Charles Rohde

Plasmon-assisted transparency in metal-dielectric microspheres.

We present a theoretical analysis of light scattering from a layered metal-dielectric microsphere. The system consists of two spherical resonators coupled through concentric embedding. Solving for the modes of this system, we find that near an avoided crossing the scattering cross section is dramatically suppressed, exhibiting a tunable optical transparency. Similar to electromagnetically induced transparency, this phenomenon is associated with a large group delay, which in our system is manifest as flat azimuthal dispersion.

Coherent light scattering from semicontinuous silver nanoshells near the percolation threshold.

We report on measurements of visible extinction spectra of semicontinuous silver nanoshells grown on colloidal silica spheres. We find that thin, fractal shells below the percolation threshold exhibit geometrically tunable plasmon resonances. A modified scaling theory approach is used to model the dielectric response of such shells, which is then utilized to obtain the extinction cross section in a retarded Mie scattering formalism. We show that such spherical resonators support unique plasmon dynamics: in the visible there is a new regime of coherently driven cluster-localized plasmons, while crossover to homogeneous response in the infrared predicts a delocalized shell plasmon.

Enhanced surface plasmon resonance absorption in metal-dielectric-metal layered microspheres

We present a theoretical study of the dispersion relation of surface-plasmon resonances of mesoscopic metal-dielectric-metal microspheres. By analyzing the solutions to Maxwells equations, we obtain a simple geometric condition for which the system exhibits a band of surface-plasmon modes whose resonant frequencies are weakly dependent on the multipole number. Using a modified Mie calculation, we find that a large number of modes belonging to this flat-dispersion band can be excited simultaneously by a plane wave, thus enhancing the absorption cross section. We demonstrate that the enhanced absorption peak of the sphere is geometrically tunable over the entire visible range.

Coupled-plasmon-controlled transmission in distributed Bragg structures

Using the finite element method, we investigate plasmon-mediated transmission in a periodically modulated metal-insulator-metal grating. We compute the eigenmodes of a silver-silica-silver conformal coating atop an array of close-packed silica rods, and correlate them with extinction and transmission characteristics of the structure. We observe efficient coupling of impinging plane-waves to gap plasmon modes, allowing control of both bandwidth and intensity of the transmitted radiation.

Extinction properties and left-handedness of spherical layered metallodielectric resonators

We present a theoretical study of the extinction properties of spherical metal-dielectric-metal layered structures. Our analysis shows tunable absorption and transparency, as well as possible left-handed behavior in the visible spectral range.

Plasmon-enhanced absorption and transmission in spherical Bragg resonators

We present a theoretical study of the dispersion relation of surface plasmon resonances of mesoscopic metal-dielectric-metal microspheres. These are spherically symmetric Bragg resonators comprising thin, alternating layers of dielectric and metal shells around spherical metal cores. By analyzing the solutions to Maxwells equations, we obtain a simple geometric condition for which the system exhibits a band of surface plasmon modes whose resonant frequencies are weakly dependent on the multipole number. Using a modified Mie calculation, we investigate the effect of this flat-dispersion band on the absorption and scattering cross-sections of the layered particle. We find that a large number of modes belonging to this band can be excited simultaneously by a plane wave, thus enhancing the absorption cross-section. Moreover, we observe a narrow transmission resonance due to the metallodielectric shells behaving as a transparent coating in a narrow spectral range. We demonstrate that the enhanced absorption and transmission of the sphere are geometrically tunable over the entire visible range.

Coherent Light Scattering in Percolative Metallodielectric Nanoshells

We present results from linear light scattering experiments using percolative silver nanoshells on dielectric silica cores. Combining scaling theory with core-shell Mie scattering formalism we obtain a new model for the observed signals

Percolative Metal Nanoshells for Metallodielectric Photonic Crystals

We present fabrication of percolative silver nanoshells on dielectric silica sphere cores, and their subsequent self-assembly into three dimensional metallodielectric photonic crystals. Results of light scattering experiments and modeling of the optical response are discussed.

Percolation-Enhanced Supercontinuum and Second-Harmonic Generation from Metal Nanoshells

ABSTRACT We present results for linear and nonlinear light scattering experiments from percolative silver nanoshells on dielectric silica cores. Using ultrashort pulsed laser illumination we observe strong nonlinear optical (NLO) responses from single metallodielectric core-shell (MDSC) spheres and disordered MDSC sphere aggregates. Finally, combining scaling theory with core-shell Mie scattering formalism we obtain a new model for the observed linear extinction signals. INTRODUCTION Metal-dielectric interfaces are known to support the propagation of surface electromagnetic (EM) waves with a broad spectral range.[1] These modes, known as surface-plasmon polaritons (SPPs), are coupled modes of charge-density waves and photons, and are guided at the surface of the metal-dielectric interface. The excitation and propagation of these modes are highly sensitive to the interfacial environment, and may be strongly altered with only slight perturbations to the interface. The optical response of noble metals changes dramatically when fabricated into nanoparticles.[2] The optical features which evolve with decreasing size of the metal particles may be further controlled through the materials topology. In particular, properly designed noble metal nanoshells allow accurate control of electromagnetic (EM) field distributions and subsequently their surface plasmon resonances (SPRs).[3] Metal nanoshells consist of a nano-scale metal shell (typically 10-30nm) surrounding a dielectric core, thus forming a metallodielectric core-shell (MDCS) structure. These systems exhibit unique extinction spectra, which are geometrically tunable through their core-shell thickness ratios. For core diameters in the sub-micrometer range, the optical response of the composite particles may be tuned over the entire visible and near infrared spectrum. We have developed a method, based on the Tollens process[4] to fabricate dense, highly uniform nanocrystalline silver shells with thicknesses of 25-100 nm on colloidal silica cores. The thinnest silver shells grown with this method are highly fractured and form disordered, percolative films. These thinnest films are treated as two dimensional (2d), but may also be grown into thicker 3d shells. Below we model the linear optical response of these thin shells by combining standard core-shell Mie scattering theory [5] with a scaling theory (ST) description[6] of their dielectric response. We show that for thin (2d) percolative shells the ST approach is better suited than the more commonly used Bruggeman effective medium theory (EMT)[7], while thicker (3d) shells are better modeled by the latter.[8] In addition to their linear response, nanometer-sized metal particles have been the focus of extensive studies owing to their greatly amplified nonlinear optical (NLO) response. These amplifications are attributed mainly to large enhancements of surface-induced electric fields at the nanoparticles’ plasma resonance. In a related context, localization of plasmon fields in the