Longitudinal Plasmon Modes Research Articles

Chiral Response of Twisted Bilayer Graphene.

We present an effective (minimal) theory for chiral two-dimensional materials. These materials possess an electromagnetic coupling without exhibiting a topological gap. As an example, we study the response of doped twisted bilayers, unveiling unusual phenomena in the zero frequency limit. An in-plane magnetic field induces a huge paramagnetic response at the neutrality point and, upon doping, also gives rise to a substantial longitudinal Hall response. The system also accommodates nontrivial longitudinal plasmonic modes that are associated with a longitudinal magnetic moment, thus endowing them with a chiral character. Finally, we note that the optical activity can be considerably enhanced upon doping and our general approach would enable systematic exploration of 2D material heterostructures with optical activity.

Characterization of emission from aggregated gold nanoparticles excited nonlinearly by 1560-nm femtosecond laser

Nonlinear emission properties of aggregated 50-nm gold nanoparticles (GNPs) excited by a femtosecond laser at 1560 nm are characterized. Aggregate forms are correlated to emission by scanning electron microscope imaging and pattern matching. Broad spectra in the visible region are obtained from aggregated GNPs, and their emission power exhibits a quadratic power dependence and an exponential decay in time due to morphology change. Polarization analysis reveals that longitudinal plasmonic modes play important roles for nonlinear emission. Relationships between brightness and morphology show that a large number of aggregates produce luminescence enhancement but with associated photo damage. It is proposed that characteristics of nonlinear emission from GNPs are explained by plasmon enhanced polarized hot electrons.

Light incoupling tolerance of resonant and nonresonant metal nanostructures embedded in perovskite medium: effect of various geometries on broad spectral resonance

In this report, the surface plasmon effect of various metal nanostructures embedded in perovskite (methylammonium lead iodide) medium has been analysed for better light incoupling. Discrete dipole approximation as a numerical approach has been used to evaluate the extinction spectra and plasmon mode induced field intensity of Au, Ag and Al nanostructures (sphere, oblate spheroid, cylinder and rectangular) of an effective radii (aeff) of 30 nm. Spatial field distribution analysis primarily elucidates that the rectangular shape is more promising as the oscillating charge confinement is high at the corners. However, the oblate shape provides broad spectral enhancement among the considered shapes for the calculated polychromatic spectrum (300 nm–800 nm) due to the occurrence of both longitudinal and transverse plasmon modes. In addition, SPR peak position and the extinction magnitude of nano oblates have been discussed for varying size (5 nm–100 nm) and aspect ratio (0.1–0.9). Obtained results show the potentiality of Al nano oblates concerning broadband light trapping into perovskite material compared to Au and Ag nano oblates. This study gives an insight for the upcoming improvement of perovskite photovoltaics due to the plasmon induced light incoupling to the host medium near the band edge.

Facile Design of a Plasmonic Nanolaser

A spaser consists of a plasmonic noble-metal nanostructure that acts as nanocavity, when incorporated or surface-coupled two-level emitters constitute the nanoscale gain medium. Suited two-level emitters are, for instance, laser dyes. Optical pumping may provide efficient excitation energy transfer between the two-level emitters in the gain medium and the surface plasmons sustained in the nanocavity. Strong resonant coupling of the surface plasmon modes to the gain medium may establish an inherent feedback amplification mechanism which finally drives the spaser action. In this contribution, we demonstrate that spaser emission can be generated by amplifying longitudinal surface plasmon modes in gold nanorods by optically pumping surface-attached resonantly-coupled laser dyes. Therefore, we synthesized gold nanorods whose longitudinal surface plasmon resonance peak was adjusted between 680 and 700 nm. The gain medium was realized by electrostatically attaching the laser dye phthalocyanine tetrasulfonate via the positively-charged CTAB (cetyltrimethylammonium bromide) bilayer to the gold-nanorod surface. Phthalocyanine tetrasulfonate exhibits fluorescence at 700 nm. Fluorescence quenching experiments unambiguously gave indication of resonant excitation energy transfer. The fluorescence intensity ratio I F 0 / I F follows the Stern–Volmer relationship, and the Stern–Volmer coefficient was determined as KSV = 1.22 × 106 M−1. The spaser emission was observed in fs transient absorption spectra as an ultrafast decaying narrow emission peak around 716 nm.

Energy transfer and depolarization in the photoluminescence of a plasmonic molecule.

We report a comprehensive experimental study of the polarization dependence between excitation and photoluminescence (PL) emission from single dolmen-like metallic nanostructures that exhibit both Fano-like and Lorentz-like plasmon resonances. Though the PL spectra of this plasmonic "molecule" also exhibit the Fano and Lorentz signature, the emitted photons do not maintain the same polarization as the excitation. Surprisingly, the degree of depolarization correlates closely to the resonant excitation of the constituent atoms (single nanorod). More specifically, the excitation of a transverse plasmon mode results in a depolarized emission through the longitudinal plasmon modes of the constituent nanorods. In view of the recent evidence of on-resonant plasmon induced excitations in generating hot electrons, our results suggest that depolarized PL emissions could be enhanced through hot-electron decay. Both macroscopic and microscopic mechanisms are proposed to well-understand the excitation wavelength dependent depolarized photoluminescence behaviors in the plasmonic molecule. Our results lay a foundation for applying the depolarized photoluminescence of complex plasmonic nanostructures in polarization engineering.

Near-Field Nonlinear CD Imaging of Single Gold Nanostructures

We demonstrate near-field nonlinear circular dichroism (CD) imaging of single rectangular (achiral) gold nanostructures using a two-photon excitation method. The gold rectangles were illuminated by pulses of circularly polarized light (CPL) to generate two-photon excitation images of the longitudinal plasmon modes, and the images consisted of oval-shaped spatial features (corresponding to the antinodes of the plasmon modes) tilted from the long axis of the rectangles. The tilting direction depended on the handedness (left or right) of the CPL used for illumination, which resulted in the observation of a strong local dissymmetry of the two-photon excitation signals. The tilts of the oval features were not observed under linearly polarized pulse illumination with any polarization direction. The nonlinear CD images constructed from the differential two-photon excitation probability for left- and right-CPL pulses exhibited spatial features that were reasonably explained by the multipolar characters of the excited plasmon modes.

Hybrid Plasmonic Cavity Modes in Arrays of Gold Nanotubes

Plasmonic structures are known to confine light at the nanometer scale, and they exhibit enhanced electromagnetic fields localized in small mode volumes. Here, plasmonic resonators based on a metamaterial consisting of periodic arrays of gold nanotubes embedded into anodic aluminum oxide are studied and strong confinement of local fields with low losses is demonstrated. Higher‐order resonance modes of surface plasmons localized in gold nanotubes when the nanotube length exceeds some critical values are observed. The numerical simulations suggest that, the electric fields associated with some higher‐order longitudinal modes for the long nanotubes and some lower‐order longitudinal modes for the short nanotubes or the nanotubes with thin walls, are mainly localized at the interfaces between aluminum oxide and gold in the form of the standing‐wave longitudinal plasmonic modes, partially localized in the pores and at two ends of the nanotubes owing to the strong coupling of the Fabry–Pérot resonances with extraordinary optical transmission effect in the periodical structures through the inner nanochannels of the nanotubes, so that the nanotubes play a role of efficient cavity resonators. The existence of hybrid plasmonic resonant cavity modes with asymmetrical distributions of the electric field resulting from the near‐field coupling of both transversal and longitudinal modes in the gold nanotube metamaterials is revealed.

Onset of negative dispersion in one-component-plasma revisited

A simple approach to describe the long-wavelength dispersion of the longitudinal (plasmon) mode of the classical one-component-plasma (OCP), with the main objective to correctly capture the onset of negative dispersion, is discussed. The approach is applicable to both three-dimensional and two-dimensional OCP. The predicted onset of negative dispersion compares well with the available results from numerical simulations and more sophisticated theoretical models.

Spectroscopic Properties of Gold Curvilinear Nanorod Arrays

We designed and fabricated gold curvilinear nanorod periodical arrays using microfabrication techniques. The gold curvilinear nanorods had two distinct resonant peaks in the near-infrared region between 1630 nm and 3000 nm. Similar peak was observed in gold straight nanorods at specific lengths. At lengths identical to the arc length of the curvilinear nanorod, the peak was in the relative range of 3000 nm, which corresponds to the longitudinal plasmon mode (L-mode). At lengths identical to half of the arc length of the curvilinear nanorod, the peak was close to 1630 nm. Plasmon resonant peaks were tunable in the infrared region by changing the arc length of the curve, the line width, and distance between the curvilinear nanorods. In particular, when two curvilinear nanorods were closely packed in a range of less than 100 nm, the peak wavelength of curvilinear nanorod was shifted due to the plasmonic coupling of each mode.

Z-shape nanostructured array deposited by substrate cooling method

The substrate cooling method was used in glancing angle deposition to grow a slanted silver nanorod array (NRA). Liquid nitrogen was allowed to flow under the substrate during deposition, and we compared the morphologies of Ag NRAs deposited with and without cooling. The cooling reduced the average width of the nanorods. A Z-shaped nanostructure array composed of three sections of silver NRAs was then deposited under the same cooling conditions. The average tilt angle of the nanorods from the surface normal was varied from the bottom section to the top section with the nanorods of the top section made to lie almost parallel to the substrate. Under normal illumination, the rods in the top section exhibit distinct longitudinal and transverse plasmon modes that cause strong polarization-dependent transmittance and reflectance.

Deviating from the nanorod shape: Shape-dependent plasmonic properties of silver nanorice and nanocarrot structures

Noble metallic nanostructures exhibit special optical properties resulting from excitation of surface plasmons. Among the various metallic nanostructures, nanorods have attracted particular attention because of their unique and intriguing shape-dependent plasmonic properties. Nanorods can support transverse and longitudinal plasmon modes, the latter ones depending strongly on the aspect ratio of the nanorod. These modes can be routinely tuned from the visible to the near-infrared spectral regions. Although nanorods have been investigated extensively, there are few studies devoted to nanostructures deviating from the nanorod shape. This review provides an overview of recent progress in the development of two kinds of novel quasi-one-dimensional silver nanostructures, nanorice and nanocarrot, including their syntheses, crystalline characterizations, plasmonic property analyses, and performance in plasmonic sensing applications.

Gold nanorod libraries enhancement for optical imaging

Efficient far-field coupling of longitudinal plasmon modes of gold nanorods makes them prospective for optical imaging. A facile one-step hydrothermal synthesis of stabilized gold nanorods in an aqueous solution at 85°C is reported. High aspect ratio of these nanorods ranging from 10 to 57 is achieved using methyltrioctylammonium chloride (TOMAC) as a phase transfer reagent. The growth of {100} and {110} facets being more favorable to conception is facilitated via the capping agent (TOMAC). These highly elongated nanorods reveal visible luminescence at around 410nm, which is attributed to the ligand–metal charge transition mechanism. We affirm that such plasmonic nanorods acquiring superior nonlinear optical properties can be used as a vehicle for single-particle measurements.

Germanium - the superior dopant in n-type GaN

We present an experimental study about the influence of Si and Ge doping in GaN with focus on the occurring strain levels and overall crystalline quality. Extremely high quality samples were examined by means of Raman spectroscopy, demonstrating effective, n-type doping concentrations up to the 5 × 1019 cm–3 regime. By studying the full width at half maximum (FWHM) of the E2(high) Raman mode with rising doping concentration, Ge is approved as the by far superior dopant if compared to Si. Even elevated nominal Ge concentrations yield corresponding FWHM values of just 3 cm–1, a most competitive value even for bare bulk GaN samples. At the same time, the biaxial, compressive stress that is introduced by such high Ge doping amounts to just 0.2 GPa, in clear contrast to the particular case of silicon. Here, even moderate doping levels lead to tensile stress up to 1 GPa and consequently to a serious degeneration of the overall crystal quality as approved by our Raman analysis. Additionally, the examined high doping concentrations enable the observation of longitudinal optical phonon plasmon (LPP) modes in the Raman spectra, which serve as a direct tool for the determination of the effective doping concentration. A careful analysis of the LPP coupling at cryogenic and room temperature yields within the error interval identical free carrier concentrations in all germanium doped samples, pointing towards an energetically shallow nature of the dopant. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Single Anisotropic Plasmonic Nanoparticle Three-Dimensional Orientation Determination Based on Fano-Like Resonance and Universal 3D Orientation-Dependent Scattering Trait

Single-nanoparticle orientation determination plays a vital role in studying complex nanoscale motion. In the present paper, we systematically investigate the three-dimensional (3D) orientation-dependent far- and near-field optical properties of single (Au core)-(dielectric shell) nanorice and gold nanorod. It shows that the scattering spectrum of the single anisotropic nanoparticle with arbitrary orientation is a linear superposition of a set of basic scattered spectra. And the scattering spectra of the single nanorice show a polarization-dependent Fano-like resonance which can be well described by a model with two coupled oscillators. Furthermore, by means of coordinate transformation, a universal analytic formula for description single nanoparticle 3D orientation- and polarization- dependent scattering intensity is proposed. Then we develop a new method to detect the 3D orientation of single nanoparticle, that is, by fitting the scattering intensity under different incident polarization directions based on our analytic formula and the 3D orientations could be resolved explicitly. Both of the experimental and numerical simulation data can be well described by our analytical formula. Our method for single particle 3D orientation determination has high precision only with subdegree uncertainty. In addition, based on the Fano resonances caused by the efficient coupling between the dielectric elliptical shell and the gold nanoellipsoid core, we found that the 3D orientation of single nanorice could be confirmed from either the transverse or longitudinal plasmon mode polarization-dependent scattering trait, while it is almost impossible for single gold nanorod based on the transverse plasmon mode. It is worth noting that the transverse plasmon mode of the nanorice is mostly insensitive to the aspect ratio then it allows nanorices with different lengths to be 3D orientation sensors without altering the incident laser wavelength.

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...

Transverse plasmon mode in a screened two-dimensional electron system

The dispersion and damping of the transverse plasmon mode in a screened two-dimensional (2D) electron system are theoretically studied. It is shown that the transverse plasmon mode has a much larger quality factor and a smaller retardation factor at terahertz (THz) frequencies in comparison with those known for the longitudinal plasmon mode. In addition, the electric dipole moment of the transverse plasmon mode in the screened 2D electron system can be comparable with the dipole moment of the longitudinal plasmon mode. These properties of the transverse plasmon mode make it attractive for use in plasmon devices of the THz frequency range.

Quantum nonlocal effects on optical properties of spherical nanoparticles

To study the scattering of electromagnetic radiation by a spherical metallic nanoparticle with quantum spatial dispersion, we develop the standard nonlocal Mie theory by allowing for the excitation of the quantum longitudinal plasmon modes. To describe the quantum nonlocal effects, we use the quantum longitudinal dielectric function of the system. As in the standard Mie theory, the electromagnetic fields are expanded in terms of spherical vector wavefunctions. Then, the usual Maxwell boundary conditions are imposed plus the appropriate additional boundary conditions. Examples of calculated extinction spectra are presented, and it is found that the frequencies of the subsidiary peaks, due to quantum bulk plasmon excitations exhibit strong dependence on the quantum spatial dispersion.

Topological edge plasmon modes between diatomic chains of plasmonic nanoparticles.

We study the topological edge plasmon modes between two "diatomic" chains of identical plasmonic nanoparticles. Zak phase for longitudinal plasmon modes in each chain is calculated analytically by solutions of macroscopic Maxwell's equations for particles in quasi-static dipole approximation. This approximation provides a direct analogy with the Su-Schrieffer-Heeger model such that the eigenvalue is mapped to the frequency dependent inverse-polarizability of the nanoparticles. The edge state frequency is found to be the same as the single-particle resonance frequency, which is insensitive to the separation distances within a unit cell. Finally, full electrodynamic simulations with realistic parameters suggest that the edge plasmon mode can be realized through near-field optical spectroscopy.

Standing wave plasmon modes interact in an antenna-coupled nanowire.

In a standing wave optical cavity, the coupling of cavity modes, for example, through a nonlinear medium, results in a rich variety of nonlinear dynamical phenomena, such as frequency pushing and pulling, mode-locking and pulsing, modal instabilities, even complex chaotic behavior. Metallic nanowires of finite length support a hierarchy of longitudinal surface plasmon modes with standing wave properties: the plasmonic analog of a Fabry-Pérot cavity. Here we show that positioning the nanowire within the gap of a plasmonic nanoantenna introduces a passive, hybridization-based coupling of the standing-wave nanowire plasmon modes with the antenna structure, mediating an interaction between the nanowire plasmon modes themselves. Frequency pushing and pulling, and the enhancement and suppression of specific plasmon modes, can be controlled and manipulated by nanoantenna position and shape.

Giant enhancement of second harmonic generation by engineering double plasmonic resonances at nanoscale.

We have investigated second harmonic generation (SHG) from Ag-coated LiNbO₃(LN) core-shell nanocuboids and found that giant SHG can occur via deliberately designed double plasmonic resonances. By controlling the aspect ratio, we can tune fundamental wave (FW) and SHG signal to match the longitudinal and transverse plasmonic modes simultaneously, and achieve giant enhancement of SHG by 3 × 10(5) in comparison to a bare LN nanocuboid and by about one order of magnitude to the case adopting only single plasmonic resonance. The underlying key physics is that the double-resonance nanoparticle enables greatly enhanced trapping and harvesting of incident FW energy, efficient internal transfer of optical energy from FW to the SHG signal, and much improved power to transport the SHG energy from the nanoparticle to the far-field region. The proposed double-resonance nanostructure can serve as an efficient subwavelength coherent light source through SHG and enable flexible engineering of light-matter interaction at nanoscale.