Periodic Nanoparticle Arrays Research Articles

Charge transfer plasmons in nanoparticle arrays on graphene: Theoretical development

The properties of charge transfer plasmons (CTPs) in periodic metallic nanoparticle arrays (PMNPAs) on the single-layer graphene surface are studied within a computationally efficient original hybrid quantum-classical model. The model is based on the proven assumption that the carrier charge density in doped graphene remains unchanged under plasmon oscillations. Calculated CTP frequencies for two PMNPA geometries are shown to lie within the THz range and to be factorized, i.e., presented as a product of two independent factors determined by the graphene charge density and the PMNPA geometry. Equations are derived for describing the CTP frequencies and eigenvectors, i.e., oscillating nanoparticle charge values. It is shown that the CTP plasmons having a band structure containing a wave vector and a band number, like to phonons in periodic media, can be divided into an acoustic mode and optical CTP modes. For the acoustic modes, the CTP group velocity tends to zero at k→0, but reaches a value of ∼VFermi in graphene inside the Brillouin zone, while for the optical modes, the group velocity dispersion is extremely weak, although their energy is higher than the acoustic plasmon energies. It is shown that the calculated dependence of CTP frequencies on the carrier concentration in graphene is in good agreement with experimental data. We believe that the proposed model can help in designing various graphene-based terahertz nanoplasmonic devices of complex geometry due to very high computational efficiency.

Hybrid Plasmonic Modes for Enhanced Refractive Index Sensing

AbstractCompared to single nanoparticles, strongly coupled plasmonic nanoparticles provide attractive advantages owing to their ability to exhibit multiple resonances with unique spectral features and higher local field intensity. These enhanced plasmonic properties of coupled metal nanoparticles have been used for various applications including realization of strong light‐matter interaction, photocatalysis, and sensing. In this article, the basic physics of hybrid plasmonic modes in coupled metallic nanodimers is reviewed and their potentials for refractive index sensing are assessed. In particular, the spectral line shapes of various modes of hybrid plasmons including bonding and antibonding modes in symmetric nanodimers, Fano resonances in asymmetric nanodimers, charge transfer plasmons in linked nanoparticle dimers, hybrid plasmon modes in nanoshells, gap modes in particle‐on‐mirror configurations, and hybrid magnetoplasmonic modes in heterodimers are overviewed. Beyond the dimeric nanosystems, the potentials of surface lattice resonances in periodic nanoparticle arrays for sensing applications are also showcased. Finally, based on the critical assessment of the recent research on coupled plasmonic modes, the outlook on the future prospects of hybrid plasmon‐based refractometric sensing are discussed. Given their tunable resonances and ultranarrow spectral features, coupled metal nanoparticles are expected to play key roles in developing precise plasmonic nanodevices with extreme sensitivity.

Silver Nanoantenna Stickers for Photoluminescence Control

Nanoantenna stickers, in which the periodic array of nanoparticles is embedded in an elastomer film, can be easily attached to and detached from the surface of target materials and work as nanoantennas to control light. The stickers can be fabricated by nanoimprint lithography followed by a transfer process. While aluminum nanoantenna stickers have been prepared with poly(vinylpyrrolidone) (PVP) as the sacrificial layer that is dissolved in water during the transfer process, it has been difficult to transfer the silver nanoantenna because of the weak adhesion between silver and PVP. In this work, upon placement of a thin aluminum layer between silver and PVP, a silver nanoantenna is successfully transferred into an elastomer film. We demonstrate a photoluminescence outcoupling by placing a sticker on the luminous layer. The emission pattern is modulated both spatially and spectrally in a way defined by the nanoantenna sticker. The ability to enhance photoluminescence in the forward direction is stronger than that of the aluminum nanoantenna sticker with an identical geometry.

Laser-driven self-organized evolution of 1D- and 2D-Nanostructures from metal thin-films on silicon: Influence of alloying and oxidation

One- and two-dimensional laser-induced periodic surface structure (LIPSS) formation of various metal (Ag, Cu, Fe, Zn, Ti) thin films on silicon wafers is achieved by irradiation with circularly polarized 532 nm nanosecond laser pulses. The occurrence and the evolution of pseudo-1D linear periodic, 2D honeycomb-like structures, and 2D periodic nanoparticle arrays were mapped for each element in dependence on the energy applied, i.e. the effective pulse number, and the metal thin film thickness. Samples series with increasing pulse counts monitor, like snapshots, the progressive pattern evolution. Because the metal thin films are of nanometer thickness only, the co-structuring of the silicon substrate must be taken into account in all cases. Even more complex are LIPSS formations on samples carrying two stacked thin films. The LIPSS formation process of thin films becomes rather complex as soon as co-structuring of the substrate, oxidation of the material, and alloy formation with substrate material or a potential second thin film layer are considered. All these effects affect the morphology of the formed LIPSS patterns.

Generalization of Self-Assembly Toward Differently Shaped Colloidal Nanoparticles for Plasmonic Superlattices.

Periodic superlattices of noble metal nanoparticles have demonstrated superior plasmonic properties compared to randomly distributed plasmonic arrangements due to near-field coupling and constructive far-field interference. Here, a chemically driven, templated self-assembly process of colloidal gold nanoparticles is investigated and optimized, and the technology is extended toward a generalized assembly process for variously shaped particles, such as spheres, rods, and triangles. The process yields periodic superlattices of homogenous nanoparticle clusters on a centimeter scale. Electromagnetically simulated absorption spectra and corresponding experimental extinction measurements demonstrate excellent agreement in the far-field for all particle types and different lattice periods. The electromagnetic simulations reveal the specific nano-cluster near-field behavior, predicting the experimental findings provided by surface-enhanced Raman scatteringmeasurements. It turns out that periodic arrays of spherical nanoparticles produce higher surface-enhancedRamanscatteringenhancement factors than particles with less symmetry as a result of very well-defined stronghotspots.

Surface lattice resonance effect of double-ring array of metallic nano-particles

Surface lattice resonances due to regular periodic array of metallic nanoparticles can be attributed to the mutual coupling between the localized surface plasmon resonances of different nanoparticles. A comparison of resonant effect between the single particle and the array shows that the resonance line width can be significantly reduced. In this paper, we extend the coupled dipole approximation to solving the electromagnetic characteristics of the particle ring structures with rotational symmetry, and propose an analytical model for the double ring array of metallic nano-particles. Furthermore, we derive the general resonant condition of the double ring array and investigate some concrete cases in detail. It shows that the full resonance of the whole array depends crucially on the structural parameters, whose enhancement factor can be extremely high. But a slight change in the structural parameter willlead the enhancement factor to decrease sharply. We also find that the radiation field of the full resonance effect will be independent of the external field, which provides us a simple approach to producing a localized optical field with complex space distribution. This proposed structure can possess potential applications in various fields such as metasurface, optoelectronics, optical manipulation, communication, and biosensing.

Lattice Resonances for Thermoplasmonics

Thanks to their ability to support localized surface plasmons, metallic nanostructures have emerged as ideal tools to transduce light into heat at the nanoscale, giving birth to the field of thermoplasmonics. When arranged in a periodic array, the localized plasmons of metallic nanostructures can interact coherently to generate a collective mode known as a lattice resonance. This collective mode, whose wavelength is controlled by the periodicity of the array, produces a stronger and more spectrally narrow optical response than that of the localized plasmons supported by the individual nanostructures. Motivated by the exceptional properties of the lattice resonances of periodic arrays of metallic nanoparticles, here, we investigate their use for applications in thermoplasmonics. Through a comprehensive analysis based on a coupled dipole model, we show that arrays supporting a lattice resonance absorb more energy per nanoparticle, and thus achieve a much larger increase in temperature under pulsed illumination conditions, than those that do not support such a mode. On the contrary, for continuous wave illumination conditions, we find that the temperature increase is mostly independent of the array period for the systems under consideration. Furthermore, by analyzing arrays with two nanoparticles per unit cell, we show that it is possible to engineer their lattice resonances to selectively absorb light in one of the nanoparticles without exciting the other. The results of this work pave the way for the development of thermoplasmonics applications exploiting the exceptional optical response and tunability provided by lattice resonances.

Growth of Highly-Ordered Metal Nanoparticle Arrays in the Dimpled Pores of an Anodic Aluminum Oxide Template.

A reliable, scalable, and inexpensive technology for the fabrication of ordered arrays of metal nanoparticles with large areal coverage on various substrates is presented. The nanoparticle arrays were formed on aluminum substrates using a two-step anodization process. By varying the anodization potential, the pore diameter, inter-pore spacing, and pore ordering in the anodic aluminum oxide (AAO) template were tuned. Following a chemical etch, the height of the pores in the AAO membrane were reduced to create a dimpled membrane surface. Periodic arrays of metal nanoparticles were subsequently created by evaporating metal on to the dimpled surface, allowing for individual nanoparticles to form within the dimples by a solid state de-wetting process induced by annealing. The ordered nanoparticle array could then be transferred to a substrate of choice using a polymer lift-off method. Following optimization of the experimental parameters, it was possible to obtain cm2 coverage of metal nanoparticles, like gold and indium, on silicon, quartz and sapphire substrates, with average sizes in the range of 50-90 nm. The de-wetting process was investigated for a specific geometry of the dimpled surface and the results explained for two different film thicknesses. Using a simple model, the experimental results were interpreted and supported by numerical estimations.

Evolutionary optimization of the short-circuit current enhancement in organic solar cells by nanostructured electrodes

Using a particle swarm optimization algorithm (a population-based stochastic optimization technique) combined with 3D finite-difference time-domain simulations, we inverse design periodic arrays of metallic nanoparticles on indium-tin-oxide electrodes and nanoholes in metallic thin films working as electrodes in P3HT (Poly(3-hexylthiophene-2,5-diyl)):PCBM ([6,6]-Phenyl C61 butyric acid methyl ester) organic solar cells to achieve the maximum short-circuit currents (Jsc). Nanohole-array electrodes have large optical losses, leading to a net reduction of Jsc compared to a reference solar cell. On the other hand, nanoparticle arrays can lead to a significant enhancement of Jsc of up to 20%. Detailed simulations show that this enhancement is caused by the grating coupling of the incident light to surface plasmon polaritons at the interface of the metal electrode and the hole transport layer, leading to the enhancement of the electromagnetic field in the organic blend.

Inverse designed plasmonic metasurface with parts per billion optical hydrogen detection

Plasmonic sensors rely on optical resonances in metal nanoparticles and are typically limited by their broad spectral features. This constraint is particularly taxing for optical hydrogen sensors, in which hydrogen is absorbed inside optically-lossy Pd nanostructures and for which state-of-the-art detection limits are only at the low parts-per-million (ppm) range. Here, we overcome this limitation by inversely designing a plasmonic metasurface based on a periodic array of Pd nanoparticles. Guided by a particle swarm optimization algorithm, we numerically identify and experimentally demonstrate a sensor with an optimal balance between a narrow spectral linewidth and a large field enhancement inside the nanoparticles, enabling a measured hydrogen detection limit of 250 parts-per-billion (ppb). Our work significantly improves current plasmonic hydrogen sensor capabilities and, in a broader context, highlights the power of inverse design of plasmonic metasurfaces for ultrasensitive optical (gas) detection.

Engineering Bound States in the Continuum at Telecom Wavelengths with Non‐Bravais Lattices

AbstractVarious optical phenomena can be induced in periodic arrays of nanoparticles by the radiative coupling of the local dipoles in each particle. Probably the most impressive example is bound states in the continuum (BICs), which are electromagnetic modes with a dispersion inside the light cone but infinite lifetime, that is, modes that cannot leak to the continuum. Symmetry‐protected BICs appear at highly symmetric points in the dispersion of periodic systems. Although the addition of nonequivalent lattice points in a unit cell is an easy and straightforward way of tuning the symmetry, BICs in such particle lattice, that is, non‐Bravais lattice, are less explored among periodic systems. Starting from a periodic square lattice of Si nanodisks, three non‐Bravais lattices are prepared by detuning size and position of the second disk in the unit cell. Diffraction‐induced coupling excites magnetic/electric dipoles in each nanodisk, producing two surface lattice resonances at the Γ point with a band gap in between. The high/low energy branch becomes a BIC for the size/position‐detuned array, respectively, while both branches are bright (or leaky) when both size and position are detuned simultaneously. The unexplored role of the interplay between magnetic and electric dipoles in dielectric nanoparticles in connection with the change of BIC to bright character in the detuned arrays is discussed with the aid of a coupled electric and magnetic dipole model. This study gives a simple way of tuning BICs at telecom wavelengths in non‐Bravais lattices, including both plasmonic and dielectric systems, thus scalable to a wide range of frequencies.

Nanoparticle-on-Mirror Metamaterials for Full-Spectrum Selective Solar Energy Harvesting.

Most broadband metamaterial absorbers are realized by patterning periodic arrays of plasmonic nanoparticles (>100 nm) on dielectric/metallic substrates to enable both electric and magnetic resonances. These metamaterials, however, require costly nanolithographic top-down techniques for fabrication. Here, we demonstrate new-concept nanoparticle-on-mirror (NoM) metamaterial absorbers by densely packing plasmonic nanoparticles of much smaller size (∼30 nm) on metal films directly. Such a simple but rational design enables the use of all-solution-based bottom-up processes. Because of the decoupling of electric and magnetic polarizations in these ultrasmall nanoparticles, excellent impedance match and near-perfect light absorption can be achieved in a broad band over the solar spectrum with weak thermal emission. Proof-of-concept large-area NoM metamaterial absorbers that offer a solar absorptance of 94% but a low IR emittance of 2% are experimentally demonstrated. The outstanding performance, bottom-up process, and great compatibility render the design promising for efficient and large-scale solar energy harvesting.

Applicability of multipole decomposition to plasmonic- and dielectric-lattice resonances.

Periodic nanoparticle arrays have attracted considerable interest recently since the lattice effect can lead to spectrally narrow resonances and tune the resonance position in a broad range. Multipole decomposition is widely used to analyze the role of the multipoles in the resonance excitations, radiation, and scattering of electromagnetic waves. However, previous studies have not addressed the validity and accuracy of the multipole decomposition around the lattice resonance. The applicability of the exact multipole decomposition based on spherical harmonics expansion has not been demonstrated around the lattice resonance with the strong multipole coupling. This work studies the two-dimensional periodic arrays of both plasmonic and dielectric nanospheres and compares the multipole decomposition results with the analytic ones around their lattice resonances. We study both the effective polarizabilities of multipoles and the scattering spectra of the structures. The analytical results are calculated from the coupled dipole-quadrupole model. This study demonstrates that the exact multipole decomposition agrees well with the numerical simulation around lattice resonances. Only a small number of multipoles are required to represent the results accurately.

Design and simulation of high figure of merit plasmonic resonance sensor

Periodic metallic nanoparticle arrays have a wide range of applications in sensors due to the generation of the plasmonic lattice resonance (PLR). To increase the figure of merit (FOM) of periodic metallic nanoparticle arrays, schemes to reduce full-width at half maximum (FWHM) are necessary. Introducing silicon and dielectric nanopillars under metallic nanoparticles has been suggested as a feasible route to reducing FWHM and thus enhancing FOM. However, many factors may lead to a reduction in FOM. Thus, different factors, such as the diameter and height of metallic nanoparticles, the diameter and height of silicon nanopillars, the height of dielectric nanopillars, and the period of the arrays, should be explored to improve FOM of this structure. We investigate the influences of different structural parameters on FOM of Ag / Si / SiO2 nanopillar array sensors. Results show that a larger diameter of the Ag nanopillars widens the FWHM and weakens the FOM. The optimal diameter of Si nanopillars and the optimal height of Ag nanopillars to obtain a high FOM are all 100 nm. The period of larger arrays corresponds to a narrower FWHM and higher FOM. A larger height of Si nanopillars or a larger height of SiO2 nanopillars decreases the FWHM and increases the FOM. These results are relatively instructive and meaningful for the design of PLR sensors with a high FOM.

Anomalous Absorption in Arrays of Metallic Nanoparticles: A Powerful Tool for Quantum Dot Optoelectronics.

Periodic arrays of noble metal nanoparticles are emblematic nanostructures in photonics. Their ability to sustain localized surface plasmon resonances has been used throughout the years to demonstrate a variety of passive and active functionalities such as enhanced luminescence in dipolar media and LEDs as well as higher responsivities in photoconductive detectors. Here, we show that additional magnetic resonances, associated with inductive current loops between the nanoparticles and accessible with transverse electric waves, emerge in the limit of dense arrays with subwavelength periods. Moreover, their interplay with the plasmons of the system results in spectrally sharp analogues of electromagnetically induced absorption (EIA). We use these metasurfaces to induce changes and enhancements in the emission, absorption, photoconduction, and polarization properties of active layers of PbS nanocrystals, illustrating the potential of EIA beyond the passive functionalities demonstrated so far in literature.

Multiply-resonant second-harmonic generation using surface lattice resonances in aluminum metasurfaces.

Nonlinear metamaterials show potential for realizing flat nonlinear optical devices but are generally lacking in terms of achievable conversion efficiencies. Recent work has focused on enhancing nonlinear processes by utilizing high quality factor resonances, such as collective responses known as surface lattice resonances (SLRs) taking place in periodic metal nanoparticle arrays. Here, we investigate how the dispersive nature of SLRs affects the nonlinear responses of SLR-supporting metasurfaces. Particularly, we measure second-harmonic generation from aluminum nanoparticle arrays and demonstrate that by tilting the sample along two orthogonal directions, the sample can be made multiply-resonant for several pump and second-harmonic signal wavelength combinations. Characterized metasurfaces are estimated to exhibit a second-order susceptibility value of 0.40 pm/V, demonstrating aluminum as a potential material for nonlinear metasurfaces.

Localized surface plasmon and transferred electron enhanced UV emission of ZnO by periodical aluminum nanoparticle arrays

Periodical Al nanoparticle (NP) arrays were applied to enhance the ultraviolet (UV) light emission of ZnO by localized surface plasmon resonance (LSPR) effect and electron transfer mechanism. By applying corresponding anodic aluminum oxide (AAO) templates, periodical Al NP arrays with different diameters (D) and spaces (S) were manufactured by magnetron sputtering. Theoretical simulation by FDTD-solution indicates that local electric field around Al NPs is significantly enhanced, and the surface plasmon resonance wavelength is determined by D and S. Al NP arrays with D of 70 nm and S of 125 nm enhanced near band-edge emission of ZnO by 2.4 times. By adding Al NP arrays, the photonic lifetime of ZnO decreased by 0.09 ns according to time-resolved photoluminescence spectra, indicating that the enhancement in UV luminescence intensity of ZnO is ascribed to the coupling between surface plasmons of Al NPs and ZnO excitons. Deep-level emission spectra analysis indicates that there lies another mechanism of electron transfer from the Al NPs to surface plasmon resonance (SPR) level and then further to ZnO conduction band.

Green Tensor Analysis of Lattice Resonances in Periodic Arrays of Nanoparticles

When arranged in a periodic geometry, arrays of metallic nanostructures are capable of supporting collective modes known as lattice resonances. These modes, which originate from the coherent multiple scattering between the elements of the array, give rise to very strong and spectrally narrow optical responses. Here, we show that, thanks to their collective nature, the lattice resonances of a periodic array of metallic nanoparticles can mediate an efficient long-range coupling between dipole emitters placed near the array. Specifically, using a coupled dipole approach, we calculate the Green tensor of the array connecting two points and analyze its spectral and spatial characteristics. This quantity represents the electromagnetic field produced by the array at a given position when excited by a unit dipole emitter located at another one. We find that, when a lattice resonance is excited, the Green tensor is significantly larger and decays more slowly with distance than the Green tensor of vacuum. Therefore, in addition to advancing the fundamental understanding of lattice resonances, our results show that periodic arrays of nanostructures are capable of enhancing the long-range coupling between collections of dipole emitters, which makes them a promising platform for applications such as nanoscale energy transfer and quantum information processing.

The Light Absorption Enhancement in Graphene Monolayer Resulting from the Diffraction Coupling of Surface Plasmon Polariton Resonance.

In this study, we investigate a physical mechanism to improve the light absorption efficiency of graphene monolayer from the universal value of 2.3% to about 30% in the visible and near-infrared wavelength range. The physical mechanism is based on the diffraction coupling of surface plasmon polariton resonances in the periodic array of metal nanoparticles. Through the physical mechanism, the electric fields on the surface of graphene monolayer are considerably enhanced. Therefore, the light absorption efficiency of graphene monolayer is greatly improved. To further confirm the physical mechanism, we use an interaction model of double oscillators to explain the positions of the absorption peaks for different array periods. Furthermore, we discuss in detail the emerging conditions of the diffraction coupling of surface plasmon polariton resonances. The results will be beneficial for the design of graphene-based photoelectric devices.

Photoluminescence coupled to electric and magnetic surface lattice resonance in periodic arrays of zirconia nanoparticles

Zirconia is transparent from near-ultraviolet through the entire visible region, and here we utilize the periodic arrays of zirconia nanoparticle to excite electric and magnetic SLRs in the visible.