Trimeric plasmonic molecules: the role of symmetry

Quantitative Plasmon Mode and Surface-Enhanced Raman Scattering Analyses of Strongly Coupled Plasmonic Nanotrimers with Diverse Geometries

Here, we quantitatively monitored and analyzed the spectral redistributions of the coupled plasmonic modes of trimeric Au nanostructures with two ∼1 nm interparticle gaps and single-dye-labeled DNA in each gap as a function of varying trimer symmetries. Our precise Mie scattering measurement with the laser-scanning-assisted dark-field microscopy allows for individual visualization of the orientations of the radiation fields of the coupled plasmon modes of the trimers and analyzing the magnitude and direction of the surface-enhanced Raman scattering (SERS) signals from the individual plasmonic trimers. We found that the geometric transition from acute-angled trimer to linear trimer induces the red shift of the longitudinally polarized mode and the blue shift of the axially polarized mode. The finite element method (FEM) calculation results show the distinct "on" and "off" of the plasmonic modes at the two gaps of the trimer. Importantly, the single-molecule-level systematic correlation studies among the near-field, far-field, and surface-enhanced Raman scattering reveal that the SERS signals from the trimers are determined by the largely excited coupled plasmon between the two competing plasmon modes, longitudinal and axial modes. Further, the FEM calculation revealed that even 0.5 nm or smaller discrepancy in the sizes of two gaps of the linear trimer led to >10-fold difference in the SERS signal. Granted that two gap sizes are not likely to be completely the same in actual experiments, one of two gaps plays a more significant role in generating the SERS signal. Overall, this work provides the knowledge and handles for the understanding and systematic control of the magnitude and polarization direction of the both plasmonic response and SERS signal from trimeric nanostructures and sets up the platform for the optical properties and the applications of plasmonically coupled trimers and higher multimeric nanostructures.

Plasmonic properties of triangle-shaped silver trimers selectively fabricated by near-field photo-reduction using an apertured cantilever for an atomic force microscope

On an AgNO3 crystal, an equilateral or a right-angle triangle-shaped Ag trimer was selectively fabricated through near-field photo-reduction and observed in situ by using an apertured cantilever coupled with an atomic force microscope. By using the different triangle-shaped Ag trimers, irradiation wavelength and polarization dependence of surface-enhanced Raman scattering were investigated.

Fractal nanoparticle plasmonics: the Cayley tree

There has been strong, ongoing interest over the past decade in developing strategies to design and engineer materials with tailored optical properties. Fractal-like nanoparticles and films have long been known to possess a remarkably broad-band optical response and are potential nanoscale components for realizing spectrum-spanning optical effects. Here we examine the role of self-similarity in a fractal geometry for the design of plasmon line shapes. By computing and fabricating simple Cayley tree nanostructures of increasing fractal order N, we are able to identify the principle behind how the multimodal plasmon spectrum of this system develops as the fractal order is increased. With increasing N, the fractal structure acquires an increasing number of modes with certain degeneracies: these modes correspond to plasmon oscillations on the different length scales inside a fractal. As a result, fractals with large N exhibit broad, multipeaked spectra from plasmons with large degeneracy numbers. The Cayley tree serves as an example of a more general, fractal-based route for the design of structures and media with highly complex optical line shapes.

Giant local circular dichroism within an asymmetric plasmonic nanoparticle trimer

We investigated the near-field response in silver nanoparticle aggregates to the excitation of circular polarized light. In a right-angle trimer system, the local field intensity excited by right-hand circularly polarized light is almost one thousand times larger than the left-hand case. By analyzing the polarization and phase of the local field in plasmonic hotspots, we found this local circular dichroism is originated from the near-field interference excited by orthogonal polarized incident lights. The local circular dichroism can be tuned by the rotation of the third particle, the interparticle distance, and the dielectric environment. This phenomenon could also widely exist in more complicated nanoaggregates. These findings would benefit for resolving light handedness, and enhancing circular dichroism and optical activity.

Selective TERS detection and imaging through controlled plasmonics

Enhanced Raman spectroscopy offers capabilities to detect molecules in the complex molecular environments and image chemical heterogeneity in a wide range of samples. It has been shown that plasmonic interactions between a TERS tip and a metal surface produce significant enhancements. In this report we show how SERS spectra from purified molecules can be used to selectively image proteins on surfaces and in cell membranes. The SERS response from the purified protein can be used to create a multivariate regression model that can be applied to nanoparticles that bind to protein receptors. Filtering the observed TERS spectra with the regression model can then selectively image the protein receptor. Experiments with mutant proteins suggest that key amino acids provide significant contributions to the observed TERS signal, which enables the differentiation of protein receptors. These results demonstrate the selectivity that can be obtained in TERS images through a controlled plasmonic interaction. This approach has further implications for identifying membrane receptors that bind specific molecules relevant to drug targeting and chemical signaling.

Harnessing the interparticle J-aggregate induced plasmonic coupling for surface-enhanced Raman scattering

The kinetic linkage between surface plasmon resonance absorption and surface-enhanced Raman scattering data highlights an effective pathway for harnessing the plasmonic coupling of gold nanoparticlesviaJ-aggregation in solutions.

Plasmonic nanosnowmen with a conductive junction as highly tunable nanoantenna structures and sensitive, quantitative and multiplexable surface-enhanced Raman scattering probes

The precise design and synthesis of plasmonic nanostructures allow us to manipulate, enhance, and utilize the optical characteristics of metallic materials. Although many multimeric structures (e.g., dimers) with interparticle nanogap have been heavily studied, the plasmonic nanostructures with a conductive junction have not been well studied mostly because of the lack of the reliable synthetic methods that can reproducibly and precisely generate a large number of the plasmonic nanostructures with a controllable conductive nanojunction. Here, we formed various asymmetric Au-Ag head-body nanosnowman structures with a highly controllable conductive nanojunction and studied their plasmon modes that cover from visible to near-infrared range, electromagnetic field enhancement, and surface-enhanced Raman scattering (SERS) properties. It was shown that change in the plasmonic neck region between Au head and Ag body nanoparticles and symmetry breaking using different sizes and compositions within a structure can readily and controllably introduce various plasmon modes and change the electromagnetic field inside and around a nanosnowman structure. The charge-transfer and capacitive coupling plasmon modes at low frequencies are tunable in the snowman structure, and subtle change in the conductive junction area of the nanosnowman dramatically affects the resulting electromagnetic field and optical signal. The relationships between the electromagnetic field distribution and enhancement in the snowman structure, excitation laser wavelength, and Raman dye were also studied, and it was found that the strongest electromagnetic field was observed in the crevice area on the junction and synthesizing a thinner and sharper neck junction is critical to generate the stronger electromagnetic field in the crevice area and to obtain the charge-transfer mode-based near-infrared signal. We have further shown that highly reproducible SERS signals can be generated from these nanosnowman structures with a linear dependence on particle concentration (5 fM to 1 pM) and the SERS-enhancement factor values of >10(8) can be obtained with the aid of the resonance effect in SERS. Finally, a wide range of LSPR bands with high tunability along with high structural reproducibility and high synthetic yield make the nanosnowman structures as very good candidates for practically useful multiple-wavelength-compatible, quantitative and sensitive SERS probes, and highly tunable nanoantenna structures.

Ultrasensitive size-selection of plasmonic nanoparticles by Fano interference optical force

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core-shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication.

The dark side of plasmonics

Plasmonic dark modes are pure near-field modes that can arise from the plasmon hybridization in a set of interacting nanoparticles. When compared to bright modes, dark modes have longer lifetimes due to their lack of a net dipole moment, making them attractive for a number of applications. We demonstrate the excitation and optical detection of a collective dark plasmonic mode from individual plasmonic trimers. The trimers consist of triangular arrangements of gold nanorods, and due to this symmetry, the lowest-energy dark plasmonic mode can interact with radially polarized light. The experimental data presented confirm the excitation of this mode, and its assignment is supported with an electrostatic approximation wherein these dark modes are described in terms of plasmon hybridization. The strong confinement of energy in these modes and their associated near fields hold great promise for achieving strong coupling to single photon emitters.

Plasmonics in nanostructures

Plasmonics has developed into one of the rapidly growing research topics for nanophotonics. With advanced nanofabrication techniques, a broad variety of nanostructures can be designed and fabricated for plasmonic devices at nanoscale. Fundamental properties for both surface plasmon polaritons (SPP) and localized surface plasmons (LSP) arise a new insight and understanding for the electro-optical device investigations, such as plasmonic nanofocusing, low-loss plasmon waveguide and active plasmonic detectors for energy harvesting. Here, we review some typical functional plasmonic nanostructures and nanosmart devices emerging from our individual and collaborative research works.

Optically isotropic responses induced by discrete rotational symmetry of nanoparticle clusters

Fostered by the recent progress of the fields of plasmonics and metamaterials, the seminal topic of light scattering by clusters of nanoparticles is attracting enormous renewed interest gaining more attention than ever before. Related studies have not only found various new applications in different branches of physics and chemistry, but also spread rapidly into other fields such as biology and medicine. Despite the significant achievements, there still exists unsolved but vitally important challenges of how to obtain robust polarisation-invariant responses of different types of scattering systems. In this paper, we demonstrate polarisation-independent responses of any scattering system with a rotational symmetry with respect to an axis parallel to the propagation direction of the incident wave. We demonstrate that the optical responses such as extinction, scattering, and absorption, can be made independent of the polarisation of the incident wave for all wavelengths. Such polarisation-independent responses are proven to be a robust and generic feature that is purely due to the rotational symmetry of the whole structure. We anticipate our finding will play a significant role in various applications involving light scattering such as sensing, nanoantennas, optical switches, and photovoltaic devices.

Fano resonance of nanoparticles embedded in Fabry-Perot cavities

We present an optical structure, which consists of metal nanoparticles embedded in Fabry-Perot (F-P) cavity, to investigate the Fano resonance, which originates from the interaction between F-P mode and the plasmon modes supported by the nanoparticles. The coupling system is modeled theoretically by coupled-mode theory in time domain and the transmission properties are demonstrated numerically by the finite-difference time-domain method. The charge distribution features of the nanoparticle plasmon modes are further characterized by using boundary integral equation technology. Results show that the F-P modes can be used to active the optical inactive surface plasmon modes by breaking the mode symmetry.

Experimental verification of the spectral shift between near- and far-field peak intensities of plasmonic infrared nanoantennas

Theory predicts a distinct spectral shift between the near- and far-field optical response of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy.

Maximal Raman optical activity in hybrid single molecule-plasmonic nanostructures with multiple dipolar resonances

We show that a hybrid system built of a plasmonic nanoparticle cluster and a single molecule can attain maximal Raman optical activity (ROA), converting linearly polarized light into purely circularly polarized light at the Raman-scattered frequency. In contrast to standard molecular ROA, the effect described here does not involve magnetic modes and is attributed to off-resonance excitation of electric-dipole plasmon modes of the nanoparticle cluster. A model based on a combination of harmonic oscillators excited at the frequency of the Raman-scattered light is shown to successfully capture the physics of the effect.

Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas

We directly visualize and identify the capacitive coupling of infrared dimer antennas in the near field by employing scattering-type scanning near-field optical microscopy (s-SNOM). The coupling is identified by (i) resolving the strongly enhanced nano-localized near fields in the antenna gap and by (ii) tracing the red shift of the dimer resonance when compared to the resonance of the single antenna constituents. Furthermore, by modifying the illumination geometry we break the symmetry, providing a means to excite both the bonding and the "dark" anti-bonding modes. By spectrally matching both modes, their interference yields an enhancement or suppression of the near fields at specific locations, which could be useful in nanoscale coherent control applications.

Single-molecule and single-particle-based correlation studies between localized surface plasmons of dimeric nanostructures with ~1 nm gap and surface-enhanced Raman scattering

Understanding the detailed electromagnetic field distribution inside a plasmonically coupled nanostructure, especially for structures with ~ 1 nm plasmonic gap, is the fundamental basis for the control and use of the strong optical properties of plasmonic nanostructures. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between localized surface plasmon (LSP) and surface-enhanced Raman scattering (SERS), the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au-Ag core-shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ~ 1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ~ 1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures.

A plethora of plasmonics from the laboratory for nanophotonics at Rice University

The study of the surface plasmons of noble metals has emerged as one of the most rapidly growing and dynamic topics in nanoscience. Key advances in the synthesis of noble metal nanoparticles and nanostructures have resulted in a broad variety of structures whose geometries can be controlled systematically at the nanoscale. Arising from these efforts is a new level of insight and understanding regarding the fundamental properties of localized plasmons supported by these structures, and, in particular, the properties of interacting plasmon systems. This additional insight has led to the design of plasmonic systems that support coherent phenomena, such as Fano resonances. A broad range of applications are emerging for these structures: single- nanoparticle and nanogap chemical sensors, low-loss plasmon waveguides, and active plasmonic devices and detectors. Applications in biomedicine that exploit the strong photothermal response of nanoparticle plasmons have developed and advanced into clinical trials. The Laboratory for Nanophotonics at Rice has been home to many of these advances. Here, we showcase a variety of functional plasmonic materials and nanodevices emerging from our individual and collaborative efforts.

Multiple Fano resonances in plasmonic heptamer clusters composed of split nanorings

Fano resonances in plasmonic nanostructures are important for plasmon line shaping. Compared to a single Fano resonance, multiple Fano resonances can modify plasmon lines at several spectral positions simultaneously, but they often suffer from weak modulation depths. In this paper, plasmonic heptamer clusters comprising split nanorings are designed to form multiple Fano resonances. Three prominent Fano resonances are observed in the spectra due to the formation of multiple narrow subradiant resonances, and the multiple Fano resonances can be switched on and off by adjusting the polarization direction. Particularly, by modifying the geometry parameters, there is a large tunability of the modulation depth of each Fano resonance. Heptamer clusters comprising split nanorings are highly suitable for plasmon line shaping, and it is expected that they are useful for multiwavelength biosensing and surface-enhanced Raman scattering.

Surface plasmon resonance properties of single elongated nano-objects: gold nanobipyramids and nanorods

The spectral characteristics (wavelength and line width) and the optical extinction cross-section of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanobipyramids have been quantitatively measured using the spatial modulation spectroscopy technique. The morphology of the same individual nanoparticles has been determined by transmission electron microscopy (TEM). The experimental results are thus interpreted with a numerical model using the TEM measured sizes of the particles as an input, and either including the substrate or assuming a mean homogeneous environment. Results are compared to those obtained for individual nanorods and also show the importance of the local environment of the particle on the detailed description of its spectral position and extinction amplitude.

Nanoplasmonics for dual-molecule release through nanopores in the membrane of red blood cells

A nanoplasmonics-based opto-nanoporation method of creating nanopores upon laser illumination is applied for inducing diffusion and triggered release of small and large molecules from red blood cells (RBCs). The method is implemented using absorbing gold nanoparticle (Au-NP) aggregates on the membrane of loaded RBCs, which, upon near-IR laser light absorption, induce release of encapsulated molecules from selected cells. The binding of Au-NPs to RBCs is characterized by Raman spectroscopy. The process of release is driven by heating localized at nanoparticles, which impacts the permeability of the membrane by affecting the lipid bilayer and/or trans-membrane proteins. Localized heating and temperature rise around Au-NP aggregates is simulated and discussed. Research reported in this work is relevant for generating nanopores for biomolecule trafficking through polymeric and lipid membranes as well as cell membranes, while dual- and multi-molecule release is relevant for theragnostics and a wide range of therapies.

Dynamic tuning and symmetry lowering of Fano resonance in plasmonic nanostructure

We present dynamic tuning and symmetry lowering of Fano resonances in gold heptamers accomplished by applying uniaxial mechanical stress. The flexible heptamer structure was obtained by embedding the seven-gold-nanocylinder complex in a polydimethylsiloxane membrane. Under uniaxial stress, the Fano resonance exhibited opposite spectral shifts for the two orthogonal polarizations parallel and perpendicular to the mechanical stress. Furthermore, a new resonance was observed for polarization parallel to the mechanical stress but not for the perpendicular polarization. The experimental results showed good agreement with the numerical simulations. A detailed group theoretical analysis showed that the symmetry lowering caused by the mechanical stress not only splits the originally degenerate mode but also modifies the originally optically inactive mode into an optically active mode, which then interacts strongly with a closely spaced mode and exhibits anticrossing behavior. The symmetry tuning enabled by applying mechanical stress is a simple and efficient way to engineer the nature of coupled plasmon resonances in complex nanostructures. The mechanically tunable plasmonic nanostructures also provide an excellent platform for dynamically tunable nanophotonic devices such as tunable filters and sensors.

Discriminating nanoparticle dimers from higher order aggregates through wavelength-dependent SERS orientational imaging

Surface-enhanced Raman scattering (SERS) orientational imaging is a recently developed all-optical technique able to determine SERS-active silver nanoparticle dimer orientations by observing lobe positions in SERS emission patterns formed by the directional polarization of SERS along the longitudinal axis of the dimer. Here we extend this technique to discriminate nanoparticle dimers from higher order aggregates by observing the wavelength dependence of SERS emission patterns, which are unchanged in nanoparticle dimers but show differences in higher order aggregates involving two or more nanoparticle junctions. The ability of SERS orientational imaging to identify stacked nanoparticles in higher order aggregates is also demonstrated. The shape of the SERS emission patterns originating from trimers labeled with low and high concentrations of dye is investigated, showing that the emission pattern lobes become less defined as the dye concentration increases. Dynamic fluctuations in the SERS emission pattern lobes are observed in aggregates labeled with low dye concentrations, as molecules diffuse into regions of higher electromagnetic enhancement in multiple nanoparticle junctions.

SERS Substrates by the Assembly of Silver Nanocubes: High-Throughput and Enhancement Reliability Considerations

Small clusters of nanoparticles are ideal substrates for SERS measurements, but the SERS signal enhancement by a particular cluster is strongly dependent on its structural characteristics and the measurement conditions. Two methods for high-throughput assembly of silver nanocubes into small clusters at predetermined locations on a substrate are presented. These fabrication techniques make it possible to study both the structure and the plasmonic properties of hundreds of nanoparticle clusters. The variations in SERS enhancement factors from cluster to cluster were analyzed and correlated with cluster size and configuration, and laser frequency and polarization. Using Raman instruments with 633 nm and 785 nm lasers and linear clusters of nanocubes, an increase in the reproducibility of the enhancement and an increase in the average enhancement values were achieved by increasing the number of nanocubes in the cluster, up to 4 nanocubes per cluster. By examining the effect of cluster configuration, it is shown that linear clusters with nanocubes attached in a face-to-face configuration are not as effective SERS substrates as linear clusters in which nanocubes are attached along an edge.

Design and Implementation of Noble Metal Nanoparticle Cluster Arrays for Plasmon Enhanced Biosensing

Nanoparticle Cluster Arrays (NCAs) are a class of electromagnetic materials that comprise chemically defined nanoparticles assembled into clusters of defined size in an extended deterministic arrangement. NCAs are fabricated through integration of chemically synthesized building blocks into predefined patterns using a hybrid top-down/bottom-up fabrication approach that overcomes some of the limitations of conventional top-down fabrication methods with regard to minimum available feature size and structural complexity. NCAs can sustain near-field interactions between nanoparticles within individual clusters as well as between entire neighboring clusters. The availability of near-field interactions on multiple length scales - together with the ability to further enhance the coupled plasmon modes through photonic modes in carefully designed array morphologies - leads to a multiscale cascade electromagnetic field enhancement throughout the array. This feature article introduces the design and fabrication fundamentals of NCAs and characterizes the electromagnetic coupling mechanisms in the arrays. Furthermore, it reviews how the optical properties of NCAs can be tuned through the size and shape of the nanoparticle building blocks and the geometry, size, and separation of the assembled clusters. NCAs have potential applications in many different areas; this feature article focuses on plasmon enhanced biosensing and surface enhanced Raman spectroscopy (SERS), in particular.

Removing a wedge from a metallic nanodisk reveals a fano resonance

A wide variety of complex, multicomponent plasmonic nanostructures have been shown to possess Fano resonances. Here we introduce a remarkably simple planar nanostructure, a single metallic nanodisk with a missing wedge-shaped slice, that also supports a Fano resonance. In this geometry, the Fano line shape arises from the coupling between a hybridized plasmon resonance of the disk and a narrower quadrupolar mode supported by the edge of the missing wedge slice. As a consequence, both disk size and wedge angle control the properties of the resonance. A semianalytical description of plasmon hybridization proves useful for analyzing the resulting line shape.

Real-space mapping of Fano interference in plasmonic metamolecules

An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochemical sensing and surface-enhanced Raman spectroscopy. Here we use scattering-type near-field optical microscopy to map the spatial field distribution of Fano modes in infrared plasmonic systems. We observe in real space the interference of narrow (dark) and broad (bright) plasmonic resonances, yielding intensity and phase toggling between different portions of the plasmonic metamolecules when either their geometric sizes or the illumination wavelength is varied.