Jun Jun Xiao

Optical switching in graded plasmonic waveguides

A mechanism of longitudinal confinement of optical energy via coupled plasmon modes is proposed in chains of noble metal nanoparticles embedded in a graded dielectric medium, which is analogous to the confinement of electrons in semiconductor quantum wells. In these systems, one can control the transmission of optical energy by varying the graded refractive index of the host medium or the separation between the nanoparticles to realize the photonic analog of electronic transistors. Possible passband tunability by nanoparticle spacing and modulation of the refractive index in the host medium have been presented explicitly and compared favorably with numerical calculations.

Dispersion and transitions of dipolar plasmon modes in graded plasmonic waveguides

Coupled plasmon modes are studied in graded plasmonic waveguides, which are periodic chains of metallic nanoparticles embedded in a host with gradually varying refractive indices. The authors identify three types of localized modes called “light,” “heavy,” and “light-heavy” plasmonic gradons outside the passband, according to various localizations. The authors also demonstrate different transitions among extended and localized modes when the interparticle separation d is smaller than a critical dc, whereas the three types of localized modes occur for d>dc, with no extended modes. The transitions can be explained with phase diagrams constructed for the lossless metallic systems.

Global phase diagram of one-dimensional graded diatomic elastic chains: a diagrammatic approach to identifying vibrational normal modes

We study vibrational normal modes in graded diatomic chains wherein the masses m1 of one type of atom vary linearly with the gradient c while those of the other type m2 remain constant, in order to examine the diatomic effect on one-dimensional graded elastic chains. By means of a band overlapping picture—a convenient diagrammatic approach—we found six distinct kinds of vibrational mode, four of which are localized and two extended. Depending on their characteristics, we are also able to categorize these modes into acoustic and optical modes. Furthermore, investigating transitions among these rich normal modes we construct the global phase diagrams of vibrational modes in the ω–c space. All results are verified by numerical calculations in finite size systems.

Giant enhanced optical nonlinearity of colloidal nanocrystals with a graded-index host

The effective linear and third-order nonlinear optical properties of metallic colloidal crystal immersed in a graded-index host fluid are investigated theoretically. The local electric fields are extracted self-consistently based on the layer-to-layer interactions, which are readily given by the Lekner summation method. The resultant optical absorption and nonlinearity enhancement show a series of sharp peaks, which merge in a broadened resonant band. The sharp peaks become a continuous band for increasing packing density and number of layers. We believe that the sharp peaks arise from the in-plane dipolar interactions and the surface plasmon resonance, whereas the continuous band is due to the presence of the gradient in the host refractive index. These results have not been observed in homogeneous and randomly dispersed colloids, and thus would be of great interest in optical nanomaterial engineering.

Variety of normal modes and their transition behaviors in graded elastic networks : Square networks with a diagonal gradient

We study vibrational excitations in graded elastic lattices modelled by coupled harmonic oscillators in a square lattice, in which the force constants or the vibrating masses vary along a diagonal ...

Energy Relaxation in Damped Two-Dimensional Graded Elastic Lattices

We have studied the relaxation rate of vibrational modes in damped two-dimensional graded mass lattices. The relaxation rate spectrum in the weak damping limit can be obtained analytically through a perturbation theory based on the semiclassical quantum analogue envelope function. We found dip or peak structures on the relaxation rate spectrum. The dip or peak structures can be described quantitatively by the asymptotic behavior of relaxation rate at the transition frequencies. The frequency dependence of the relaxation rate is qualitatively explained by the mode patterns of gradon modes. The validity of the analytic results is confirmed by numerical solution with weak damping. In the strong damping case, we need to retain higher-order perturbations. These results can be applied to the energy relaxation in analogous systems.

Electrostatic resonances and optical responses of cylindrical clusters

We developed a Green function formalism (GFF) for computing the electrostatic resonance in clusters of cylindrical particles. In the GFF, we take advantage of a surface integral equation to avoid matching the complicated boundary conditions on the surfaces of the particles. Numerical solutions of the eigenvalue equation yield a pole spectrum in the spectral representation. The pole spectrum can in turn be used to compute the optical response of these particles. For two cylindrical particles, the results are in excellent agreement with the exact results from the multiple image method and the normal mode expansion method. The results of this work can be extended to investigate the enhanced nonlinear optical responses of metal-dielectric composites, as well as optical switching in plasmonic waveguides.