Plasmonic Metamolecules Research Articles

Coupling phenomena and collective effects in resonant meta-atoms supporting both plasmonic and (opto-)magnetic functionalities: an overview on properties and applications [Invited

We review both the fundamental aspects and the applications of functional magneto-optic and opto-magnetic metamaterials displaying collective and coupling effects on the nanoscale, where the concepts of optics and magnetism merge to produce unconventional phenomena. The use of magnetic materials instead of the usual noble metals allows for an additional degree of freedom for the control of electromagnetic field properties, as well as it allows light to interact with the spins of the electrons and to actively manipulate the magnetic properties of such nanomaterials. In this context, we explore the concepts of near-field coupling of plasmon modes in magnetic meta-molecules, as well as the effect of excitation of surface lattice resonances in magneto-plasmonic crystals. Moreover, we discuss how these coupling effects can be exploited to artificially enhance optical magnetism in plasmonic meta-molecules and crystals. Finally, we highlight some of the present challenges and provide a perspective on future directions of the research towards photon-driven fast and efficient nanotechnologies bridging magnetism and optics beyond current limits.

Gated Graphene Enabled Tunable Charge–Current Configurations in Hybrid Plasmonic Metamaterials

An active hybrid toroidal metasurface is demonstrated based on artificially engineered plasmonic metamolecules deposited on a gate-controlled graphene layer. Our numerical predictions are confirmed by room-temperature time-domain spectroscopy characterizations in the terahertz frequency range. We quantitatively and qualitatively show that the gate-controlled active graphene metamaterial allows for an active tuning of the dynamic nonradiating toroidal dipole by tuning the essential mismatch in the spinning magnetic fields in the strongly coupled resonators. It is demonstrated that this feature can be obtained by varying the resistivity of gate-controlled graphene and tuning the capacitive coupling at the critical openings of the system.

Chiral Metamolecules with Active Plasmonic Transition.

Energy-dissipating self-assembly is at the basis of many important cellular processes, such as cell organization, proliferation, and morphogenesis. Beyond equilibrium self-assembled molecular systems and materials, it is increasingly recognized that the control of assembly kinetics provides great opportunity for the next generation of molecular materials with intelligent behavior including programmed spatiotemporal organization. Here we show the transient self-assembly of active chiral plasmonic metamolecules (CPMs), which is controlled by the proton flux generated from a positive-feedback chemical reaction network. The fuel-conversion kinetics allows for temporal control and adaptive tuning of multiple structures of plasmonic metamolecules (PMs). This approach enables autonomous tuning of chiroptical properties of metamolecules with dynamic behavior. Moreover, we show that 11 types of spatial configurations of PMs are assembled, and 9 types of temporal configurations of CPMs are differentiated.

Magnetic hot-spot generation at optical frequencies: from plasmonic metamolecules to all-dielectric nanoclusters

Abstract The weakness of magnetic effects at optical frequencies is directly related to the lack of symmetry between electric and magnetic charges. Natural materials cease to exhibit appreciable magnetic phenomena at rather low frequencies and become unemployable for practical applications in optics. For this reason, historically important efforts were spent in the development of artificial materials. The first evidence in this direction was provided by split-ring resonators in the microwave range. However, the efficient scaling of these devices towards the optical frequencies has been prevented by the strong ohmic losses suffered by circulating currents. With all of these considerations, artificial optical magnetism has become an active topic of research, and particular attention has been devoted to tailor plasmonic metamolecules generating magnetic hot spots. Several routes have been proposed in these directions, leading, for example, to plasmon hybridization in 3D complex structures or Fano-like magnetic resonances. Concurrently, with the aim of electromagnetic manipulation at the nanoscale and in order to overcome the critical issue of heat dissipation, alternative strategies have been introduced and investigated. All-dielectric nanoparticles made of high-index semiconducting materials have been proposed, as they can support both magnetic and electric Mie resonances. Aside from their important role in fundamental physics, magnetic resonances also provide a new degree of freedom for nanostructured systems, which can trigger unconventional nanophotonic processes, such as nonlinear effects or electromagnetic field localization for enhanced spectroscopy and optical trapping.

Fundamental and Practical Limits of Achieving Artificial Magnetism and Effective Optical Medium by Using Self-Assembly of Metallic Colloidal Clusters

The self-assembly of metallic colloidal clusters (so called plasmonic metamolecules) has been viewed as a versatile, but highly effective approach for the materialization of the metamaterials exhibiting artificial magnetism at optical frequencies (including visible and near infrared (NIR) regimes). Indeed, several proofs of concepts of plasmonic metamolecules have been successfully demonstrated in both theoretical and experimental ways. Nevertheless, this self-assembly strategy has barely been used and still remains an underutilized method. For example, the self-assembly and optical utilization of the plasmonic metamolecules have been limited to the discrete unit of the structure; the materialization of effective optical medium made of plasmonic metamolecules is highly challenging. In this work, we theoretically exploited the practical limits of self-assembly technology for the fabrication of optical magnetic metamaterials.

Modular Assembly of Plasmonic Nanoparticles Assisted by DNA Origami.

Arraying noble metal nanoparticles with nanoscale features is an important way to develop plasmonic devices with novel optical properties such as plasmonic chiral metamolecules, optical waveguides, and so forth. Along with top-down methods of fabricating plasmonic nanostructures, solution-based self-assembly provides an alternative approach. There are mainly two routes to organizing metal nanoparticles via self-assembly. One is directly linking nanoparticles through linker molecules, and the other is using nanoparticles to decorate a preformed template. We combine these two routes and herein report a strategy for the DNA origami-assisted modular assembly of gold nanoparticles into homogeneous and heterogeneous plasmonic nanostructures. For each module, we designed W-shaped DNA origami with two troughs as two domains. One domain is used to host a gold nanoparticle, and the other domain is designed to capture another gold nanoparticle hosted on a different module. By simply tuning the sequences of capture DNA strands on each module, gold nanoparticles including spherical and rod-shaped gold nanoparticles (denoted as AuNPs and AuNRs) could be well organized in a predefined manner to form versatile plasmonic nanostructures. Since the interparticle distances could be precisely controlled at the nanoscale, we also studied the plasma coupling among the assembled plasmonic nanostructures. This modular assembly strategy represents a simple yet general and effective design principle for DNA-assembled plasmonic nanostructures.

DNA Origami‐Guided Assembly of the Roundest 60–100 nm Gold Nanospheres into Plasmonic Metamolecules

AbstractDNA origami can provide programmed information to guide the self‐assembly of gold nanospheres (Au NSs) into higher‐order supracolloids. Molecularly precise and truly 2D/3D integration of Au NSs is possible using DNA origami‐enabled assembly, and the resulting assemblies have potential applications in plasmonics and metamaterials. However, the relatively small size (<60 nm) and randomly faceted Au NSs that have been used thus far in DNA origami‐enabled assembly have limited their nanophotonic applications. Here, the robust self‐assembly of the 60–100 nm roundest Au NSs into metamolecular assemblies using 3D DNA origami is described. These Au NSs are successfully conjugated with DNA oligonucleotides and are therefore stable at high salt concentrations even without backfilling using organic ligands. The roundest Au NSs are successfully assembled into supracolloidal metamolecules and chains via 3D DNA origami. These plasmonic metamolecules and chains display strong electric and unnatural magnetic resonances that can be deterministically controlled.

Extreme sensitive metasensor for targeted biomarkers identification using colloidal nanoparticles-integrated plasmonic unit cells

Engineered terahertz (THz) plasmonic metamaterials have emerged as promising platforms for quick infection diagnosis, cost-effective and real-time pharmacology applications owing to their non-destructive and harmless interaction with biological tissues in both in vivo and in vitro assays. As a recent member of THz metamaterials family, toroidal metamaterials have been demonstrated to be supporting high-quality sharp resonance modes. Here we introduce a THz metasensor based on a plasmonic surface consisting of metamolecules that support ultra-narrow toroidal resonances excited by the incident radiation and demonstrate detection of an ultralow concertation targeted biomarker. The toroidal plasmonic metasurface was designed and optimized through extensive numerical studies and fabricated by standard microfabrication techniques. The surface then functionalized by immobilizing the antibody for virus-envelope proteins (ZIKV-EPs) for selective sensing. We sensed and quantified the ZIKV-EP in the assays by measuring the spectral shifts of the toroidal resonances while varying the concentration. In an improved protocol, we introduced gold nanoparticles (GNPs) decorated with the same antibodies onto the metamolecules and monitored the resonance shifts for the same concentrations. Our studies verified that the presence of GNPs enhances capturing of biomarker molecules in the surrounding medium of the metamaterial. By measuring the shift of the toroidal dipolar momentum (up to Δω~0.35 cm-1) for different concentrations of the biomarker proteins, we analyzed the sensitivity, repeatability, and limit of detection (LoD) of the proposed toroidal THz metasensor. The results show that up to 100-fold sensitivity enhancement can be obtained by utilizing plasmonic nanoparticles-integrated toroidal metamolecules in comparison to analogous devices. This approach allows for detection of low molecular-weight biomolecules (≈13 kDa) in diluted solutions using toroidal THz plasmonic unit cells.

Self-Assembly of Heterogeneously Shaped Nanoparticles into Plasmonic Metamolecules on DNA Origami.

Fabrication of plasmonic metamolecules (PMs) with rationally designed complexity is one of the major goals of nanotechnology. Most self-assembled PMs, however, have been constructed using single-component systems. The corresponding plasmonic assemblies still suffer from the lack of complexity, which is required to achieve a high degree of functionality. Here, we report a general applicable strategy that can realize a series of high-ordered hetero-PMs using bottom-up DNA self-assembly. DNA-functionalized differently shaped nanoparticles were deliberately arranged in prescribed positions on 3D triangular DNA origami frames to form various hetero-PMs. Importantly, we showed that the optical properties of assembled PMs could be facially tuned by selectively regulating the position of each component. This method provides a promising pathway for manufacturing more complex and advanced materials by integrating diverse nanocomponents with particular properties.

Resolving the bond angle of a plasmonic metamolecule

Plasmonic metamolecules have many peculiar properties and have been applied in biological and materials science such as in reconfigurable 3D building blocks for complex nano-architectures and imaging probes for high-resolution sensing. In these applications, fast detection of the bond angles of the sub-wavelength metamolecules is highly desired. However, angle detection is not the same as orientation detection. The two orientations must be determined simultaneously, and common orientation sensors can only measure one. In this work, we propose and demonstrate a method to resolve the bond angle of a plasmonic metamolecule composed of three spherical nanoparticles. The detection of the bond angle is achieved via modulation depth analysis of polarization-resolved dark-field images. The underlying mechanism is found to be the opposing responses of the longitudinal and transversal bonding modes to the polarization variation of the incident light. In addition, the spectrally degenerate structures are further distinguished by the spot center localization method. This method may pave the way for practical application of plasmonic metamolecules.

Assembly of \u201c3D\u201d plasmonic clusters by \u201c2D\u201d AFM nanomanipulation of highly uniform and smooth gold nanospheres

Atomic force microscopy (AFM) nanomanipulation has been viewed as a deterministic method for the assembly of plasmonic metamolecules because it enables unprecedented engineering of clusters with exquisite control over particle number and geometry. Nevertheless, the dimensionality of plasmonic metamolecules via AFM nanomanipulation is limited to 2D, so as to restrict the design space of available artificial electromagnetisms. Here, we show that “2D” nanomanipulation of the AFM tip can be used to assemble “3D” plasmonic metamolecules in a versatile and deterministic way by dribbling highly spherical and smooth gold nanospheres (NSs) on a nanohole template rather than on a flat surface. Various 3D plasmonic clusters with controlled symmetry were successfully assembled with nanometer precision; the relevant 3D plasmonic modes (i.e., artificial magnetism and magnetic-based Fano resonance) were fully rationalized by both numerical calculation and dark-field spectroscopy. This templating strategy for advancing AFM nanomanipulation can be generalized to exploit the fundamental understanding of various electromagnetic 3D couplings and can serve as the basis for the design of metamolecules, metafluids, and metamaterials.

Self-Assembled Plasmonic Metamolecules Exhibiting Tunable Magnetic Response at Optical Frequencies

A bottom-up self-assembly approach is used to synthesize pseudospherical clusters of Au nanoparticles with low size and shape polydispersity. Such plasmonic metamolecules (PMMs) with various diameters are prepared from two different AuNP building block sizes, enabling the investigation of the size-dependent optical properties of these structures. Trends in the UV–vis spectra are reproduced theoretically using generalized multiparticle Mie theory as either the AuNP or PMM diameter are changed. The calculated results indicate that features in the spectra of large PMMs can be associated with magnetic dipole resonances, which strengthen and red-shift with increasing PMM size. Finally, effective medium theory is used to demonstrate that materials composed of these PMMs could exhibit a bulk isotropic magnetic response at optical frequencies.

Magnetic Fano resonance-induced second-harmonic generation enhancement in plasmonic metamolecule rings.

The "artificial magnetic" resonance in plasmonic metamolecules extends the potential application of magnetic resonance from terahertz to optical frequency bypassing the problem of magnetic response saturation by replacing the conduction current with the ring displacement current. So far, the magnetic Fano resonance-induced nonlinearity enhancement in plasmonic metamolecule rings has not been reported. Here, we use the magnetic Fano resonance to enhance second-harmonic generation (SHG) in plasmonic metamolecule rings. In the spectra of the plasmonic metamolecule, an obvious Fano dip appears in the scattering cross section, while the dip does not appear in the absorption cross section. It indicates that at the Fano dip the radiative losses are suppressed, while the optical absorption efficiency is at a high level. The largely enhanced SHG signal is observed as the excitation wavelength is adjusted at the magnetic Fano dip of the plasmonic metamolecule rings with stable and tunable magnetic responses. We also compare the magnetic Fano dip with the electric case to show its advantages in enhancing the fundamental and second harmonic responses. Our research provides a new thought for enhancing optical nonlinear processes by magnetic modes.

Controlling the nonlinear optical properties of plasmonic nanoparticles with the phase of their linear response

We numerically investigate the second harmonic generation from different plasmonic systems and evidence the key role played in their nonlinear response by the phase at the fundamental wavelength. In the case of a single plasmonic nanorod, the interference between the second harmonic dipolar and quadrupolar emission modes depends on their relative phase, which is deeply related to the excitation wavelength. The knowledge obtained in this simple case is then used to describe and understand the nonlinear response from a more complex structure, namely a gold nanodolmen. The complex phase evolution associated with a Fano resonance arising at the fundamental wavelength enables dramatically modifying the second harmonic emission patterns from plasmonic metamolecules within minute wavelength shifts. These results emphasize the importance of the phase in the nonlinear optical processes arising in plasmonic nanostructures, in addition to the increase in conversion yield associated with the excitation of localized surface plasmon resonances.

Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules.

The smallness of natural molecules and atoms with respect to the wavelength of light imposes severe limits on the nature of their optical response. For example, the well-known argument of Landau and Lifshitz and its recent extensions that include chiral molecules show that the electric dipole response dominates over the magneto-electric (bianisotropic) and an even smaller magnetic dipole optical response for all natural materials. Here, we experimentally demonstrate that both these responses can be greatly enhanced in plasmonic nanoclusters. Using atomic force microscopy nanomanipulation technique, we assemble a plasmonic metamolecule that is designed for strong and simultaneous optical magnetic and magneto-electric excitation. Angle-dependent scattering spectroscopy is used to disentangle the two responses and to demonstrate that their constructive/destructive interplay causes strong directional scattering asymmetry. This asymmetry is used to extract both magneto-electric and magnetic dipole responses and to demonstrate their enhancement in comparison to ordinary atomistic materials.

Plasmonic Toroidal Metamolecules Assembled by DNA Origami

We show hierarchical assembly of plasmonic toroidal metamolecules that exhibit tailored optical activity in the visible spectral range. Each metamolecule consists of four identical origami-templated helical building blocks. Such toroidal metamolecules show a stronger chiroptical response than monomers and dimers of the helical building blocks. Enantiomers of the plasmonic structures yield opposite circular dichroism spectra. Experimental results agree well with the theoretical simulations. We also show that given the circular symmetry of the structures s distinct chiroptical response along their axial orientation can be uncovered via simple spin-coating of the metamolecules on substrates. Our work provides a new strategy to create plasmonic chiral platforms with sophisticated nanoscale architectures for potential applications such as chiral sensing using chemically based assembly systems.

Hydrogen-Regulated Chiral Nanoplasmonics.

Chirality is a highly important topic in modern chemistry, given the dramatically different pharmacological effects that enantiomers can have on the body. Chirality of natural molecules can be controlled by reconfiguration of molecular structures through external stimuli. Despite the rapid progress in plasmonics, active regulation of plasmonic chirality, particularly in the visible spectral range, still faces significant challenges. In this Letter, we demonstrate a new class of hybrid plasmonic metamolecules composed of magnesium and gold nanoparticles. The plasmonic chirality from such plasmonic metamolecules can be dynamically controlled by hydrogen in real time without introducing macroscopic structural reconfiguration. We experimentally investigate the switching dynamics of the hydrogen-regulated chiroptical response in the visible spectral range using circular dichroism spectroscopy. In addition, energy dispersive X-ray spectroscopy is used to examine the morphology changes of the magnesium particles through hydrogenation and dehydrogenation processes. Our study can enable plasmonic chiral platforms for a variety of gas detection schemes by exploiting the high sensitivity of circular dichroism spectroscopy.

Subgroup decomposition analyses of D3h and D4h plasmonic metamolecule Fano resonance spectrum

In recent decades, research about surface plasmon polariton (SPP) has earned its popularity in nanotechnology with many theoretical achievements, much progress in metal nanostructure manufacturing, spectral analyzing, biomedicine ultrasensing, etc. Group theory is an effective tool for analyzing the spectra of symmetrical organized multiparticles (dubbed as plasmonic metamolecule). Recently, SPP Fano resonance in nanostructure either from plasmonic metamolecules or from symmetry-breaking has attracted much attention. Regarding to the subgroup decomposition analysis of the D3h and D4h plasmonic metamolecule surface plasmon resonance spectra and the mechanism of forming the Fano resonance spectral dip, this paper proposes an explanation method based on group theory.By using a similar group theory approach to constructing the molecular vibration normal modes, the method to build the dipolar SPP symmetric modes of plasmonic metamolecules is established. It is confirmed that under the linear polarization excitation there exists only two dipolar SPP symmetric modes for a ring shaped Dnh plasmonic metamolecule, while adding the center particle will merely add an extra independent symmetric mode. For the D3h and D4h plasmonic metamolecule, it is found that there are two dominant eigenmodes i. e., one is composed by adding two symmetric modes and the other by subtracting two symmetric modes. The decomposition analysis further reveals that the negative coefficient of the symmetric mode for forming the short wavelength eigenmode for D3h tetramer plasmonic metamolecules is much smaller than that for D4h pentamer plasmonic metamolecules, thereby explaining that the Fano resonance dip of the pentamer is sharper than that of the tetramer. It is worth noting that the group theory can provide some guidance for building the symmetric modes and the SPP eigenmodes, but is unable to determine the coefficient of each symmetric mode.As for the origin of Fano resonance dip, so far there have existed two different perspectives: one is the traditional viewpoint, i.e., the Fano resonance dip is formed due to the coupling of the wideband superradiant bright mode with the narrowband subradiant dark mode, and the other is that the Fano resonance dip is formed by the destructive interference between two neighboring eigenmodes. The decomposition analysis described in this paper actually can unify these two perspectives.

Evolution of Plasmonic Metamolecule Modes in the Quantum Tunneling Regime.

Plasmonic multinanoparticle systems exhibit collective electric and magnetic resonances that are fundamental for the development of state-of-the-art optical nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. While electric dipolar modes have been investigated in both the classical and quantum realm, little attention has been given to magnetic and other "dark" modes at the smallest dimensions. Here, we study the collective electric, magnetic, and dark modes of colloidally synthesized silver nanosphere trimers with varying interparticle separation using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). This technique enables direct visualization and spatially selective excitation of individual trimers, as well as manipulation of the interparticle distance into the subnanometer regime with the electron beam. Our experiments reveal that bonding electric and magnetic modes are significantly impacted by quantum effects, exhibiting a relative blueshift and reduced EELS amplitude compared to classical predictions. In contrast, the trimer's electric dark mode is not affected by quantum tunneling for even Ångström-scale interparticle separations. We employ a quantum-corrected model to simulate the effect of electron tunneling in the trimer which shows excellent agreement with experimental results. This understanding of classical and quantum-influenced hybridized modes may impact the development of future quantum plasmonic materials and devices, including Fano-like molecular sensors and quantum metamaterials.

Giant Nonlinearity of an Optically Reconfigurable Plasmonic Metamaterial.

Metamaterial nanostructures actuated by light give rise to a large optical nonlinearity. Plasmonic metamolecules on a flexible support structure cut from a dielectric membrane of nanoscale thickness are rearranged by optical illumination. This changes the optical properties of the strongly coupled plasmonic structure and therefore results in modulation of light with light.