Delocalized Surface Plasmon Research Articles

Ultrafast Dynamics of Strong Near‐Field Coupled Localized and Delocalized Surface Plasmons

AbstractThis paper reports transient dynamics of strong near‐field coupling behavior of plasmons in a multilayer metal nanostructure that enables ultrahigh‐efficient optical switching. Particle plasmon on the top array units couples with localized/delocalized plasmons on the bottom Au film to produce dipolar and high‐order hybrid modes with nearly suppressed reflection. The mode coupling can be tuned from near‐field to far‐field regime readily mediated by the spacer thickness. The dipolar mode inherits the high dispersive property of the delocalized plasmon, but the higher‐order one shows limited dispersion. Transient spectroscopy illustrates both hybrid modes have a picosecond timescale redshift under on‐ and off‐resonance excitation. However, the higher‐order mode exhibits a longer decay lifetime than the dipolar one because of resonance characteristics and cavity properties. An ultrafast optical switching effect with a response time of τ = 3.2 ps and an ultrahigh efficiency of 5100 mOD mJ−1 cm2 is achieved under an extremely low pump fluence.

Bandwidth tunability of graphene absorption enhancement by hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons

The bandwidth-tunable absorption enhancement of monolayer graphene is theoretically studied in the near-infrared wavelengths. The monolayer graphene is placed on the silver substrate surface with a periodic array of one-dimensional slits. Two absorption peaks are found to result from the hybridization of delocalized surface plasmon polaritons and localized magnetic plasmons. The positions of absorption peaks are accurately predicted by a coupling model of double oscillators. The full width at half maximum of absorption peaks is largely tuned from about 1–200 nm by changing the array period of slits. The effect of the slit size on absorption peaks is also investigated in detail. Our work is promising in applications for photoelectric devices.

Simultaneously achieving narrowband and broadband light absorption enhancement in monolayer graphene

We study theoretically how to simultaneously achieve narrowband and broadband light absorption enhancement of monolayer graphene, which is placed on a metallic substrate surface with a periodic array of small holes. By a couple model of double resonances, the narrowband and broadband absorption peaks of monolayer graphene are proven to stem from the hybrid modes of delocalized surface plasmon polartions propagating on the substrate surface and localized magnetic plasmons confined within individual holes. The positions, maxima, and bandwidths of the absorption peaks can be tuned conveniently through changing the hole array period. For an optimized period, the maximum absorption values at the narrowband and broadband peaks are able to reach up to 65% and 85%, respectively. The bandwidth of the narrowband absorption peak can be reduced to only several nanometers, and the bandwidth of the broadband absorption peak can be expanded to tens of nanometers. Our work will be helpful to design graphene-based photoelectric nanodevices whose optical responses are in the visible and near-infrared regions.

Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates.

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Two-dimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton-plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule-gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.

Random lasing based on a nanoplasmonic hybrid structure composed of (Au core)-(Ag shell) nanorods with Ag film

Metallic films are widely used as electrodes in micro-cavities. However, the quenching effect of metal films is generally considered fatal for lasing, and presents a major obstacle to the development of electrically pumped lasers. In this article, we report on the dramatic performance enhancement of random lasing assisted by a plasmonic hybrid structure composed of (Au core)-(Ag shell) nanorods deposited on Ag film (Au@Ag NRs-Ag film). This study reveals that the Au@Ag NRs-Ag film hybrid structure can more effectively enhance the lasing properties than independent Au@Ag NRs or Ag film. Besides, compared with hybrid structures composed of Ag film with Au nanorods or Au nanospheres, the gain medium deposited on Au@Ag NRs-Ag film has the lowest lasing threshold: only 12.5% of that of the neat gain medium. As a unique plasmonic hybrid nanostructure, Au@Ag NRs-Ag film exhibits a stronger localized electrical field and scattering effect than the hybrid structures composed of Ag film with regular Au nanoparticles. This is attributed to the broader and stronger plasmonic absorption of Au@Ag NRs, as well as to the stronger plasmonic coupling between the localized surface plasmons of Au@Ag NRs and the delocalized surface plasmon polariton of Ag film. Our results could provide a simple way to effectively avoid the negative effects of metal films and realize a lower pumped threshold.

Tuning of longitudinal plasmonic coupling in graphene nanoribbon arrays/sheet hybrid structures at mid-infrared frequencies

Coupling and hybridization of plasmon polaritons can commonly occur in graphene plasmonic nanostructures, providing new possibilities for developing many novel plasmonic optoelectronic devices. Here we have theoretically investigated the longitudinal plasmonic coupling between localized and delocalized surface plasmon polaritons in graphene nanoribbon arrays and monolayer structures in the mid-infrared region. It has been demonstrated that vertical plasmonic coupling can be actively controlled by either the geometric parameters or the Fermi energy in graphene, allowing for strong light–matter interaction. Thanks to the strong plasmon coupling, dual-band perfect absorption with A≈100% and large Rabi splitting exceeding 17.2 meV have been obtained in the absorption spectra of this hybrid system. More intriguingly, we found, by varying the distance between the graphene sheet and the metallic substrate, that periodic spectral nodes can emerge in absorption response of the hybrid mode, which was explained by the mechanism of longitudinal microcavity resonance in this coupled system. The controllable plasmonic coupling and ultrahigh dual-band absorption capability offered by this coupled structure open new avenues for designing tunable multi-channel graphene optoelectronic devices with high performance.

Controlling the interference between localized and delocalized surface plasmons via incident polarization for optical switching

Surface plasmons supported by various metallic nanostructures have given rise to several significant breakthroughs in the field of integrated photonic devices due to its ability to effectively confine and enhance optical field in subwavelength volume. In particular, the demand to actively control optical responses of plasmonic systems becomes urgent for the miniaturization of signal processing devices, surface-enhanced Raman scattering (SERS) substrates and biochemical sensors. In this paper, we systematically investigate the plasmon modes as well as their interaction in a layered nanostructure composed of a periodically-arranged radiative nanoring and a metallic ground plane, as well as a thin insulating spacer. A tunable transparent peak on the background of the broadband plasmon resonance emerges in the reflection spectrum as changing the periodicity of nanoparticle array, a plasmonic analogue of electromagnetically induced transparency (EIT). Owing to the structural symmetry of the rings, we demonstrate a new scheme of controlling the interference between localized and delocalized plasmons by means of incident polarization and believe that the proposed metasurface may find applications in optical switching if the polarization-controlled components are introduced.

Localized versus delocalized surface plasmons: dual nature of optical resonances on a silver circular wire and a silver tube of large diameter

We investigate the resonances in the scattering and absorption of TE-polarized light by a solid circular silver wire and a silver tube larger than the wavelength radius and nanoscale thickness using the analytical solution and experimental data for the silver complex permittivity. We find that the resonances are well observed both in the visible and infra-red ranges if the scatterer radius is smaller than a certain value. They can be explained as the resonances on the standing surface-plasmon waves running along a curved metal-air interface or a metal layer, respectively. Still, each of them corresponds to one of the multipole localized surface plasmon modes of the circular wire or tube. This bridges the gap between localized and delocalized plasmons, which are complementary descriptions of the same physical effect intrinsic for metals in the visible range.

Tunable metasurface with two non-coplanar and inter-perpendicular graphene nanoribbon arrays for the coupling between localized and delocalized surface plasmon polaritons

Graphene metasurface has attracted a lot of attentions due to the unique tunability for exotic electromagnetic properties. In this work, we propose and numerically investigate a tunable metasurface with two non-coplanar and inter-perpendicular graphene nanoribbon arrays. The variation of transmission at different substrate thickness and the coupled mode are analyzed. It is shown that the Rabi-like splitting can be achieved by the coupling between localized and delocalized graphene surface plasmon polaritons. Tunable coupling strength and positions with different gate-voltages have been discussed. The effect of relaxation time and oblique incidences to resonant responses are also investigated. Additionally, we find an optical analogue of a spring, where the spectral dip vibrates around its equilibrium position at a certain wavelength. Our study suggests that the proposed structure is potentially attractive for realization of tunable double-channel filter, optical switch, and variable optical attenuator based on the graphene metasurface.

Light Source of Multilayer Graphene to Control Interactions Between Localized an Delocalized Surface Plasmon Polaritons in the THZ Band

Light Source of Multilayer Graphene to Control Interactions Between Localized an Delocalized Surface Plasmon Polaritons in the THZ Band

All-plasmonic Optical Phased Array Integrated on a Thin-film Platform

Optical phased arrays have been demonstrated to enable a variety of applications ranging from high-speed on-chip communications to vertical surface emitting lasers. Despite the prosperities of the researches on optical phased arrays, presently, the reported designs of optical phased arrays are based on silicon photonics while plasmonic-based optical phased arrays have not been demonstrated yet. In this paper, a passive plasmonic optical phased array is proposed and experimentally demonstrated. The beam of the proposed plasmonic optical phased array is steerable in the far-field area and a high directivity can be achieved. In addition, radio frequency phased array theory is demonstrated to be applicable to the description of the coupling conditions of the delocalized surface plasmons in optical phased arrays and thus the gap between the phased arrays at two distinctly different wavelengths can be bridged. The potential applications of the proposed plasmonic phased arrays include on-chip optical wireless nanolinks, optical interconnections and integrated plasmonic lasers.

Broadband plasmonic metamaterial absorber with fish-scale structure at visible frequencies

We propose an ultrathin fish-scale metamaterial structure for broadband absorption with an averaged value of higher than 90% over the entire visible spectrum (400-700 nm) under normal incidence. The performance preserves under a wide range of incident angle from 0° to 60°. Light absorption of such a plasmonic metamaterial absorber (PMA) relies on the localized and delocalized surface plasmon resonances of the cascaded fish-scale meta-atoms in the structure. The PMA presents excellent structural tunability for absorption band. This study with rich finding is helpful for light harvesting with metamaterial strategy.

Manipulating the interaction between localized and delocalized surface plasmon-polaritons in graphene

The excitation of localized or delocalized surface plasmon polaritons in nanostructured or extended graphene has attracted a steadily increasing attention due to their promising applications in sensors, switches, and filters. These single resonances may couple and intriguing spectral signatures can be achieved by exploiting the entailing hybridization. Whereas thus far only the coupling between localized or delocalized surface plasmon polaritons has been studied in graphene nanostructures, we consider here the interaction between a localized and a delocalized surface plasmon polariton. This interaction can be achieved by two different schemes that reside on either evanescent near- field coupling or far-field interference. All observable phenomena are corroborated by analytical considerations, providing insight into the physics and paving the way for compact and tunable optical components at infrared and terahertz frequencies.

Two-Dimensional Array of Silica Particles as a SERS Substrate

We present the observation of size-dependent surface-enhanced Raman spectra (SERS) for crystal violet (CV) adsorbed on a monolayer of hexagonally close-packed silica spheres. Enhancement factors on the order of 104 are estimated based on the ratio of the number of CV molecules excited in solution to those on the surface of the monolayer array. The proposed enhancement mechanism is that of delocalized surface plasmon resonance over the monolayer array. Plasmon resonances of silica spheres are calculated using Mie theory for light scattering and experimentally validated with UV–vis absorption spectra. There is an excellent agreement between the observed spectrum from the monolayer arrays and a single particle calculated spectrum. This work points the way to utilize silica particles as a viable alternative to current SERS substrates as well as illustrates the importance of surface plasmon resonances from photonic crystals in the SERS of semiconductors.

Enhanced Sensitivity of Delocalized Plasmonic Nanostructures

This work reports on the observation of a delocalized surface plasmon resonance (DSPR) phenomenon in linear chains of square-shaped silver nanoparticles (NP) as a function of the chain length and the distance between the nanoparticles in the chain. Transmission spectra of the silver nanoparticle chains reveal the emergence of new, red-shifted extinction peaks that depend strongly on the spacing between the nanoparticles and the polarization of the exciting light with respect to the chain axis. As the spacing between the nanoparticles in the linear chain decreases and the number of nanoparticles in the linear chain increases, the strength of the new extinction features increase strongly. These changes can be described by a tight-binding model for the coupled chain, which indicates that the origin of the phenomenon is consistent with an increased coupling between the nanoparticles. FDTD calculations reveal that the electric field is strongly enhanced between the nanoparticles in the chain. The DSPR response is found to be much more sensitive to dielectric changes than the localized surface plasmon resonance (LSPR).

Generation of Multiple Plasmon Resonances in a Nanochannel

In this paper, we investigate the coupling of surface plasmon based on Kretschmann configuration (KC) to localized and delocalized surface plasmon modes supported by metallic nanowire grating (MG) at the oblique incidence. The designed structure consists of MG deposited on a substrate positioned upside down above a conventional KC at a certain distance. Mode coupling and resonant interactions between KC and MG create two new hybrid plasmon modes with narrow bandwidths and unique spectra features, resulting in two obvious dips in the reflection spectrum. Although both dips respond nonlinearly to the refractive index (RI) of the dielectric layer, surprisingly, the sum of two resonance wavelengths has a good linear approximation in a wider RI range. The simultaneous measurement of two resonance dips makes it more attractive as a surface plasmon resonance analysis technology, which is valuable for high-sensitivity optical sensors.

Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.

Enhancing four-wave-mixing processes by nanowire arrays coupled to a gold film

We consider the process of four-wave mixing in an array of gold nanowires strongly coupled to a gold film. Using full-wave simulations, we perform a quantitative comparison of the four-wave mixing efficiency associated with a bare film and films with nanowire arrays. We find that the strongly localized surface plasmon resonances of the coupled nanowires provide an additional local field enhancement that, along with the delocalized surface plasmon of the film, produces an overall four-wave mixing efficiency enhancement of up to six orders of magnitude over that of the bare film. The enhancement occurs over a wide range of excitation angles. The film-coupled nanowire array is easily amenable to nanofabrication, and could find application as an ultra-compact component for integrated photonic and quantum optic systems.

Planar-localized surface plasmon resonance device by block-copolymer and nanoimprint lithography fabrication methods

The authors report on the integration of delocalized surface plasmon resonances (SPRs) and localized surface plasmon resonances (LSPRs) on a single device. The submicron SPR device was fabricated with nanoimprint lithography (NIL). Gold nanoparticles for LSPR generation were created and deposited via three methods and analyzed with rhodamine 6 G and surface-enhanced Raman spectroscopy (SERS). Compared to drop-cast and thin film annealing methods, gold nanoparticles fabricated from a diblock-copolymer NIL template produced the most significant effect on the charge-transfer component of the SERS enhancement mechanism due to near-field interactions at the 10 nm inter-particle separation region. The authors also report a 26% enhancement of optical resonance with an integrated SPR-LSPR plasmonic device consisting of a two-dimensional submicron aluminum grating fully coupled with gold nanoparticles measuring 20.4 nm in diameter in a water medium. If the 2D aluminum grating were coupled to an optimized nanoparticle SERS device fabricated from a DBCP NIL template, the coupled nanoparticle-grating device could exhibit an even higher enhancement and optical resonance performance.

Plasmonic Coherent Drive of an Optical Trap

We demonstrate that optical trapping can be driven by delocalized surface plasmon modes resonantly excited within a standing wave trap. Dynamical modifications are shown to be determined by the near-field symmetry of the plasmonic modes with negligible thermal effect. With low trapping powers and polarization control, remarkable stiffness enhancements are recorded, the larger the smaller the particle. The results can be simply modeled accounting for a coherent interaction between the plasmon field and the Gaussian standing wave of the trap.