Plasmonic abilities of gold and silver spherical nanoantennas in terms of size dependent multipolar resonance frequencies and plasmon damping rates

Absorbing and emitting optical properties of a spherical plasmonic nanoantenna are described in terms of the size dependent resonance frequencies and damping rates of the multipolar surface plasmons (SP). We provide the plasmon size characteristics for gold and silver spherical particles up to the large size retardation regime where the plasmon radiative damping is significant. We underline the role of the radiation damping in comparison with the energy dissipation damping in formation of receiving and transmitting properties of a plasmonic particle. The size dependence of both: the multipolar SP resonance frequencies and corresponding damping rates can be a convenient tool in tailoring the characteristics of plasmonic nanoantennas for given application. Such characteristics enable to control an operation frequency of a plasmonic nanoantenna and to change the operation range from the spectrally broad to spectrally narrow and vice versa. It is also possible to switch between particle receiving (enhanced absorption) and emitting (enhanced scattering) abilities. Changing the polarization geometry of observation it is possible to effectively separate the dipole and the quadrupole plasmon radiation from all the non-plasmonic contributions to the scattered light.     

Coherent phonon excitation in bismuth

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A_1g optical phonon.     

Underdamped soft mode in bismuth titanate (Bi4Ti3O12)

We have observed an underdamped soft mode in the Raman spectra of bismuth titanate. The soft mode decreases its frequency on approaching 200°C from both above and below, but the frequency does not drop to zero at this temperature. There seems to be additional quasi-elastic scattering near 200°C. Another phase transition could be expected in bismuth titanate.     

Optimal structure for resonant THz detection of plasmons–polaritons in the 2D quantum wells

We investigate terahertz plasmon?Cpolariton (PP) resonances for hetero-structures (AlGaN/GaN, SiGe/Si/SiGe, AlGaAs/GaAs, and InAlN/GaN) with a grating coupler in order to find the overall optimal structure showing the strongest absorption for terahertz detection (THz). We show by a parametric study (influence of geometric dimensions, electron concentration, temperature, etc.) that refined and intense resonances can be obtained at specific frequency. GaN based heterostructures present the higher PP resonances at room temperature. The roles of the finite thicknesses of lossy metal grating and a two-dimensional gas (2DEG) layer on observed absorption are also investigated. Absorption spectra for three kinds of heterogeneous charge density profiles (piecewise, linear, and parabolic) of 2DEG was investigated and compared for an AlGaAs/GaAs structure because some physical parameters such as the Fermi level pinning at the interface semiconductor/air are well established only for this heterostructure. We show that the PP resonance (amplitude and frequency position) is modulated by the charge concentration but also by the metallization biasing.     

外围介质介电常数对金纳米棒光学性质影响的数值模拟

采用电化学方法制备出悬浮胶体金纳米棒。在室温下测得其紫外-可见吸收光谱和荧光发射光谱。结合准静电场理论与散射理论对金纳米棒的光学特性进行了数值模拟。计算结果表明,由于金纳米棒形状的各向异性,吸收截面与散射截面均出现两个共振峰,分别对应于横截方向与长轴方向的表面等离子共振。对于几何形状一定的金纳米棒,在外围介质介电常数从1.5增加到2.5的过程中,两个共振吸收峰均增强并非线性红移。然而对于散射光谱,短波方向的共振峰线性红移,强度变化微弱;长波方向的共振峰减弱并非线性红移。     

Ab-initio calculation of optical properties of AA-stacked bilayer graphene with tunable layer separation

Density functional theory based calculations revealed that optical properties of AA-stacked bilayer graphene are anisotropic and highly sensitive to the interlayer separation. In the long wave length limit of electromagnetic radiation, the frequency dependent response of complex dielectric function becomes vanishingly small beyond the optical frequency of 25.0 eV. Besides, static dielectric constant shows a saturation behaviour for parallel polarization of electric field vector when interlayer spacing is greater than 2.75 Å. As a consequence, an appropriate modification of effective fine structure constant is observed as a function of layer separation. Moreover, the bilayer systems are highly transparent in the optical frequency range of 7.0–10.0 eV. The electron energy loss function exhibits two different in-plane collective (plasmon) excitations and a single out of plane plasmon excitation. The spectral nature of different frequency dependent optical properties is observed to be very similar to that of the monolayer pristine graphene apart from their exact numerical values.     

Why metallic surfaces with grooves a few nanometers deep and wide may strongly absorb visible light

It is theoretically shown that nanometric silver lamellar gratings present very strong visible light absorption inside the grooves, leading to electric field enhancement of several orders of magnitude. It is due to the excitation of quasistatic surface plasmon polaritons with particular small penetration depth in the metal. This may explain the abnormal optical absorption observed a long time ago on almost flat Ag films. Surface enhanced Raman scattering in rough metallic films could also be due to the excitation of such quasistatic plasmon polaritons in grain boundaries or notches of the films.     

Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer: (II) Binding‐site and quantum‐size effects

We describe quantum‐size and binding‐site effects on the chemical and local field enhancement mechanisms of surface‐enhanced resonance Raman scattering (SERRS), in which the pyridine molecule is adsorbed on one of the vertices of the Ag₂₀ tetrahedron. We first investigated the influence of the binding site on normal Raman scattering (NRS) and excited state properties of optical absorption spectroscopy. Second, we investigated the quantum‐size effect on the electromagnetic (EM) and chemical mechanism from 300 to 1000 nm with charge difference density. It is found that the strong absorption at around 350 nm is mainly the charge transfer (CT) excitation (CT between the molecule and the silver cluster) for large clusters, which is the direct evidence for the chemical enhancement mechanism for SERRS; for a small cluster the strong absorption around 350 nm is mainly intracluster excitation, which is the direct evidence for the EM enhancement mechanism. This conclusion is further confirmed with the general Mie theory. The plasmon peak in EM enhancement will be red‐shifted with the increase of cluster size. The influence of the binding site and quantum‐size effects on NRS, as well as chemical and EM enhancement mechanisms on SERRS, is significant. Copyright © 2009 John Wiley & Sons, Ltd.     

Plasmonic tomography of optical vortices

We present a method for analyzing the wavefront of optical vortices that does not involve interferometry but rather uses surface plasmon polaritons (SPPs). We employ a subwavelength slit in a gold film to cut slices from an optical vortex beam and measure the diffraction of the generated SPPs by scattering them off a second slit. By moving the slits across the vortex beam, we create a tomogram, from which we can determine the vortex charge of the incident beam at a glance. We present results for vortex beams of integer- and half-integer-vortex charge.     

Theory of optical spectra involving charge transfer states: dynamic localization predicts a temperature dependent optical band shift

The influence of charge transfer states on the optical line shape of chromophore complexes is investigated in a minimal model that includes a coupling between an excited state and an optically dark charge transfer state. In the calculations of the absorption spectrum, an intensity borrowing by the charge transfer state, strong vibrational sidebands, and a temperature dependent shift of optical transitions are obtained. The theory is applied to the bacterial photosynthetic reaction center to explain a 30 nm blueshift of the low-energy absorption band with increasing temperature.     

Theory of Raman scattering in Europium-chalcogenides

The intensity of the spin assisted Raman scattering in the Eu-chalcogenides is calculated using the excited states which were used in the analysis of the optical absorption. Various mechanisms are examined for the Raman scattering. In these mechanisms, the cross effect of the 4f spin-orbit interaction and the exciton-phonon interaction is found to be the most important for the scattering intensity. The characteristics of the Raman scattering from this mechanism are as follows: When the frequency of the incident light is in the tail region of the absorption peak, the polarization of the scattered light is perpendicular to that of the incident light; when the light in the middle region of the absorption peak is applied, the polarization of the incident light is depolarized in the scattered light; the scattering intensity decreases when the spin fluctuation is suppressed by an application of magnetic field or by lowering temperature through the Curie temperature. These characteristics in the Raman scattering have been observed in the experiments.     

The influence of temperature on the optical properties of gold nanoparticles

The optical properties of gold nanoparticles in a transparent matrix are studied at temperatures from 300 to 1000 K within the spectral range from 450 nm to 1.5 μm. It is shown that, in the case of small nanoparticles and short wavelengths in the plasmon resonance band, an increase in temperature leads to a decrease in the absorption efficiency factor. The light absorption efficiency factor of gold nanoparticles with a radius exceeding 40 nm increases with increasing temperature in the entire spectral range studied. The single scattering efficiency factor always decreases with increasing temperature. The effects observed are related to a change in the refractive indices of gold and the matrix with comparable contributions. It is shown that results of calculations agree qualitatively with available experimental data. The results are necessary to optimize the composition of the actuators, including gold nanoparticles, in the transparent matrices.     

Optical properties of Ag nanoparticle embedded silicate glass prepared by field-assisted diffusion

Ag nanoparticle embedded silicate glass was prepared by field-assisted diffusion, combined with post-annealing process. Surface plasmon resonance (SPR) bands were observed in the optical absorption measurement. The properties of optical absorption were strongly affected by the process parameters. With increasing electric field intensity and diffusion time, the SPR peak position and the full width at half maximum (FWHM) of the plasmon band were almost invariable. With increasing post-annealing time, the SPR peak was blue-shifted and the FWHM of SPR band decreased. The experimental results could be well simulated using modified Mie scattering theory considering the spill-out effects and the limitation of the mean free path.     

Temperature dependence of the optical properties of ion-beam sputtered ZrN films

The reflectivity of sputtered Zirconium nitride films on glass substrate has been investigated in the spectral energy range of 0.8–6.1 eV as a function of deposition temperature varying between 373 and 723 K. Optical constants of the prepared films have been determined using the Drude analysis. Experimental results showed strong dependency of optical properties of the films, such as optical resistivity on the substrate temperature. The temperature increase of the substrate has shown an increase in both the plasmon frequency and electron scattering time. The electrical behavior of the films showed a good agreement between their optical and electrical resistivity.     

Light scattering by a medium with a spatially modulated optical conductivity: the case of graphene

We describe light scattering from a graphene sheet having a modulated optical conductivity. We show that such modulation enables the excitation of surface plasmon polaritons by an electromagnetic wave impinging at normal incidence. The resulting surface plasmon polaritons are responsible for a substantial increase of electromagnetic radiation absorption by the graphene sheet. The origin of the modulation can be due either to a periodic strain field or to adatoms (or absorbed molecules) with a modulated adsorption profile.     

Simultaneous low extinction and high local field enhancement in Ag nanocubes

We theoretically investigate surface plasmon resonance properties in Au and Ag cubic nanoparticles and find a novel plasmonic mode that exhibits simultaneous low extinction and high local field enhancement properties. We analyse this mode from different aspects by looking at the distribution patterns of local field intensity, energy flux, absorption and charge density. We find that in the mode the polarized charge is highly densified in a very limited volume around the corner of the nanocube and results in very strong local field enhancement. Perturbations of the incident energy flux and light absorption are also strongly localized in this small volume of the corner region, leading to both low absorption and low scattering cross section. As a result, the extinction is low for the mode. Metal nanoparticles involving such peculiar modes may be useful for constructing nonlinear compound materials with low linear absorption and high nonlinearity.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Heat-generating property of a local plasmon resonator under illumination

We have investigated the heat generation from gold nanoparticles resulting from their local plasma resonance. We have demonstrated the self-assembly of Au nanoparticle arrays/dielectric layer/Ag mirror sandwiches, i.e., a local plasmon resonator, using a dynamic oblique deposition technique. The thicknesses of the Au and dielectric layers were changed combinatorially on a single substrate. As a result, local plasmon resonator chips were successfully fabricated. Because of strong interference, their optical absorption can be controlled between 0.0% and 97% in the near-IR region, depending on the thickness of the dielectric layer. We evaluated the heat generation from Au nanoparticles by measuring the temperature of water with which a cell prepared on a chip is filled under laser illumination. The change in the water temperature is proportional to the optical absorption of the local plasmon resonator chips. This suggests that the photothermal conversion efficiency can be controlled by interference. These features make the application of the local plasmon resonator to nanoheaters, which can spatiotemporally control heat generation, suitable.     

Transport and cyclotron resonance theory for GaAs-AlGaAs heterostructures

A theory for static and dynamic transport of a two-dimensional electron gas in GaAs-AlGaAs heterostructures at temperature zero is presented. Charged impurities, separated from the electron gas by a spacer layer, are considered as the dominant scattering mechanism. Finite extension of the wave function of the two-dimensional electron gas is taken into account. Multiple scattering effects are included and are shown to lead to a metal insulator transition at low electron densities. Due to plasmon dynamics the scattering is strongly frequency dependent, and this dissipative process determines the width of the cyclotron resonance. The corresponding reactive effect determines the shift of the cyclotron resonance. It is shown that a correlation between line width maximum and zero frequency shift of the cyclotron mode exists, in agreement with experimental results.     

Non-perturbative approach to the electrodynamics of rough metal surfaces

Surface plasmon (SP) effects on the optical properties of rough metal surfaces are considered. Angular and frequency dependences of radiation, arising from SP decay and diffuse light scattering, as well as the photoemission current from a rough metal surface have been calculated. The above effects have been shown to be strongly enhanced when SP attenuation lengths stipulated by SP decay into vacuum and absorption in the metal, are much larger than the attenuation length stipulated by transformations into other SP states. Possibilities of comparing theoretical calculations with experimental data are considered.