented nanorods exhibit small coupling effects. In contrast, we find that the oligomers with azimuthally oriented nanorods exhibit large coupling effects that lead to silencing of SHG from the whole structure. Our experimental results are in very good agreement with numerical calculations based on the boundary element method. The work describes a new route for studying coupling effects in complex arrangements of nano-objects and thereby for tailoring the efficiency of nonlinear optical effects in such structures.