Percolation-Enhanced Supercontinuum and Second-Harmonic Generation from Metal Nanoshells
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Percolation-Enhanced Supercontinuum and Second-Harmonic Generation from Metal Nanoshells Charles Rohde, Keisuke Hasegawa, Aiqing Chen and Miriam Deutsch Oregon Center for Optics and Department of Physics, University of Oregon, Eugene, OR 97403, USA [email protected] 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 coreshell 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 material's 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 nanoscale 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 Tollen's process[4] to fabricate dense, highly uniform nanocrystalline silver shells with thicknesses of 25100 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
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