Search

The modified emission characteristics of a source dipole near a metal-coated topological insulator sphere are studied with the application of axion electrodynamics in the long wavelength limit. Our focus is on the effect due to the topological magneto-electric (TME) response of the sphere and its influence on the multipolar plasmonic resonance of the metallic nanoshell. It is found that the TME response mainly affects the bonding modes of the metal shell plasmons with the lower order multipole resonances experiencing appreciable red-shifts. In addition, the TME effects can lead to either greater or smaller decay rates and lower emission frequencies, depending crucially on the frequency of the emitting dipole; but otherwise rather insensitive to the orientation of the dipole moments. These findings thus provide an alternative approach to probing the TME effects via molecular fluorescence experiments, as well as new possibility of controlling the emission properties of molecules using these novel nanoparticles.

The hybridization model for the coupled plasmons of a metallic nanoshell is generalized to the case with the presence of topological insulator materials via the application of axion electrodynamics. A new approach has to be adopted in the Lagrangian formulation of the coupling dynamics between the surface and cavity plasmons to account for the axion-induced charges on the boundary surfaces. It is found that, as observed previously from numerical studies in the literature, the hybridization model predicts that a topological magneto-electric effect will lead to red shifts on both the coupled bonding and anti-bonding modes–more significant for the higher-order bonding modes with the increase of the axion coupling parameter.

The optical response of charged polymeric and metallic colloids is investigated using effective medium theories for composite systems of nanoparticles. Based on the Bohren-Hunt theory for generalized Mie scattering from charged particles, an effective quasi-static dielectric function previously obtained is applied to the present study to characterize the response from the various colloidal particles. It is found that such effects are more prominent for polymeric and nonmetallic colloidal solutions in general. In addition, the effects of clustering among the colloidal particles are also studied via a fractal model available from the literature. Detailed numerical studies of the dependence of these effects on the amount of extraneous charge, as well as on the geometry and volume fraction of the colloidal particles are presented.

The recent demonstration of the plasmonic-enhanced Forster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.

Axion electrodynamics is applied to study the response of a plasmonic nanoshell with a core made of topological insulator (TI) materials. The electric polarizability of such a system is calculated in the long wavelength limit via the introduction of two scalar potentials satisfying the various appropriate boundary conditions. Our focus is on the topological magneto-electric effect (TME) as manifested in the coupled plasmonic resonances of the nanoshell. It is found that for a TI with broken time-reversal symmetry, such TME will lead to observable red-shifts in the coupled plasmonic modes, with more significant manifestation of such shifts for the bonding modes of a metallic nanoshell. It is speculated that such universal red-shift could be a manifestation of the fundamental dual symmetry as generalized for axion electrodynamics. Keywords: Surface plasmons, Axion electrodynamics, Topological insulator, Nanoshells

Motivated by the recent demonstration of asymmetric transmission of light through stratified systems of various anisotropic metamaterials, we have established generalized reciprocal relations for both transmission and reflection for these systems referring to the two cases of incidence from opposite directions. These relations reduce to those derived previously for simple isotropic systems which guarantee reciprocal symmetry in both transmission and reflection for non-dissipative medium, and provide constraint for reflection from both sides in the presence of absorption. Illustrations of our generalized relations are provided for different material systems.

The optical properties of composites with metallic nanoparticles are studied, taking into account the effects due to the nonlocal dielectric response of the metal and the coalescing of the particles to form clusters. An approach based on various effective medium theories is followed, and the modeling results are compared with those from the cases with local response and particles randomly distributed through the host medium. Possible observations of our modeling results are illustrated via a calculation of the transmission of light through a thin film made of these materials. It is found that the nonlocal effects are particularly significant when the particles coalesce, leading to blue-shifted resonances and slightly lower values in the dielectric functions. The dependence of these effects on the volume fraction and fractal dimension of the metal clusters is studied in detail.

The optical properties of tetrodotoxin (TTX), a scarce toxin with anesthetic properties, were studied using nanoparticle arrays-assisted surface-enhanced Raman scattering (SERS). The nanoparticles arrays were fabricated using nanosphere lithography and a metallic lift-off process to control the particle size, shape, and spacing in the arrays. Using density functional methods, the Raman spectrum of TTX was also calculated with Gaussian03 software. The main peaks of the spectrum are originated from the vibration of the NH2 molecule group. In the SERS experiment, we were able to measure the Raman spectrum with a TTX concentration as less as 0.9 ng/mL. This sensitivity is comparable to that from high performance liquid chromatography.

The dispersion relation for the plasmon at the interface of a metal and a nonlinear Kerr medium is studied by applying a semi-analytic theory established in the literature. Explicit analytical results are obtained and are compared to those from a certain approximate treatment appeared in the literature. It is found that for large electric field strengths, both the dispersion relation and the surface plasmon frequency from the approximate treatment deviate significantly from those obtained in the exact approach, especially for the case with a negative Kerr susceptibility.

We perform theoretical studies on the plasmonic enhancement for the Forster resonance energy transfer (FRET) between a donor and an acceptor molecule in the vicinity of a metallic particle or cavity, with focus on the possible role of the addition of a clad layer of gain material can play in such a process. The results show that while the plasmonic resonances can be shifted with higher order plasmonic enhancements emerged in the presence of such a layer of gain material, optimal enhancement of the FRET rate can be achieved when gain just balances with the loss in the metal. This then leads to the existence of an optimal thickness for the gain material layer, for both particle and cavity enhancement. In addition, it is observed that the FRET efficiency can always be increased with the coating of the gain material even at the dipole plasmonic resonance when nonradiative transfer from the donor to the metal is high, provided that the gain level is not beyond a certain critical value.

We present a theoretical analysis of optical force on a molecule near a metal sphere by taking into account the change of decay rate as well as the shift of resonance frequency in the response of the metal surface. As a result, the optical force is determined not only by the gradient of field but also the polarizability depending on the position and orientation of the molecule. In addition, we find the resulting reactive force cannot be expressed as a gradient of a scalar potential, in contrast to the usual situations.

A phenomenological model for the optical response of composite materials with metallic nanoparticles is presented. This model applies the conventional effective medium theories (EMT) but takes into account the spatial dispersion effects in the dielectric response of the metallic nanoparticles. This leads to an EMT that depends on the size of the particles. Numerical results from a model computation shows that this effect due to the nonlocal optical response of the nanoparticles can increase the resonant absorption frequency of the composite significantly for particles of very small sizes; and can lead to resonant absorption even in the Bruggeman symmetric EMT—a feature which is believed to be absent in the conventional treatment where local response for the metal particles has been assumed.

The application of surface plasmon resonance to the monitoring of the temperature of a metal film and its environment is well-established. A new feature in our present experimental work is to carry out this monitoring via the measurement of the phase difference between a s and a p-polarized wave at different wavelengths, 632.8 nm and 1.15 μm, based on a technique established previously in the literature. By monitoring the change of this phase as a function of the film temperature, it is found that this approach leads to very sensitive measurements of temperature, in comparison with previous approaches in which reflectance was measured instead. Sensitivity is 0.027 K at incident wavelength of 632.8 nm and 0.1 K at incident wavelength of 1.15 μm. A simple model based on the temperature dependence of the optical constants of the metal is applied to simulate our measurements, and it is found that the general qualitative behavior and trend of the experimental results can be reasonably accounted for using such a model.

The possible nonlocal electrodynamic effects on the optical excitation of the surface plasmon resonance (SPR) at a metal–dielectric interface are studied theoretically adapting the Kretschmann attenuated-total-reflection (ATR) geometry. Calculations of the SPR reflectivity are performed using both the Lindhard–Mermin and the hydrodynamic models for the dielectric response of the metal film. The results are compared with those obtained from the classical Drude model, as well as those from previous study for a bimetallic system. It is concluded that, unlike the bimetallic system, these nonlocal effects are relatively insignificant in the Kretschmann device, except perhaps at cryogenic temperatures when appreciable changes in dip-reflectivity values can take place due to the significance of single-particle excitation.

A microscopic model is constructed to describe the excited states of MEH-PPV in thin film. It is suggested that in the high energy region, the nature of excited states in the film is very similar to the species observed in solution phase. Moreover the processes of energy transfer, vibrational relaxation and dissipation are considered to occur simultanesously, in contrast to the usual argument that assumes exciton migration occurs after vibrational motion reaches thermal equilibrium. In the low energy region, corresponding to the red-edge of absorption, the excited state behaves differently. We assume that there exists a low energy species due to the enhancement of coplanarity of polymers in thin film and the absorption spectrum can be fitted.

In this work, we construct a microscopic model with which one can reconstruct all the experimentally observed characteristics of the RCs including temperature effects. For this purpose, we analyze the absorption spectra of Rb. sphaeroides RCs (R26 and R26.Phe-a mutants). We apply the excitonic-vibronic model with the displaced harmonic potential surface model for the nuclear freedoms and determine microscopic properties such as the electronic coupling constants, the vibrational frequencies, and the displacements among the potential surfaces of these electronic states. We also take into account inhomogeneity of RCs in our theoretical analysis. We introduce a thermal expansion model to treat temperature dependent peak shifts observed in the P band of the absorption spectra of the RCs. With the resulting information, we also calculate the rate constants of the dynamic processes occurring in RCs, determining the electronic coupling constants among the electronic states involved in electron transfer and electronic energy transfer of the RCs. Based on the excitonic-vibronic model, we also investigate the mechanism of the direct path PB * H → P + B - H recently observed in YM210W mutant.

We present a theory of photocurrent spectra for a light-emitting diode fabricated by a π-conjugated polymer system. The tunneling through a barrier induced by the DC field dominates the dissociation of excitor states (i.e., photoionization of vibronic states) below the threshold. The spectra thus obtained explain most of the experimental results and also allows us to more easily position the S2 state and other higher states in the conjugated polymer.

In this paper, the vibronic structure of a dimer system is studied both theoretically and numerically. To construct adiabatic potential surfaces and electronic and vibrational wave functions for a dimer system, the adiabatic approximation is applied to two identical molecules, each of which has two electronic states with one vibrational mode. In this scheme, the excitonic splitting results not only from the electronic coupling of two molecules, but also from the vibronic coupling in each molecule. By using the resulting wavefunctions and the corresponding energies, the absorption and fluorescence spectra are studied. The effect of temperature on these spectra is also studied.

We investigate herein the propagation and tunneling of monochromatic light through a tapered fiber tip, modeled as a tapered perfectly conducting waveguide. The opening aperture of the waveguide is much smaller than the wavelength of light. The transmission rate through the tip as well as the profiles of electric fields are obtained. A detailed formulation for calculating the fluorescent spectra of a single molecule under near-field excitation is also presented. In addition, the limitations of fluorescence measurement for obtaining near-field pattern is discussed on the basis of the present formulation.

The effect of extraneous charge on the optical interactions with metallic nanoshells is studied theoretically based on an effective medium model, with the electrodynamic theory also formulated to justify the accuracy of this model. Both optical far-field and near-field interactions will be studied. Furthermore, the hybridization model is generalized to include the charge effects yielding results for the multipole resonances consistent with those from the effective medium model. It is found that while the extraneous charges will lead to blueshifted resonances, their effect on the bonding modes will be most significant, especially for localized near-field sources. In addition, it is pointed out that the applicability of our theory can go beyond the metallic nanoshell to include graphene-coated core–shell particles studied recently in the literature.

We show that optical solitons in a broad spectral range can be excited by propagating an off-axis Gaussian beam in nonlinear graded-index (GRIN) waveguides. Owing to periodical variation of both the beam size and dispersion coefficient in GRIN waveguides, the solitons belong to a newly discovered class called guiding-center solitons. In planar GRIN waveguides, solitons may be arranged to interact in both the temporal and spatial domains, thus the waveguide may serve as a novel platform for soliton interactions.

An analysis of group-velocity dispersion in graded-index (GRIN) lenses is presented. The analysis shows that continuously adjustable negative group-velocity dispersion up to hundreds of square femtoseconds can be produced by propagating the optical beam off the axis of a GRIN lens. Compared with the well-known prism-pair and grating-pair methods for producing negative dispersion, the method described here is advantageous because it is 100-fold smaller, it has low insertion loss, and it is compatible with integrated optics.

The optical properties of tetrodotoxin (TTX), a scarce toxin with anesthetic properties, were studied using nanoparticle arrays-assisted surface-enhanced Raman scattering (SERS). The nanoparticles arrays were fabricated using nanosphere lithography and a metallic lift-off process to control the particle size, shape, and spacing in the arrays. In the SERS experiment, we were able to measure the Raman spectrum with a TTX concentration as less as 0.9 ng/mL. This sensitivity is comparable to that from high performance liquid chromatography.

Theoretical studies of the optical response of metallic nanoshells have been carried out, where quantum effects are partially accounted for through the application of a nonlocal response model for shells of mesoscopic dimensions. Both far field and near field interactions are considered, with the incident source being a plane wave and an emitting molecular dipole, respectively. It is found that these nonlocal effects can lead to significant deviations from macroscopic electrodynamic theory, for shells of ultrasmall dimensions $(l10\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ or ultrathin thickness $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, and are particularly significant for processes involving higher multipolar responses of the nanoshells. It is further concluded that these effects can still be observable, even in the presence of possibly large interfacial scattering for the free electrons in these nanoshells.

In strong coupling limit, a molecule on the surface of a metallic nanosphere is studied and the fluorescence and Raman spectra are evaluated using density matrix method.

The optical response of metallic nanoshell which includes coated dielectric particles is investigated in the longwavelength limit in which the nonlocal response of the metal is taken into account. Using our recent formalism which calculates the nonlocal multipole polarizability of such a shell, we have studied how these nanoshells interact with both the far field and near field of a light source. For the far field case, both absorption and scattering cross sections for an incident plane wave are calculated in the dipole approximation. For the near field case, we study how the fluorescence properties of an emitting molecule in the vicinity of a nanoshell are affected, by including all higher multipolar response of the shell. It is found that the nonlocal effects are most prominent for higher order multipoles, and hence for the description of molecules in close proximity interacting with the nanoshell.

We have recently demonstrated that ultra high resolution of angular measurement down to 10 -6 degree can be achieved via surface-plasmon-resonance heterodyne interferometry, in which the phase difference between p- and s- polarized reflected waves is monitored as a function of the incident angle. Here we give a brief summary of this technique and the rationale based on which such a measurement is possible. As a further study, we have also investigated, via simulation, how the change in environmental temperature will affect the resolution limit of this very versatile technique.

It is demonstrated that ultrahigh-resolution angular measurement can be achieved via surface-plasmon-resonance excitation in which the phase difference between p- and s-polarized reflected waves is monitored as a function of the incidence angle. Resolutions down to 1.9 x 10(-6) deg are obtained by performing the measurements at optimal incident wavelengths. This represents an order of magnitude improvement compared with previously reported values.

We report here theoretical investigations on the resonance fluorescence of a two‐level molecule, probed by the near field optical microscopy technique in illumination mode. The proposed model considers the influence of a coated tapered fiber probe on the radiative properties of the molecule. It is observed that both the radiative decay rate as well as the fluorescence power depend sensitively on the position and direction of the transition dipole of the molecule.

We theoretically study the exciton--LO-phonon interaction in a semiconductor quantum dot in the presence of an electric field. The confinement potential of the dot in the direction of the field is assumed to be a quantum well with various barrier heights while that in the direction transverse to the field is modeled by a parabolic form. The adiabatic approximation is employed to treat the effect of LO phonon. A trial wave function with a parameter describing the electron-hole correlation is used in the calculation of the ground-state energy of the exciton by the variational method. The magnitude of the exciton--LO-phonon interaction energy $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ and the corresponding Huang-Rhys factor S are evaluated as functions of field strength and the size of the quantum dot for different barrier heights. With the field strength, the separation between electron and hole is enhanced and thus $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ increases. Moreover, lowering of the barrier height further increases the separation, resulting in greater values of $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|.$ With a fixed strength of field, the increase of size of the quantum dot leads to both delocalization of wave packets for electrons and holes and an increase of their separation simultaneously. Consequently the size dependence of $|{E}_{\mathrm{e}\mathrm{x}\ensuremath{-}\mathrm{p}\mathrm{h}}|$ is complicated. For comparison, an additional calculation is performed for a quantum dot with the shape of a cylindrical disk and barrier of infinite height in all directions and the contrast feature is discussed.

Strong coupling between a molecule and a metallic nanosphere clad by a layer of gain medium, which provides compensation for losses in the metal, is investigated theoretically. We self-consistently solve the classical model of a molecule which is described as a damped oscillating dipole, driven by the field reflected back from the metallic surface. It is found that in a broad range of gain level, the frequencies of this coupled system exhibit a splitting substantially larger than the associated damping rate, a signature of strong coupling. As the gain–dissipation balance is optimized, strong coupling occurs even if the molecule is separated from the nanoparticle by a distance as large as several tens of nanometers. Thus through the aid of a gain medium, experimental observation of strong coupling can be easily achieved.

The theory of pump-probe femto-second time-resolved experiment will be presented. For the case in which the pump and probe pulses do not overlap, the either can be referred to as the generalized linear response theory. The fs time-resolved spectra consist of the contributions from the dynamics of both population and coherence (or phase) of the nonstationary system. Unless the dephasing is fast, the quantum beat is often observed in fs time-resolved spectra. Recently, it has been found that some polymers could exhibit semiconducting properties. In particular, the conjugated polymers, poly (phenylene-vinylene), i.e., PPV, show strong photoluminescence and can form electroluminescent layer in light-emitting diodes (LED). To study the mechanisms of the photoluminescence of PPV, fs time-resolved experiments have been performed. In this paper, the theoretical analysis of these fs time-resolved spectra will be presented. 15

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Recent application of tapered fiber tips in near field optical microscopy has demonstrated a resolution far exceed the diffraction limit. We investigate the propagation and tunneling of monochromatic light through a tapered fiber tip near a dielectric surface. The transmission rate through the tip is given for various circumstances and found to increase significantly with the dielectric constant of the sample.

We present a theoretical study on the dispersion relation for surface plasmons at a metal–Kerr medium interface where the metal is described by a nonlocal dielectric function according to the hydrodynamic model. The Kerr effect leads to a shift of the surface plasmon frequency depending on the electric field intensity, the magnitude as well as the sign of the Kerr coefficient, and the wavevector. We further observe that this frequency shift is largely quenched for large wavenumbers for which the nonlocal effect is significantly manifested. We account for this quenching quantitatively by a calculation showing how the nonlocality weakens the sensitivity in the metal's response to the variation of the dielectric function of the Kerr medium. A possible experimental observation of this new effect is briefly discussed.

The dynamical behavior of counterpropagating light beams interacting with a nonlinear medium is studied numerically. When the incident intensities are increased, this system becomes unstable and exhibits oscillations undergoing complex behavior, including quasiperiodic motion and chaos. We analyze the output fields by using Fourier spectra, phase portraits, and autocorrelation functions. In the chaotic regime, the Lyapunov exponents and dimensions are calculated. A physical interpretation of the quasiperiodic motion is also given by an exact calculation of the absorption spectrum of a two-level atom.

The principle of optical reciprocity is examined in the long wavelength limit in the presence of an isotropic nonlocal medium, by generalizing a previous result established in the literature on the symmetry of the Green's function. Our focus here is in its possible application to plasmonics, i.e.?optics with metallic nanostructures. It is shown that reciprocity will still hold as long as the dielectric function is symmetric with . Hence, under this symmetric condition, anisotropic dielectric response is necessary for the breakdown of reciprocity in the long wavelength limit. We further show in the appendix that this breakdown only occurs for an asymmetric dielectric tensor, in both the local and nonlocal response cases. An example is given to illustrate this symmetry in the problem of an emitting dipole interacting with a metallic nanoparticle whose response is described by certain dielectric functions which are both frequency and wavevector dependent.

After the publication of the above article, we have becomeaware of a key reference which had completely skipped ourattention previously (even with the help of a ‘key word’ searchusing the science citation index). This is:G.S. Agarwal and R. Inguva, Phys. Rev. B 30 (1984) 6108.Agarwal and Inguva had essentially studied the sameproblem and obtained similar results as ours. The two modelswe have studied—Maxwell-Garnett and Bruggeman—areessentially contained in the two theories of theirs: theaverage-T-matrix approximation and coherent-potentialapproximation, respectively.The only additional (relatively minor) things we have donein our paper include:(i) Study of the nonlocal (NL) effects beyond thehydrodynamic model by using the Lindhard–Mermin RPAmodel; (ii) study of also the NL effects on dispersion (besidesabsorption) of the composite material; and (iii) elucidation ofthe mechanism for the appearance of resonance in absorptionfor large metallic volume fraction in the Bruggeman modelcaused by the NL effects.We apologize for our unintentional omission of the work ofAgarwal and Inguva.

The near-field intensity distribution perpendicular to the light-propagation direction was measured by photochemical processes on conjugated-polymer thin films. The shape of the distribution is elliptical, with the long axis along the direction of the incident polarization. The results are compared with calculations based on the realistic tapered probe geometry. The asymmetry distribution is due to the simultaneous presence of horizontal and vertical electric fields in the near field.

The density matrix method is well known for being useful to theoretically treat complicated phenomena. In this work, this method will be applied to study the ultrafast energy and electron transfers in photosynthetic reaction centers. We present a microscopic model to describe the spectroscopy and dynamics for photosynthetic bacterial reaction centers (RCs). In this model, we propose eight vibrational modes and their Huang–Rhys factors for different electronic states, and the couplings between the electronic states. As applications, we have constructed steady-state and ultrafast time-resolved spectra for the wild-type RC of Rhodobacter (Rb.) sphaeroides. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1043–1054, 2000