Dipolar Localized Surface Plasmon Resonance Research Articles

Seedless synthesis of Au nanoplates with tunable plasmonic peaks

Au nanoplates with tunable in-plane dipolar localized surface plasmon resonance peaks in a broad range from the visible to near-infrared region were obtained in high yield using a seedless wet chemical growth method after purification. Cetyltrimethylammonium chloride was used as a surfactant, while hydrogen peroxide and sodium borohydride were used as the weak and strong reducing agents, respectively. The edge length and in-plane dipolar localized surface plasmon resonance peak of the Au nanoplates could be adjusted by varying the amounts of hydrogen peroxide and sodium borohydride. The Au nanoplates were further used as the saturable absorber to generate pulsed laser output in a passively Q-switched solid-state laser at approximately 2 µm. Our study offers a new method for obtaining Au nanoplates with tunable plasmonic peaks over a broad range.

Nanoscale mapping of shifts in dark plasmon modes in sub 10 nm aluminum nanoantennas

In this work, we report the fabrication and spectroscopic characterization of subwavelength aluminum nanocavities—consisting of hexamer or tetramer clusters of sub 10 nm width Al nanorods—with tunable localized surface plasmon resonance (LSPR) energies on suspended SiN x membranes. Here the volume plasmon (VP) and LSPR modes of lithographically-fabricated Al nanocavities are revealed by low-loss electron energy-loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM). We show that the existence of grain boundaries (GBs) in these nanocavities results in shifts in the VP energy and a reduction in the VP lifetime. We map the VP energy and lifetime across GBs and we observe a decrease in VP energy and lifetime at GBs that is consistent with a reduction in free carrier density and increased plasmon scattering at these locations. Dipolar LSPR modes resonant in the UV and blue regions of the electromagnetic spectrum as well as higher-energy optically dark quadrupolar and hexapolar LSPR modes are also observed and mapped by STEM and EELS. All LSPR modes are confirmed via electromagnetic simulations based on the boundary element method. Both tetramer and hexamer structures support the excitation of dipolar bright and dipolar dark modes. Finally, we find that asymmetries in fabricated nanorod hexamer and tetramer nanocavities result in a mode mixing leading to a shift in dipolar dark LSPR modes.

In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures.

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

Nonlinear Nanophotonic Circuitry: Tristable and Astable Multivibrators and Chaos Generator

AbstractThe concept of lumped optical nanoelements (or metactronics), wherein nanometer‐scale structures act as nanoinductors, nanocapacitors, and nanoresistors, has attracted a great deal of attention as a simple toolbox for engineering different nanophotonic devices in analogy with microelectronics. While recent studies of the topic have been predominantly focused on linear functionalities, nonlinear dynamics in microelectronic devices plays a crucial role and provides a majority of functions, employed in modern applications. Here, the metactronics paradigm is extended and nonlinear dynamical modalities are added to those nanophotonic devices that have never been associated with optical nanoantennas. Specifically, it is shown that nonlinear dimer nanoantennae can operate in the regimes of tristable and astable multivibrators as well as chaos generators. The physical mechanism behind these modalities relies on the Kerr‐type nonlinearity of nanoparticles in the dimer enhanced by a dipolar localized surface plasmon resonance. This allows one to provide a positive nonlinear feedback at moderate optical intensities, leading to the desired dynamical behavior via tuning the driving field parameters. The findings shed light on a novel class of nonlinear nanophotonic devices with a tunable nonlinear dynamical response.

Numerical investigation of plasmon sensitivity and surface-enhanced Raman scattering enhancement of individual TiN nanosphere multimers

Titanium nitride (TiN) nanoparticles have recently been considered as potential candidate plasmonic materials; such materials support localized surface plasmon resonances (LSPRs) and show excellent thermal stability with a high melting point. The electromagnetic (EM) field coupling and gap distance between components of individual TiN nanosphere multimers are critical parameters affecting their plasmonic sensitivity and surface-enhanced Raman scattering (SERS) performance, both of which are numerically investigated by the finite element method. It is demonstrated that the fractional shifts of both the dipolar LSPR wavelength and the refractive index sensitivity factor S follow the universal ‘plasmon ruler’ behavior, which is explained well in terms of EM field distribution. The response of the obtained S to is also presented and elucidated in terms of the optical response of the dielectric constants of TiN. The maximum S and SERS enhancement (excited by three normally available lasers in experiments) are also predicted; both are comparable to the values for Au dimeric nanoparticles. The present work holds great promise for the development of non-noble metal plasmonic materials in both SERS and plasmonic sensing applications.

Nanomechanical modulation cavities of localized surface plasmon resonance with elastic whispering-gallery modes

The dispersion of mechanical vibration limits nano-optomechanical modulation. In this work, we propose an optomechanical modulation exploiting elastic local resonances, also called whispering-gallery modes (WGMs). We find that our structure supports two quadripolar and two hexapolar elastic WGMs, which are nondispersive to avoid losses where the displacement field is localized on the gold nanodisks (AuNDs). We numerically demonstrate that the coupling between localized surface plasmon resonance (LSPR) and WGMs are relative both to the symmetry displacement of the elastic modes and to the strong isolation of phononic modes in the AuNDs. The amplitude of the modulation is evaluated by computing the wavelength shift of dipolar LSPR under different deformations by four WGMs. A detailed comparison between the four WGMs allows us to determine the ones with more efficient coupling. Furthermore, this simultaneous confinement gives a large acousto-plasmonic coupling that can be used to design a new mechanical sensor with the plasmonic response as a potential application and innovation toward new acousto-plasmonic devices.

Role of Depolarization Factors in the Evolution of a Dipolar Plasmonic Spectral Line in the Far- and Near-Field Regimes

In this article, the excitation of dipolar localized surface plasmon resonances (LSPRs) in both the far- and near-field regions is described in terms of the relevant static, dynamic, and radiative depolarization factors. This approach offers a direct relationship between the evolution of the LSPR spectral line and the depolarization components in an analogous sense to a harmonic oscillator. The static, dynamic, and radiative terms reflect the coefficients of the “stiffness”, effective mass, and damping in the oscillator system, respectively. Hence, one can immediately perceive that the static part of the depolarization factor is mainly responsible for the shifts in the resonant frequency, and the radiative part is responsible for the change in bandwidth. Additionally, the dynamic part behaves like an effective mass, acting as an inertial weighting factor that decides how significant the changes taking place in the system are. From this model, we can rationalize that the qualitative behavior of the far-field efficiency primarily depends on the shifting resonant frequencies, and the corresponding near-field efficiency is highly sensitive to the presence of damping. The model also clarifies the discrepancy in the resonant frequency and bandwidth between the far- and near-field spectra, which is due to the significant presence of the radiative component. These basic descriptions can be used as a guiding principle for handling more sophisticated structures and gaining more rationalized designs for novel applications related to the LSPR mechanism.

Tailored Engineering of Bimetallic Plasmonic Au@Ag Core@Shell Nanoparticles.

A distinctive synthetic method for the efficient synthesis of multifunctional bimetallic plasmonic Au@Ag core@shell nanoparticles (NPs) with tunable size, morphology, and localized surface plasmon resonance (LSPR) using Triton X-100/hexanol-1/deionized water/cyclohexane-based water-in-oil (W/O) microemulsion (ME) is described. The W/O ME acted as a “true nanoreactor” for the synthesis of Au@Ag core@shell NPs by providing a confined and controlled environment and suppressing the nucleation, growth, agglomeration, and aggregation of the NPs. High-resolution transmission electron microscopic analysis of the synthesized Au@Ag core@shell NPs revealed an “unusual core@shell” contrast, and the selected area electron diffraction and Moiré patterns showed that Au layers are paralleled to Ag layers, thus indicating the formation of Au@Ag core@shell NPs. Interestingly, the UV–visible spectrum of the Au@Ag core@shell NPs exhibited enthralling plasmonic properties by introducing a high-frequency quadrupolar LSPR mode originated from the isolated Au@Ag NPs along with a low-frequency dipolar LSPR mode originated from the coupled Au@Ag NPs. The effective plasmonic enhancement of the Au@Ag core@shell NPs is attributed to the extreme enhancement of the localized electromagnetic field by coupling of the localized surface plasmons of the Au core and Ag shell. The mechanisms for the nucleation and growth of Au@Ag core@shell NPs in W/O ME have been proposed. A unique electron transfer phenomenon between the Au core and Ag shell is elucidated for better understanding and manipulation of the electronic properties, which evinced the development of Au@Ag core@shell NPs through suppression of the galvanic replacement reaction.

Detailed balance analysis of plasmonic metamaterial perovskite solar cells.

Whether dispersal of plasmonic nanoparticles (NPs) within a perovskite active layer can increase the efficiency of solar cells is a long-standing question. It is well known that inclusion of metallic NPs in an active layer can boost the surrounding near-field intensity around them owing to the dipolar localized surface plasmon resonance (LSPR, also called antenna effect), which can increase light absorption by solar cells. However, the use of plasmonic NPs in perovskite solar cells has been barely reported, and it is not known whether inserting plasmonic NPs into a perovskite active layer produces any performance advantage compared with a pure perovskite counterpart. We explore the fundamental and practical limits of "plasmonic metamaterial" perovskite solar cells by applying effective medium theory and a detailed balance analysis. Our results indicate that an increase in effective refractive index of perovskite through dispersed plasmonic NPs can in principle enhance the performance of solar cells.

In Situ Contact Angle Tuning of Silver Nanostructures by Laser Heating in Different Fluid Ambient.

It has been theoretically suggested by Yan et al. (Colloids Surf., A 2013, 429, 142-148) that the contact angle θc of a liquid droplet on any given surface can be controlled by immersing it in an appropriate surrounding environment. Here, we report the first experimental demonstration of such an in situ contact angle tuning of silver (Ag) nanoparticles on quartz substrates via nanosecond pulsed laser heating under various fluid ambients, like air, water, and glycerol. Nanosphere lithography (NSL) was used to deposit Ag nanopyramids on quartz substrates. This system was subsequently melted by laser heating inside the various fluids to form nanoparticles. By using a combination of top and side view scanning electron microscopy (SEM) imaging, we show that the contact angle of Ag nanoparticles could be increased by going from heating in air to heating under fluids, with a near hemispherical shape (θc ∼ 99°) under air irradiation to nearly spherical (θc ∼ 167°) under glycerol irradiation. The mechanism of this contact angle change could be explained qualitatively by the changing interfacial energies of the substrate and metal in the various fluids. Similar contact angle control was also achieved for nanoparticles created by dewetting of Ag thin films in the various fluids. One practical implication of this contact angle tunability is the ability to change the intensity ratio of the quadrupolar to dipolar localized surface plasmon resonances in the Ag nanoparticles. This work also has implications to those applications in which nanoparticles on a substrate are heated in various gas or fluid ambients, such as in catalysis, as the ensuing shape change can modify properties.

Optimal geometry parameter for plasmonic sensitivities of individual Au nanopoarticle sensors.

The shape, aspect ratio and size are key parameters governing the plasmon sensitivities of individual Au nanoparticle bio/chemical sensors. It is crucial to unveil the general geometry parameters to optimize their corresponding sensitivity applications. In this work, the geometry-dependent refractive-index sensitivity factor (S) and figure of merit (FOM) of individual Au nanoparticle sensors (including a nanodisc, nanorod, nanoellipsoid and hexagonal nanoplate) are numerically investigated by discrete dipole approximation (DDA). S is revealed to increase quadratically/linearly with aspect ratio, while FOM reaches a maximum at an optimized aspect ratio of about 3.0/8.0 for the studied prolate/oblate nanoparticles, respectively, reflecting their shapes and aspect ratios and, hence, their size effects. However, their responses to shape factors are shown to follow nearly the same trend regardless of their different detailed geometries, demonstrating that their shape factors provide the general geometry parameters governing the plasmon sensitivities of the concerned individual Au nanoparticle sensors. This can be analytically explained well under dipolar localized surface plasmon resonance (LSPR) conditions. Their optimal FOM is predicted to be about 12.5 RIU-1 at a shape factor of 10.5; the underlying reason for this is analytically discussed as well. The obtained results in this work are believed to hold great promise for choosing appropriate nanoparticle geometry parameters for individual Au nanoparticle LSPR-based bio/chemical sensor design and applications as well as to access the corresponding optimal geometry parameters and FOM simultaneously.

Magnetic circular dichroism (MCD) in silver nanocubes with different sizes

Magnetic circular dichroism (MCD) spectroscopy is demonstrated for Ag nanocubes with different edge-lengths. A large bisignated MCD response is typical in the dipolar localized surface plasmon resonance (LSPR) region, which can be explained within a model on the magnetic Lorentz force imparted on the collective electrons that circularly move in the nanocube. In contrast, the MCD signal attributed to multipolar (including quadrupolar) LSPR exhibits a sharp simple (single) peak, but it never corresponds to the extinction peak position. This discrepancy is discussed on the basis of energetically unequal MCD responses for the left- and right-circularly polarized light excitations.

Plasmonic Coupling Effects on the Refractive Index Sensitivities of Plane Au-Nanosphere-Cluster Sensors

Plasmonic coupling effects (between neighboring components) are able to red shift the peak wavelengths of dipolar-localized surface plasmon resonances (LSPRs) and increase the corresponding refractive index sensitivity of nanoparticle sensors. The coupling effects on plane Au-nanosphere-cluster (including nanosphere dimer, trimer, pentamer, and heptamer) sensors are numerically investigated by finite element method (FEM). We found that the coupling does not violate the quadratic response characteristics of LSPR peak wavelengths, hence the linear responses of the sensitivities to the bulk refractive index of Au cluster sensors. Yet, for nanosphere dimer sensors, they contribute to the exponential decrease of sensitivities with their gap distances, which follow the universal plasmon ruler behavior. The amplitude of their fractional sensitivity shift is revealed to be bulk refractive index independent, which is different from that of their fractional LSPR peak wavelength shift. These are analytically explained well in terms of an effective nanoparticle model. The present work also gives an upper sensitivity limit for Au nanosphere dimer systems and provides a method to estimate the interparticle separation between the two component nanospheres of the dimer.

Extrinsic chirality of non-concentric plasmonic nanorings

We show how extrinsic chirality, i.e. the optical activity of achiral media exhibited at oblique light incidence, can be achieved in plasmonic nanorings by symmetry breaking. We demonstrate that even a small, 5% offset of an inner hole of a 190 nm gold ring results in a measurable circular dichroism signal in the near-infrared region. By using computer simulations, we show that optical activity arises upon excitation of a symmetric dipolar localized surface plasmon resonance mode due to the appearance of co-aligned electric and magnetic dipole moments.

Multipolar Surface Magnetoplasmon Resonances in Triangular Silver Nanoprisms studied by MCD Spectroscopy

Magneto-optical responses of triangular silver nanoprisms are demonstrated for the first time with magnetic circular dichroism (MCD) spectroscopy. In Ag nanoprisms with the mean aspect ratio ranging from 6.3 to 8.8, optical absorption exhibits a strong main peak at 650–800 nm that is ascribed to the in-plane dipolar localized surface plasmon resonance (LSPR), along with other two peaks at 450–500 nm and ∼330 nm, which can be assigned to the in-plane and out-of-plane quadrupolar LSPR, respectively. A derivative-like MCD signal is observed at the energy of in-plane dipolar LSPR, but the position crossing zero is not in correspondence with the absorption maximum. In contrast, interestingly, the MCD response for in-plane or out-of-plane quadrupolar LSPR shows a simple (single) peak that corresponds to the position of its absorption maximum, suggesting that MCD spectroscopy is very effective for easy identification (or rapid determination) whether the LSPR modes are dipolar or quadrupolar. The agreement betwee...

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.

Terahertz localized surface plasmon resonance of periodic silicon microring arrays

We demonstrate the absorption characteristics of silicon microring resonators at terahertz frequencies. Simulation and experimental data show a dipolar localized surface plasmon resonance (DLSPR) absorption peak. We demonstrate that the frequency position and magnitude of the DLSPR peak may be tuned by varying the geometry and thickness of the microring or by modification of the silicon impurity concentration. Finite difference time domain simulations reveal that there is a strong enhancement of the electric field at the resonant frequency. The absorption properties of our resonator are described in terms of effective optical constants and reveal that the silicon microring is an electric resonator. Surface plasmon resonators are efficient terahertz absorbers and have potential applications in security imaging, biological analysis, spectroscopy and nondestructive testing.

Investigation of particle shape and size effects in SERS using T-matrix calculations

The influence of particle size and shape effects on average and punctual surface-enhanced Raman scattering (SERS) enhancement factors (EFs) is investigated using exact T-matrix electrodynamic calculations of silver and gold spheroids over a large parameter space. This study extends the conventional treatment of these effects within the frameworks of the electrostatics approximation, its generalizations, or Mie theory for spheres. It confirms the qualitative features of these approaches, but provides in addition quantitative predictions of SERS EFs in the case of large non-spherical particles, where the lightning-rod effect (shape effect) and radiation damping (size effect) operate simultaneously. Finally, the localization effect at large SERS EF (hot-spots) is shown to be dictated only by shape, not size, in the case of metallic spheroids at the dipolar localized surface plasmon resonance.

Second Harmonic Excitation Spectroscopy of Silver Nanoparticle Arrays

Frequency-scanned excitation profiles of coherent second harmonic generation (SHG) were measured for silver nanoparticle arrays prepared by nanosphere lithography. The frequency of the fundamental beam did not coincide with the localized surface plasmon resonance (LSPR) of the nanoparticles and was tuned so that the coherent second harmonic (SH) emission was in the region of the LSPR at 720-750 nm. The SH emission from the arrays was compared with a smooth silver film to identify an enhancement of SH emission efficiency that peaks near approximately 650 nm for nanoparticles 50 nm in height. The polarization and orientation dependence of this enhancement suggests that it is related to a dipolar LSPR mode polarized normal to the plane of the substrate. Linear extinction spectra are dominated by in-plane dipoles and do not show this weak out-of-plane LSPR mode. The nanoparticle arrays are truncated tetrahedrons symmetrically oriented by nanosphere lithography to cancel SH from in-plane dipoles which allows observation of the weak out-of-plane component.