Characterization of plasmonic effects in thin films and metamaterials using spectroscopic ellipsometry

The importance of plasmonic nanoparticles in sensor development, imaging, photodiagnostics, and optoelectronics has resulted in a strong interest toward the development of straightforward synthetic methods for their preparation. In this article, we review some of the most significant advances that have been made in the synthesis of plasmonic bimetallic nanoparticles. Approaches to control morphology, with an emphasis on reactions that produce uniform particles with well-defined sizes and shapes and in high yields, are described. In addition, several characterization techniques that have been employed to elucidate the morphology of the particles are illustrated.

Recent Advances in the Synthesis of Plasmonic Bimetallic Nanoparticles

For two-dimensional (2D) arrays of metallic nanorods arranged perpendicular to a substrate several methods have been proposed to determine the electromagnetic near-field distribution and the surface plasmon resonances, but an analytical approach to explain all optical features on the nanometer length scale has been missing to date. To fill this gap, we demonstrate here that the field distribution in such arrays can be understood on the basis of surface plasmon polaritons (SPPs) that propagate along the nanorods and form standing waves. Notably, SPPs couple laterally through their optical near fields, giving rise to collective surface plasmon (CSP) effects. Using the dispersion relation of such CSPs, we deduce the condition of standing-wave formation, which enables us to successfully predict several features, such as eigenmodes and resonances. As one such property and potential application, we show both theoretically and in an experiment that CSP propagation allows for polarization conversion and optical filtering in 2D nanorod arrays. Hence, these arrays are promising candidates for manipulating the light polarization on the nanometer length scale.

Polarization conversion through collective surface plasmons in metallic nanorod arrays.

We demonstrate the ability to characterize independently the vector components of the electric field associated with terahertz surface plasmons. This is accomplished via electro-optic sampling, using an electro-optic crystal placed in close proximity to a corrugated metal foil. The individual electric field vector components are measured using two separate ZnTe crystals. Since ZnTe exhibits isotropic dielectric properties, all of the detection configurations obey identical phase-matching constraints. Furthermore, since ZnTe is characterized by a single independent electro-optic tensor component, the field measurements may be directly compared against one another.

Electric field vector characterization of terahertz surface plasmons

The bimetallic core–shell nanoparticles show unique plasmonic properties and their preparations and characterizations are currently under investigation. A new type of Au core–Ag shell (Au@Ag) nanoparticles is prepared by sandwiching the chemically attached Raman reporter molecules (RRMs) and a 12-base-long oligonucleotide between the 13 nm average size core-gold nanoparticles (AuNPs) and 9 nm and 21 nm average size of Ag shell. The synthesized Au@Ag nanoparticles are tested for their surface-enhanced Raman scattering (SERS) performance. It is found that the chemical attachment of the oligonucleotides along with the RRM improved the enhancement in Raman scattering more than one order of the magnitude with the Au@Ag nanoparticles with an average 9-nm shell thickness while the Au@Ag nanoparticles with 21 nm average shell thickness have poor SERS activity. A minimum enhancement factor of 1.0 × 107 is estimated for the SERS active oligonucleotide-mediated Au@Ag nanoparticles. The approach may provide new routes for preparation of highly sensitive new generation of bimetallic core–shell nanoparticles.

Oligonucleotide-Mediated Au–Ag Core–Shell Nanoparticles

Most semiconductor devices contain at least one junction between p-type and n-type material. P—n junction theory serves as the foundation of the physics of semiconductor devices. The basic theory of current—voltage characteristics of p—n junctions was established by Shockley (1949). This theory was then expanded by Sah et al. (1957) and Moll (1958).

J. J. Liou

Papers

P—N Junction