Far-field Resonance Research Articles

Boosting Light-Matter Interactions in Plasmonic Nanogaps.

Plasmonic nanogaps in strongly coupled metal nanostructures can confine light to nanoscale regions, leading to huge electric field enhancement. This unique capability makes plasmonic nanogaps powerful platforms for boosting light-matter interactions, thereby enabling the rapid development of novel phenomena and applications. This review traces the progress of nanogap systems characterized by well-defined morphologies, controllable optical responses, and a focus on achieving extreme performance. The properties of plasmonic gap modes in far-field resonance and near-field enhancement are explored and a detailed comparative analysis of nanogap fabrication techniques down to sub-nanometer scales is provided, including bottom-up, top-down, and their combined approaches. Additionally, recent advancements and applications across various frontier research areas are highlighted, including surface-enhanced spectroscopy, plasmon-exciton strong coupling, nonlinear optics, optoelectronic devices, and other applications beyond photonics. Finally, the challenges and promising emerging directions in the field are discussed, such as light-driven atomic effects, molecular optomechanics, and alternative new materials.

Probing the optical near-field interaction of Mie nanoresonators with atomically thin semiconductors

Optical Mie resonators based on silicon nanostructures allow tuning of light-matter-interaction with advanced design concepts based on complementary metal–oxide–semiconductor (CMOS) compatible nanofabrication. Optically active materials such as transition-metal dichalcogenide (TMD) monolayers can be placed in the near-field region of such Mie resonators. Here, we experimentally demonstrate and verify by numerical simulations coupling between a MoSe2 monolayer and the near-field of dielectric nanoresonators. Through a comparison of dark-field (DF) scattering spectroscopy and photoluminescence excitation experiments (PLE), we show that the MoSe2 absorption can be enhanced via the near-field of a nanoresonator. We demonstrate spectral tuning of the absorption via the geometry of individual Mie resonators. We show that we indeed access the optical near-field of the nanoresonators, by measuring a spectral shift between the typical near-field resonances in PLE compared to the far-field resonances in DF scattering. Our results prove that using MoSe2 as an active probe allows accessing the optical near-field above photonic nanostructures, providing complementary information to sophisticated near-field microscopy equipment.

Unveiling facet effects in metallic nanoparticles to design an efficient plasmonic nanostructure

We investigated the role of nanoparticle surface morphology (facets) in estimating the plasmonic properties of nanoparticle-on-a-mirror design or NPOM in the presence of a thicker (20 nm) dielectric layer. Nanoparticle surface morphology differences ranging from smoother surface to multi-facets in the form of a sphere (NSOM), cube (NCOM), and singe bottom faceted sphere (SBF-NSOM) shapes have been employed. Three significant optical properties were observed. Better longer wavelength near-field and far-field resonance spectral positions from NCOM are achieved. Near-field enhancement extracted from SBF-NSOM outperformed NCOM by more than ∼ two times. Plasmonic gap mode enhancement is absent for NSOM in the presence of a larger dielectric layer. The availability of plasmonic gap modes in NCOM and SBF-NSOM, even at a 20 nm thick dielectric layer, is highly beneficial for various plasmonic applications. These differences in optical properties are understood by the role of NP facets in influencing the plasmonic cavity region.

Broad range electric field enhancement of a plasmonic nanosphere heterodimer

Interaction between metallic nanoparticles has been widely investigated due to the rise of the enhanced local electric field inside the gap. We numerically present the broadband near- and far-field spectra from the near-ultraviolet (UV) through the visible wavelength range using plasmonic heterodimers. Both near- and far-field resonances can be manipulated by the composition of heterodimers. They show strong dependencies on gap width and particle size. Compared with Al-Au and Al-Ag heterodimers, the dipole-mode resonant peak has a redshift for the Au-Ag heterodimer. In the near-UV range, the Al-Ag heterodimer gains the strongest optical enhancement. This is due to the robust optical resonance of Al and Ag particles in the near-UV range. On the other hand, the heterodimers with Au particles exhibit a better field enhancement at longer wavelengths. The physical origin of plasmonic resonances of the bonding dipole modes and higher-order modes are revealed by the simulated mappings of local electric fields and 3D surface charge distributions. Moreover, our simulations also reveal the suitability of the plasmon ruler equation and the power law enhancement equation to quantify the optical response of heterodimers.

Broad range electric field enhancement of a plasmonic nanosphere heterodimer

Interaction between metallic nanoparticles has been widely investigated due to the rise of the enhanced local electric field inside the gap. We numerically present the broadband near- and far-field spectra from the near-ultraviolet (UV) through the visible wavelength range using plasmonic heterodimers. Both near- and far-field resonances can be manipulated by the composition of heterodimers. They show strong dependencies on gap width and particle size. Compared with Al-Au and Al-Ag heterodimers, the dipole-mode resonant peak has a redshift for the Au-Ag heterodimer. In the near-UV range, the Al-Ag heterodimer gains the strongest optical enhancement. This is due to the robust optical resonance of Al and Ag particles in the near-UV range. On the other hand, the heterodimers with Au particles exhibit a better field enhancement at longer wavelengths. The physical origin of plasmonic resonances of the bonding dipole modes and higher-order modes are revealed by the simulated mappings of local electric fields and 3D surface charge distributions. Moreover, our simulations also reveal the suitability of the plasmon ruler equation and the power law enhancement equation to quantify the optical response of heterodimers.

Reconfigurable metalattices: Combining multipolar lattice resonances and magneto-optical effect in far and near fields

The present work has elaborated the roles of near- and far-field lattice resonances (LRs) in the performance of one-dimensional metalattices composed of magneto-optically (MO) coated cylinders. By taking advantage of LR effects and MO-modified multipolar interferences, it is feasible to alter transmission or reflection with unity efficiency by turning on or off external magnetic fields. In the far field, multipolar LRs near Rayleigh anomaly (RA) can be acquired, leading to transmission suppression or enhancement for different multipolar interference mechanisms. Meanwhile, thanks to exciting degeneracy-broken multipoles, asymmetric diffractive patterns are observed despite normal incidence. However, in the diffractionless region with strong near-field couplings, we find that LR effects are capable of not only modifying scattering amplitude (>1) but also introducing phase change or even inversion. Specifically, owing to the appearance of π/2-phase rotated electric dipoles, the first and second Kerker conditions are achieved simultaneously in this work. In addition, the coupling mechanism of RA-associated LRs and MO-influenced Mie modes supported by an individual cylinder is also unveiled. Besides, a proof-of-concept example using realistic Si@InSb metalattices has also been demonstrated, showing reconfigurable performance as expected. The revealed far/near-field mechanism of interplay between LRs and MO-modified multipoles will shed new light on wavefront engineering with diffracted effects and reconfigurable features.

Resonance Scattering of an Arbitrary Bessel Beam by a Spherical Object.

In our prior work, multipole expansion coefficients of a Bessel beam of arbitrary order with respect to an object of arbitrary location was derived analytically and combined with the T-matrix method to numerically compute the scattering. The present work is extended to directly use the multipole expansion to calculate the scattering from an elastic spherical target. The incident Bessel beam is located at an arbitrary location with respect to the center of the sphere. The far-field resonance scattering in both on- and off-axis incidences is calculated by subtracting an appropriate background from the total scattering based on the resonance scattering theory. The results reveal that the ordinary or helicoidal Bessel beams with selected half-cone angle could lead to resonance suppression at some resonance frequencies for on-axis incidence, however, not for off-axis incidence. The intrinsic property of the elastic scattering contribution from a sphere is shown by the three-dimensional patterns of resonance scattering. The resonance scattering and suppression from thin and thick shells are also noted.

Mapping plasmon-enhanced upconversion fluorescence of Er/Yb-doped nanocrystals near gold nanodisks.

Fluorescence enhancement effects have many potential applications in the domain of biochemical sensors and optoelectronic devices. Here, the emission properties of up-converting nanocrystals near nanostructures that support surface plasmon resonances have been investigated. Gold nanodisks of various diameters were illuminated in the near-infrared (λ = 975 nm) and a single fluorescent nanocrystal glued at the end of an atomic force microscope tip was scanned around them. By detecting its visible fluorescence around each structure, it is found that the highest fluorescence enhancement occurs in a zone that forms a two-lobe pattern near the nanodisks and which corresponds to the map of the near-field intensity calculated at the excitation wavelength. In agreement with numerical simulations, it is also observed that the maximum fluorescence enhancement takes place when the disk diameter is around 200 nm. Surprisingly, this disk size is small when compared to that yielding the highest far-field scattering resonance, which occurs for disks with a diameter of 300-350 nm at the same excitation wavelength. This shift between the near and far-field resonances should be taken into account in the design of structures in systems that use plasmon enhanced fluorescence effects.

Graphene as a local probe to investigate near-field properties of plasmonic nanostructures

Surface-enhanced Raman scattering (SERS) describes the huge enhancement of the Raman intensity by plasmonic near-fields. The authors investigate SERS caused by a localized surface plasmon by coupling a plasmonic gold nanodimer with the nonresonant Raman scatterer graphene. They perform comprehensive Raman scattering experiments on the coupled system to understand polarization and wavelength dependence of plasmonic enhancement. Remarkably, the near-field resonance from Raman scattering differs by almost 0.2 eV from the far-field resonance measured by dark-field spectroscopy, which is well beyond the expected difference. Graphene is an excellent material to fundamentally study the effects and interactions that give rise to SERS.

EBL-Based Fabrication and Different Modeling Approaches for Nanoporous Gold Nanodisks

We report electron beam lithography (EBL) based fabrication and different modeling techniques for disk-shaped nanoporous gold nanoparticles (NPG disk). The EBL technique can provide large area 2D patterns of regularly or randomly distributed nanodisks with narrow size distribution and flexible interdisk (center to center) distance. Such flexibility is essential to obtain quasi-single NPG disk response, which typically peaks in the near-infrared (NIR) spectrum beyond 1 μm, from ensemble measurements by common UV/vis/NIR spectrometers instead of a specialized NIR spectroscopic microscope. NPG disks of 200 to 500 nm diameter and 50 nm thickness have been fabricated and characterized. To model the NPG disk and calculate its plasmonic properties, two different modeling approaches have been developed. A model based on the Bruggeman effective medium theory (B-EMT model) requires little information about the nanoporous structure. In contrast, the nanoporous model (NP model) retains the essential nanoporous structural features of NPG disk. To evaluate the performance of these models, simulated extinction spectra have been compared to the experimental data. Both the B-EMT and NP models perform well to estimate the far-field plasmon resonance peak position. However, to obtain the accurate information about the plasmon peak width/plasmon lifetime and near-field plasmonic hot-spots formation within the nanopores, the NP model is essential since the B-EMT model lacks the nanoporous network.

Far-field resonance fluorescence from a dipole-interacting laser-driven cold atomic gas

We analyze the temporal response of the fluorescence light that is emitted from a dense gas of cold atoms driven by a laser. When the average interatomic distance is comparable to the wavelength of the photons scattered by the atoms, the system exhibits strong dipolar interactions and collective dissipation. We solve the exact dynamics of small systems with different geometries and show how these collective features are manifest in the scattered light properties such as the photon emission rate, the power spectrum and the second-order correlation function. By calculating these quantities beyond the weak (linear) driving limit, we make progress in understanding the signatures of collective behavior in these many-body systems. Furthermore, we shed light on the role of disorder and averaging on the resonance fluorescence, of direct relevance for recent experimental efforts that aim at the exploration of many-body effects in dipole–dipole interacting gases of atoms.

Effects of the rotation angle on surface plasmon coupling of nanoprisms.

We studied the effects of relative orientation of bowtie nanostructures on the plasmon resonance both experimentally and theoretically in this work. Specifically, we fabricated gold bowtie nanoantennas with rotated nanoprisms, measured the near-field and the far-field resonance behaviors using Raman spectroscopy and scattering microspectroscopy, and simulated the effects of the rotation angle on the localized surface plasmonic resonance using finite-difference time-domain simulations. In addition to the widely-discussed dipolar resonance in regular bowtie nanostructures, defined as tip-mode resonance in the present study, the excitations of edge-mode resonance were discovered under certain rotation angles of nanoprisms. Because of the resonances of different modes at different wavelengths, two different incident laser sources were used to measure the Raman spectra to provide evidence for the evolution of different resonance modes. Also, both the tip-mode and edge-mode resonances were verified by the simulated charge density distribution and their trends were discussed. Based on the discovered trend, a plasmon protractor was created with a near-exponential decay relationship between the relative resonance wavelength shift and cosine of the rotation angle. A plasmon hybridization model was also proposed for rotated bowties to explain the coupling between nanoprisms during rotation.

Localized surface plasmon resonance and the size effects of magneto-optic rods

Localized surface plasmon resonance of cylindrical magneto optical particles provides an important mechanism for the formation of chiral edge states in two-dimensional magneto-optical photonic crystals. These states are an electromagnetic analogy of the so-called chiral edge state's (CESs) in a quantum Hall system where the power transmission is unidirectional due to particular topological properties of the bands. Just like their electronic counterpart, the number of optical CESs in the band gap opened by an applied magnetic field is determined by the sum of the Chern numbers of the lower bands. For a two-dimensional photonic crystal composed of ferrite rods magnetized along their axis, the coupling of the localized surface plasmon resonance states of each rod results in a narrow flat band-gap, which contains one-way edge modes arising from the circulation of the energy flow around each rod excited by the resonance with broken time-reversal symmetry. So far the interpretation of the resonance-related chiral edge states are based on the long-wavelength approximation of the localized surface plasmon resonance of an individual magneto-optical particle. Though the results agree with the experimental results qualitatively, an obvious quantitative deviation is still obvious. In this work we apply the scattering theory to analyze the resonance condition and the features of both the far-field and the near-field at resonance for cylindrical magneto-optical particles. Our calculation shows that the splitting of scattering peaks of different orders will occur due to the magneto-optical effect. Such a split is observed between an (+n)-peak and an (-n) peak, as a sign of the broken time-reversal symmetry, and also between peaks of lower-order and higher-order. Another important feature is the simultaneous occurring of the far-field resonance and the near-field resonance, where the latter is characterized by a peak of energy-flow circulation around the particle. Based on this model the effects of particle size on the resonance peaks are discussed. It is shown that the resonance peaks are moved and broadened with the particle size increasing. The results explain the obvious deviation of the position of the resonance band-gap from the predicted frequency according to the previous long-wavelength approximation. Furthermore, the calculation of a particle of moderately-large size (nearly one-tenth of the incident wavelength) demonstrates the appearance of higher-order modes up to n=4 circling around the particle surface. This implies that these higher-order modes may also make non-trivial contribution to the formation of the flat band-gap observed in a photonic crystal of ferrite-rods and affect the behaviours of the chiral-edge state existing in such a gap. Particularly, it may be helpful in realizing the multimodes of chiral edge states in magneto-optical photonic crystals.

Nanogap effects on near- and far-field plasmonic behaviors of metallic nanoparticle dimers.

In the field of plasmonics, the nanogap effect is often related to one aspect like the near-field enhancement at a single excitation wavelength or the far-field resonance shift. In this study, taking full advantage of finite element method (FEM) calculations, we present a comprehensive and quantitative analysis of the nanogap effect on the plasmonic behaviors of metallic nanoparticle dimers. Firstly, near-field spectroscopy is proposed in order to extract the near-field resonance wavelengths. Focusing on the bonding dipole mode, it is found that the near-field enhancement factors exhibit a weak power-law dependence on the gap size, while the near-field resonance shift decays nearly exponentially as the gap size increases, with a lower decay length than that for the far-field resonance shift. The spectral deviation between these two shifts is suggested to be taken into account for spectroscopy applications of plasmonic devices, although it may be negligible for dimer structures with rather small gaps.

Impact of the plasmonic near- and far-field resonance-energy shift on the enhancement of infrared vibrational signals

We report on the impact of the differing spectral near- and far-field properties of resonantly excited gold nanoantennas on the vibrational signal enhancement in surface-enhanced infrared absorption (SEIRA). The knowledge on both spectral characteristics is of considerable importance for the optimization of plasmonic nanostructures for surface-enhanced spectroscopy techniques. From infrared micro-spectroscopic measurements, we simultaneously obtain spectral information on the plasmonic far-field response and, via SEIRA spectroscopy of a test molecule, on the near-field enhancement. The molecular test layer of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) was deposited on the surface of gold nanoantennas with different lengths and thus different far-field resonance energies. We carefully studied the Fano-type vibrational lines in a broad spectral window, in particular, how the various vibrational signals are enhanced in relation to the ratio of the far-field plasmonic resonance and the molecular vibrational frequencies. As a detailed experimental proof of former simulation studies, we show the clearly red-shifted maximum SEIRA enhancement compared to the far-field resonance.

Plasmon enhanced water splitting mediated by hybrid bimetallic Au-Ag core-shell nanostructures.

In this work, we employed wet chemically synthesized bimetallic Au-Ag core-shell nanostructures (Au-AgNSs) to enhance the photocurrent density of mesoporous TiO2 for water splitting and we compared the results with monometallic Au nanoparticles (AuNPs). While Au-AgNSs incorporated photoanodes give rise to 14× enhancement in incident photon to charge carrier efficiency, AuNPs embedded photoanodes result in 6× enhancement. By varying nanoparticle concentration in the photoanodes, we observed ∼245× less Au-AgNSs are required relative to AuNPs to generate similar photocurrent enhancement for solar fuel conversion. Power-dependent measurements of Au-AgNSs and AuNPs showed a first order dependence to incident light intensity, relative to half-order dependence for TiO2 only photoanodes. This indicated that plasmonic nanostructures enhance charge carriers formed on the surface of the TiO2 which effectively participate in photochemical reactions. Our experiments and simulations suggest the enhanced near-field, far-field, and multipolar resonances of Au-AgNSs facilitating broadband absorption of solar radiation collectively gives rise to their superior performance in water splitting.

The spectral shift between near- and far-field resonances of optical nano-antennas.

Within the past several years a tremendous progress regarding optical nano-antennas could be witnessed. It is one purpose of optical nano-antennas to resonantly enhance light-matter interactions at the nanoscale, e.g. the interaction of an external illumination with molecules. In this specific, but in almost all schemes that take advantage of resonantly enhanced electromagnetic fields in the vicinity of nano-antennas, the precise knowledge of the spectral position of resonances is of paramount importance to fully exploit their beneficial effects. Thus far, however, many nano-antennas were only optimized with respect to their far-field characteristics, i.e. in terms of their scattering or extinction cross sections. Although being an emerging feature in many numerical simulations, it was only recently fully appreciated that there exists a subtle but very important difference in the spectral position of resonances in the near-and the far-field. With the purpose to quantify this shift, Zuloaga et al. suggested a Lorentzian model to estimate the resonance shift. Here, we devise on fully analytical grounds a strategy to predict the resonance in the near-field directly from that in the far-field and disclose that the issue is involved and multifaceted, in general. We outline the limitations of our theory if more sophisticated optical nano-antennas are considered where higher order multipolar contributions and higher order antenna resonances become increasingly important. Both aspects are highlighted by numerically studying relevant nano-antennas.

Far-field Fano resonance in nanoring lattices modeled from extracted, point dipole polarizability

Coupling and extinction of light among particles representable as point dipoles can be characterized using the coupled dipole approximation (CDA). The analytic form for dipole polarizability of spheroidal particles supports rapid electrodynamic analysis of nanoparticle lattices using CDA. However, computational expense increases for complex shapes with non-analytical polarizabilities which require discrete dipole (DDA) or higher order approximations. This work shows fast CDA analysis of assembled nanorings is possible using a single dipole nanoring polarizability extrapolated from a DDA calculation by summing contributions from individual polarizable volume elements. Plasmon resonance wavelengths of nanorings obtained using extracted polarizabilities blueshift as wall dimensions-to-inner radius aspect ratio increases, consistent with published theory and experiment. Calculated far-field Fano resonance energy maximum and minimum wavelengths were within 1% of full volume element results. Considering polarizability allows a more complete physical picture of predicting plasmon resonance location than metal dielectric alone. This method reduces time required for calculation of diffractive coupling more than 40 000-fold in ordered nanoring systems for 400–1400 nm incident wavelengths. Extension of this technique beyond nanorings is possible for more complex shapes that exhibit dipolar or quadrupole radiation patterns.

Hierarchical Nanogaps within Bioscaffold Arrays as a High-Performance SERS Substrate for Animal Virus Biosensing

A three-dimensional (3D) biomimetic SERS substrate with hierarchical nanogaps was formed on the bioscaffold arrays of cicada wings by one-step and reagents-free ion-sputtering techniques. This approach requires a minimal fabrication effort and cost and offers Ag nanoislands and Ag nanoflowers with four types of nanogaps (<10 nm) on the chitin nanopillars to generate a high density of hotspots (∼2000/μm2). The 3D biomimetic substrate shows a low detection limit to Rhodamine 6G (10(-13) M), high average enhancement factor (EF, 5.8×10(7)), excellent signal uniformity (5.4%), good stability, and suitability in biosensing. Furthermore, the finite-difference time-domain (FDTD) electric-field-distribution simulations illustrate that the 3D biomimetic SERS substrate provides the high-density hotspot area within a detection volumem, resulting in enormous SERS enhancement. In addition, the conspicuous far-field plasmon resonance peaks were not found to be a strong requirement for a high EF in 3D biomimetic substrates. Additionally, the novel substrate was applied in label-free animal viruses detection and differentiation with small amounts (1.0 μL) and low concentrations of analyte (1×10(3) PFU/mL), and it exhibited potential as an effective SERS platform for virus detection and sensing.

Polylogarithm-Based Computation of Fano Resonance in Arrayed Dipole Scatterers

Efficient descriptions for Fano resonant properties of plasmonic arrays of nanoparticles could support selection of particle and array characteristics to achieve specific spectral outcomes. In contrast to conventional particle-by-particle computation of Fano resonances from arrayed particles, this work uses polylogarithms to assemble a semianalytical solution to the coupled dipole approximation for far-field Fano resonances from two-dimensional arrays by superposing results from discrete, one-dimensional chains of particles that comprise the array. This approach directly depicts the effects of discrete particle chains on the overall spectral features of an array, thus enhancing intuitive understanding of these effects, and reduces the computation time 56% relative to particle-by-particle evaluation. Fano resonance spectra calculated using polylogarithms are punctuated by more frequent, sharper oscillations and fine features due to summed contributions from distant interfering dipoles not present in spectra calculated using a necessarily finite number of particles. Chains more orthogonal to the axis of polarization contribute more Fano features due to broadband, sin2 θij dependent dipole scattering coupling to narrow diffracted modes. Overall, the approach improves accessibility and increases the speed of designing nanoparticle lattices for particular outcomes.