Localized Plasmon Modes Research Articles

Broadband Omnidirectional Nearly Perfect Plasmonic Absorber For Solar Energy Harvesting

In this paper, an efficient, broadband, omnidirectional visible plasmonic absorber is presented and numerically simulated using the rigorous three-dimensional finite difference time domain (FDTD) method and the 2-D finite element method. The proposed absorber comprises hollow cylindrical layers of aluminum (Al) and silicon dioxide ( ${\rm{SiO}}_{{2}}$ ). Arranging the geometry and adjusting the dimensions of the cylindrical layers generate localized plasmonic modes at the ${\rm{Al/ SiO}}_{{2}}$ interfaces, as well as inside the gap between two Al layers, and thereby, strong optical confinement in the visible range is allowed. Therefore, the light absorbance of over 93% is observed over the whole visible regime with a relative bandwidth from 0.4 to 0.75 PHz. Further, due to the cylindrical geometry, the absorption is almost independent on the incident angles in a wide range (–90° to 90°). Two elements of the proposed absorber have been employed to function as a nanoantenna for converting the solar energy to electricity. The proposed nanoantenna offers omnidirectional harvesting characteristics with efficient harvesting efficiency that is higher than that of the conventional rectangular dipole nanoantenna by about 38%.

Wide-angled off-axis achromatic metasurfaces for visible light.

We present an approach to build multiwavelength achromatic metasurface that can work in off-axis configuration with an ultra-wide applicable incident angle range for visible light. The metasurface is constructed by combining multiple metallic nano-groove gratings, which support enhanced diffractions for transverse magnetic polarization in an ultrawide incident angle range from 10° to 80° due to the excitations of localized gap plasmon modes at different resonance wavelengths. To achieve the achromatic diffraction, the ratio between the resonance wavelength and the period of each elementary grating is fixed. Incident light at those multiple resonance wavelengths can be efficiently diffracted into the same direction with near-complete suppression of the specular reflection. Based on the similar approach, we also design a wide-angled off-axis achromatic flat lens for focusing light of different wavelengths into the same position. Our findings provide an alternative simple way to design various off-axis achromatic flat optical elements without stringent angle requirement for imaging and display applications.

Effects of edge on graphene plasmons as revealed by infrared nanoimaging

We used scattering-type scanning near-field optical microscopy (s-SNOM) to investigate the plasmonic properties of edges in well-defined graphene nanostructures, including sharp tapers, nanoribbons and nanogaps, which were all fabricated via the growth-etching chemical vapor deposition (GECVD) method. The obtained near-field images revealed the localized plasmon modes along the graphene nanoribbon; these modes strongly depended on the size of the graphene pattern, the angle of the tapered graphene and the infrared excitation wavelength. These interesting plasmon modes were verified by numerical simulations and explained by the reflection, and interference of electromagnetic waves at the graphene–SiO2 edge. The constructive interference at the graphene nanogap caused by charge accumulation was demonstrated for the first time. Using the infrared nanoimaging technique, greater plasmon broadening was observed in the zigzag edge than in the armchair edge. Our study suggests that graphene edges should be separated by an effective working distance to avoid the overlapping of localized plasmon modes, which is very important for the design of graphene-based plasmonic circuits and devices.

Symmetry breaking induced excitations of dark plasmonic modes in multilayer graphene ribbons.

Multilayer graphene can support multiple plasmon bands. If structured into graphene ribbons, they can support multiple localized plasmonic modes with interesting optical properties. However, not all such plasmonic modes can be excited directly due to the constrains of the structural symmetry. We show by numerical simulations that by breaking the symmetry all plasmonic modes can be excited. We discuss the general principles and properties of two-layer graphene ribbons and then extend to multilayer graphene ribbons. In multilayer graphene ribbons with different ribbon widths, a tunable broadband absorption can be attained due to the excitations of all plasmonic modes. Our results suggest that these symmetry-broken multilayer graphene ribbons could offer more degrees of freedom in designing photonic devices.

Morphological and optical properties of ultra‐thin nanostructured Cu films deposited by RF sputtering on nanoporous anodic alumina substrate

Nanoporous materials have attracted considerable technological interest due to their wide range of applications. In this study, highly ordered nanoporous anodic alumina substrates (NAAS) were fabricated by two-steps anodisation. Radio-frequency (RF) magnetron sputtering was used to deposit ultrathin Cu layers of different morphologies on the top surface of NAAS. From the field emission-scanning electron microscope images, the Cu/NAAS morphologies were tuned from nanoporous ultra-thin Cu layer, to nanoporous-grains of Cu aggregates, then to a continues layer of Cu nanostones as the deposition time increased from 1 to 4 min. The reflection spectra of the Cu/NAAS samples were dominated with localised surface plasmon modes; longitudinal, transverse and higher-order modes. The surface plasmon resonances of the fundamental and higher-order longitudinal modes were shifted to longer wavelengths as the deposition time increased. In addition, the transverse surface plasmon resonances were shifted to longer wavelengths for deposition time ≤3 min (nanoporous Cu film) and suddenly shifted to shorter wavelengths for deposition time ≥4 min (continuous film). The correlations between the structural parameters and the shift of surface plasmon resonance modes were discussed. The strong red-shifted of the surface plasmon modes to the NIR region suggests a unique opportunity for the design of biomedical sensor based on the proposed structures.

Enhancing Light Emission From Germanium Quantum Dots by Bowtie Antennas

Light emitters based on germanium quantum dots (QDs) coupled with array of gold bowtie nanoantennas are fabricated and characterized on silicon-on-insulator substrate. Enhanced luminescence from germanium QDs is observed at room temperature with a maximum enhancement factor of 4.2 at 1577 nm. The light emission enhancement is due to the strong interaction between localized plasmonic mode of bowtie antennas and nearby germanium QDs. The theoretical analyses show that only a small amount of germanium QDs are efficiently enhanced by localized plasmonic mode. The maximum quantum efficiency enhancement of single germanium QD is estimated to be more than two-orders of magnitude.

Broadband absorption enhancement in plasmonic nanoshells-based ultrathin microcrystalline-Si solar cells.

With the objective to conceive a plasmonic solar cell with enhanced photocurrent, we investigate the role of plasmonic nanoshells, embedded within a ultrathin microcrystalline silicon solar cell, in enhancing broadband light trapping capability of the cell and, at the same time, to reduce the parasitic loss. The thickness of the considered microcrystalline silicon (μc-Si) layer is only ~1/6 of conventional μc-Si based solar cells while the plasmonic nanoshells are formed by a combination of silica and gold, respectively core and shell. We analyze the cell optical response by varying both the geometrical and optical parameters of the overall device. In particular, the nanoshells core radius and metal thickness, the periodicity, the incident angle of the solar radiation and its wavelength are varied in the widest meaningful ranges. We further explain the reason for the absorption enhancement by calculating the electric field distribution associated to resonances of the device. We argue that both Fabry-Pérot-like and localized plasmon modes play an important role in this regard.

Energy equipartition and unidirectional emission in a spaser nanolaser

AbstractA spaser is a nanoplasmonic counterpart of a laser, with photons replaced by surface plasmon polaritons and a resonant cavity replaced by a metallic nanostructure supporting localized plasmonic modes. By combining analytical results and first‐principle numerical simulations, we provide a comprehensive study of the ultrafast dynamics of a spaser. Due to its highly‐nonlinear nature, the spaser is characterized by a large number of interacting degrees of freedom, which sustain a rich manifold of different phases we discover, describe and analyze here. In the regime of strong interaction, the system manifests an irreversible ergodic evolution towards the configuration where energy is equally shared among all the available degrees of freedom. Under this condition, the spaser generates ultrafast vortex‐like lasing modes that are spinning on the femtosecond scale and whose direction of rotation is dictated by quantum noise. In this regime, the spaser acquires the character of a nanoparticle with an effective spin. This opens up a range of interesting possibilities for achieving unidirectional emission from a symmetric nanostructure, stimulating a broad range of applications for nanoplasmonic lasers as unidirectional couplers and random information sources. image

Tunable Localized Hybrid Plasmon Modes and Fano Resonances in Au Core-Semishell

We present a numerical study of the plasmonic properties of Au core-semishell. Symmetry breaking in semishell results in a dipole–quadrupole Fano resonance without the offset of the Au core. The plasmon hybridization theory is applied to interpret the origin of the localized hybrid plasmon modes and Fano resonances. The multiple hybrid plasmon modes resonance can be excited for the extreme core offset inducing high order dark modes visible. The Fano resonances can be tuned by changing the size of the Au core, the degree of symmetry breaking of the semishell, and the dielectric properties of local environment. The tunability of the plasmon resonances and Fano resonances in Au core-semishell provides potential application for tunable nanophotonic devices, such as filters and sensors.

Longitudinal Localized Surface Plasmons in Trimer Nanocylinder System

We report the near field localized surface plasmon characteristics of triangular system of silver nanocylinders and shell nanocylinders interacting with incident plane wave, studied using finite element method. The trimer nanocylinder system possesses far greater structural tunability than either a single nanorod or a nanoshell, along with much larger local field intensity enhancements and far greater sensitivity. The effect of geometrical as well as material parameters on the longitudinal localized plasmon resonance of the trimer nanosystem was investigated. The results provide insight in to the possible tunability of the localized plasmon modes which could find application in designing of chemical and biological sensors, electron emitters, etc.

Dispersion of polarization coupling, localized and collective plasmon modes in a metallic photonic crystal mapped by Mueller Matrix Ellipsometry.

We report a spectroscopic Mueller matrix experimental study of a plasmonic photonic crystal consisting of gold hemispheroidal particles (lateral radius 54 nm, height 25 nm) arranged on a square lattice (lattice constant 210 nm) and supported by a glass substrate. Strong polarization coupling is observed for ultraviolet wavelengths and around the surface plasmon resonance for which the off-block-diagonal Mueller matrix elements show pronounced anisotropies. Due to the Rayleigh anomalies, the block-diagonal Mueller matrix elements produce a direct image of the Brillouin Zone (BZ) boundaries of the lattice and resonances are observed at the M-point in the first and at the X-point in the second BZ. These elements show also the dispersion of the localized surface plasmon resonance.

Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes

The quantum plasmonics field has emerged and been growing increasingly, including study of single emitter-light coupling using plasmonic system and scalable quantum plasmonic circuit. This offers opportunity for the quantum control of light with compact device footprint. However, coupling of a single emitter to highly localized plasmonic mode with nanoscale precision remains an important challenge. Today, the spatial overlap between metallic structure and single emitter mostly relies either on chance or on advanced nanopositioning control. Here, we demonstrate deterministic coupling between three-dimensionally nanofocused plasmonic modes and single quantum dots (QDs) without any positioning for single QDs. By depositing a thin silver layer on a site-controlled pyramid QD wafer, three-dimensional plasmonic nanofocusing on each QD at the pyramid apex is geometrically achieved through the silver-coated pyramid facets. Enhancement of the QD spontaneous emission rate as high as 22 ± 16 is measured for all processed QDs emitting over ∼150-meV spectral range. This approach could apply to high fabrication yield on-chip devices for wide application fields, e.g., high-efficiency light-emitting devices and quantum information processing.

Coupling localized plasmonic and photonic modes tailors and boosts ultrafast light modulation by gold nanoparticles.

Plasmonic nanoparticles offer a broad range of functionalities, owing to their ability to amplify light in the near-field or convert it into heat. However, their ultrafast nonlinear optical response remains too low to envisage all-optical high-rate photonic processing applications. Here, we tackle this challenge by coupling the localized plasmon mode in gold nanoparticles with a localized photonic mode in a 1D resonant cavity. Despite the nonradiative losses, we demonstrate that a strong, reversible, and ultrafast optical modulation can be achieved. By using a light pumping fluence of less than 1 mJ cm(-2), a change of signal transmittance of more than 100% is generated within a few picosecond time scale. The nanoparticle transient optical response is enhanced by a factor of 30 to 40 while its spectral profile is strongly sharpened. The large nonlinear response of such plasmonic cavities could open new opportunities for ultrafast light processing at the nanoscale.

Broadband Spectral Signature of the Ultrafast Transient Optical Response of Gold Nanorods

The ultrafast transient optical response of gold nanorods presents a complex spectral signature that is very sensitive to the nanoparticle aspect ratio. This stems from the different electronic contributions to the photoinduced dynamics of the metal dielectric function, which modify the transverse and longitudinal localized plasmon modes. Here, we analyze the physical origins of the ultrafast optical response of ensembles of nanorods over the whole visible range. Using broadband time-resolved spectroscopy, we determine within the first picoseconds after pump excitation the transient response of colloidal solutions containing gold nanorods with different mean aspect ratios. Supported by model calculation, it is shown that the contribution of interband electron transitions dominates at ultrashort times, even for photon energies far below their threshold. At longer times, a slower intraband transition component linked with the nanoparticle heating appears. We then describe how the ensemble effect modifies th...

Tunable circular polarization selective surfaces for low-THz applications using patterned graphene.

This paper demonstrates a new type of frequency tunable polarization selective surface operating at low THz, which is devised by utilizing the unique features of graphene. The device is comprised of an infinite array of identical unit cells in three layers. Multiple graphene dipoles are placed on the top and bottom layers to form the vertical and horizontal electric field filters. Using this new configuration, the proposed device exhibits reflection for the incident Left-Hand-Circular-Polarization (LHCP) waves and becomes transparent to the incoming Right-Hand-Circular-Polarization (RHCP) waves. The excited localized surface plasmonic resonance mode on the graphene based unit cells significantly reduces the physical dimension of the device. The unit cell dimension of the proposed design is in the order of 0.18 wavelengths in comparison to conventional metallic structures, where it is of order a half a wavelength. In the full wave analysis, the graphene based polarization selective surfaces exhibit an isolation of 21 dB for LHCP waves and a transmission loss of around 5.1 dB for waves with RHCP characteristics. The performance has also been examined under oblique incidence. The results fully verify that the proposed planar device operates properly for incident angles up to 40°. The tuning effect of the described device is investigated by varying the chemical potentials of graphene. Significant frequency reconfiguration capability is achieved in the isolation of LHCP incident waves, and meanwhile, for RHCP incidence, the transmission rate remains reasonably high.

Fano-resonant aluminum and gold nanostructures created with a tunable, up-scalable process

An up-scalable approach for creating Fano-resonant nanostructures on large surfaces at visible wavelengths is demonstrated. The use of processes suitable for high throughput fabrication and the choice of aluminum as a cost-efficient plasmonic material ensure that the presented insights are valuable even in consideration of typical industrial constraints. In particular, wafer-scale fabrication and the process compatibility with roll-to-roll embossing are demonstrated. It is shown that through adjustment of readily accessible evaporation parameters, the shape and position of the optical resonance can be tuned within a spectral band of more than 70 nm. The experimental data are complemented with rigorous coupled wave analysis and surface integral equation simulations. Calculated electric fields as well as surface charges shed light onto the physics behind the present resonances. In particular, a surface plasmon polariton is found to couple to a localized plasmonic mode with a hexapolar charge distribution, leading to a Fano-like resonance. Further understanding of the interactions at hand is gained by considering both aluminum and gold nanostructures.

Strong coupling between localized and propagating surface plasmon modes in a noncentrosymmetric metallic photonic slab

We have investigated the coupling behaviors between localized and propagating surface plasmon modes in a noncentrosymmetric structure consisting of an L-shaped metal nanoparticle array and a thick metal film, separated by a silica dielectric spacer layer. It is found that surface plasmon modes exhibit hybrid behaviors due to the noncentrosymmetry of the structure. The hybrid surface plasmon modes will interact with different-order localized plasmon modes in the nanoparticle in their spectrally overlapping regions. The strong coupling between the localized and propagating plasmon modes gives rise to the energy anticrossing behavior with large mode splitting. Furthermore, a narrow absorption branch is also observed between two anticrossing absorption branches, which is absent in the centrosymmetric system. The findings hold promise in applications such as photonic and energy conversion systems and the design of novel plasmonic nanodevices.

Plasmonic Spectral Engineering via Interferometric Illumination of Colloid Sphere Monolayers

A novel method is presented for complex structure fabrication, which is capable of breaking the hexagonal symmetry of the conventional colloid sphere lithography via the interferometric illumination of colloid sphere monolayers (IICSM). It is demonstrated that the perfect lateral synchronization of a linear intensity modulation originating from two-beam interference with respect to a hexagonal colloid sphere monolayer makes it possible to tune four complex structure parameters independently. Based on comparative study of hexagonal and rectangular hole doublet-arrays, which can be generated by linearly polarized light via homogeneous illumination and via IICSM, it is shown that the novel IICSM method enables plasmonic spectral engineering with higher degrees of freedom. The unique spectral properties of the complex patterns attainable via IICSM are more precisely tunable by properly selected azimuthal orientation during illumination and by the surrounding medium. It is shown that coupling phenomena between propagating and localized plasmonic modes on appropriately designed complex structures result in unique charge and near-field distribution accompanied by narrow Fano lines. Optimal configurations of complex plasmonic structures consisting of a rectangular array of hole doublets with different geometrical size parameters are presented, which ensure enhanced sensitivity in bio-detection.

Tunable Optical Antenna Effect of Palladium-Based Dimeric Nanostructures

In photo-energy conversion, photon capture is one of critical issues to determine the overall efficiency. Since the photon density is rather low in sun light, it is indispensable to achieve highly efficient photon capture in the conversion system. Recently, plasmonic metal nanostructures are expected as optical antennas capable of capturing photons efficiently. When localized plasmon modes are excited on a metal nanostructure, photon energy is resonantly trapped on its surface. Indeed, we have recently reported that the efficiency of photocurrent generation on porphyrin-linked molecular monolayers was substantially enhanced by the use of the plasmonic antennas.1-4 Plasmon resonance features of metal nanostructures are characterized by their size and shape. Therefore, it is quite challenging to realize dynamical tuning of plasmon resonances by external stimuli. The external control of the optical antenna effect could lead to development of noble optical devices. Although there have been several reports on hydrogen -induced plasmon resonance variation in Pd nanostructures,5,6such dynamical effect is still small. In the present work, we demonstrate plasmon resonance tuning of Pd-based nanodimer array structures. Metal nanodimer arrays of various metal species were fabricated on a glass substrate using angle-resolved nanosphere lithography (AR-NSL); a self-assembled monolayer of polystyrene beads with diameter of 350 nm was formed on the substrate as a shadow mask. Then, thin films of Pd or other metals with thickness of 20 nm were formed through the shadow mask by double-angle evaporation with angles of 0 and 23 degrees. Finally, the bead monolayers were removed so that a number of dimers aligned uniaxially were obtained on the substrate. A typical AFM image of the nanodimer array obtained is shown in the right panel of Fig. 1. Transmission spectra of the dimer arrays were measured in a glass cell filled with N2 gas or H2-Ar mixed gas of a specific H2percentage, which was continuously provided from a gas cylinder. Thermal control of the cell was conducted using a transparent thermoplate. The hydrogen-induced variation of plasmon resonances was investigated in details by measuring in-situ transmission spectra of Pd-Pd homo-dimer arrays. As shown in the left panel of Fig. 2, the dimer array exhibited a broad hybridized plasmon band at around 600 nm. Under hydrogen gas flow, this peak decreased in intensity. The right panel of Fig. 2 shows the degree of the hydrogen-induced variations as functions of hydrogen concentration and temperature. One can clearly see that the hydrogen-induced variation become significantly larger at the specific conditions above 3 % H2concentration and below 30 C°. This region well agrees with that of palladium hydride (b-phase) formation in Pd-H phase diagram. That is, the formation of palladium-hydrogen solid solution (a-phase) does not change plamon resonances much. On the basis of the hydrogen-induced responses in the Pd-Pd homo-dimer arrays, we have designed various types of sp-metal/Pd hetero- and homo-dimer arrays to increase both of the intensity of plasmon resonance and the ratio of the hydrogen-induced change. We will also discuss a possibility of photo-induced change of the optical antenna effect.

Spaser Made of Graphene and Carbon Nanotubes

Spaser is a nanoscale source of surface plasmons comprising a plasmonic resonator and gain medium to replenish energy losses. Here we propose a carbon-based spaser design in which a graphene nanoflake (GNF) resonator is coupled to a carbon nanotube (CNT) gain element. We theoretically demonstrate that the optically excited CNT can nonradiatively transfer its energy to the localized plasmon modes of the GNF because of the near-field interaction between the modes and the CNT excitons. By calculating the localized fields of the plasmon modes and the matrix elements of the plasmon-exciton interaction, we find the optimal geometric and material parameters of the spaser that yield the highest plasmon generation rate. The results obtained may prove useful in designing robust and ultracompact coherent sources of surface plasmons for plasmonic nanocircuits.