Benjamin M. Ross

Comparison of near- and far-field measures for plasmon resonance of metallic nanoparticles

With a systematic comparison of the near- and far-field measures of plasmon resonance, we show that significant differences arise between the measures for both gold and silver spherical particles. The difference of the peak wavelengths between the near- and far-field measures increases with increasing particle size, reaching over 200 nm for a particle radius of 100 nm for both gold and silver. We physically explain these results by applying radiation damping to the quasi-static approximation, and we provide simple phenomenonological fits, which readily convert between the peak wavelengths for each measure. We expect that taking into account these differences can provide improvement in understanding and optimizing surface-enhanced spectroscopies.

Plasmon tuning and local field enhancement maximization of the nanocrescent

We present a systematic numerical study of plasmon resonance of the nanocrescent. We show that by varying the nanocrescent geometry, the plasmon resonance peak can be tuned into the near-infrared and local field enhancement can be increased significantly, with maximum enhancement of the electric field amplitude of approximately 100 for realistic geometric parameters. Because of its wide tunability, high local field enhancement, and geometry which utilizes both sharp features and intra-particle coupling, the nanocrescent is a structure well suited for in vivo cellular imaging as well as in vitro diagnostic applications.

Disassembly of a Core–Satellite Nanoassembled Substrate for Colorimetric Biomolecular Detection

The disassembly of a core-satellite nanostructured substrate is presented as a colorimetric biosensor observable under dark-field illumination. The fabrication method described herein utilizes thiol-mediated adsorption and streptavidin-biotin binding to self-assemble core-satellite nanostructures with a sacrificial linking peptide. Biosensing functionality is demonstrated with the protease trypsin, and the optical properties of the nanoassemblies are characterized. A figure of merit is presented to determine the optimal core and satellite size for visual detection. Nanoassemblies with 50 nm cores and 30 or 50 nm satellites are superior as these structures achieve an orange to green color shift greater than 70 nm that is easily discernible by the naked eye. This colorimetric substrate may prove to be a favorable alternative to liquid-based colloidal sensors and a useful visual readout mechanism for point-of-care microfluidic diagnostic assays.

Strategies for nanoplasmonic core-satellite biomolecular sensors: Theory-based Design

We present a systematic theoretical study of core-satellite gold nanoparticle assemblies using the Generalized Multiparticle Mie formalism. We consider the importance of satellite number, satellite radius, the core radius, and the satellite distance, and we present approaches to optimize spectral shift due to satellite attachment or release. This provides clear strategies for improving the sensitivity and signal-to-noise ratio for molecular detection, enabling simple colorimetric assays. We quantify the performance of these strategies by introducing a figure of merit. In addition, we provide an improved understanding of the nanoplasmonic interactions that govern the optical response of core-satellite nanoassemblies.

Omnidirectional 3D Nanoplasmonic Optical Antenna Array via Soft-Matter Transformation

Inspired by the natural processes during morphogenesis, we demonstrate the transformation capability of active soft-matter to define nanoscale metal-on-polymer architectures below the resolution limit of conventional lithography. Specifically, using active polymers, we fabricate and characterize ultradense nanoplasmonic antenna arrays with sub-10 nm tip-to-tip nanogaps. In addition, the macroscale morphology can be independently manipulated into arbitrary three-dimensional geometries, demonstrated with the fabrication of an omnidirectional nanoplasmonic optical antenna array.

Creating high density nanoantenna arrays via plasmon enhanced particle-cavity (PEP-C) architectures.

We propose a new solution for high hot-spot density creation by coupling a particle and a cavity in a structure dubbed a plasmonic enhanced particle-cavity (PEP-C) antenna. In comparison to analogous particle-based dimer antenna structures, the PEP-C allows both a higher maximum field and an order-of-magnitude higher hot-spot density. In addition, the hot-spots of the PEP-C antenna can be precisely controlled, resulting in increased reliability. We elucidate the photonic characteristics of the PEP-C antenna and show tuning and optimization through choice of geometric parameters. These properties make the PEP-C antenna an excellent candidate for plasmonic-based biomolecular sensors.

On negative-phase-velocity propagation in the ergosphere of a charged rotating black hole

The presence of charge promotes the propensity of a rotating black hole to support the propagation of electromagnetic plane waves with negative phase velocity (NPV) in its ergosphere, whether the Kerr–Newman or the Kerr–Sen metric descriptions of spacetime are considered. Striking differences in the NPV characteristics for the Kerr–Newman and Kerr–Sen metrics emerge from numerical studies, particularly close to the outer event horizon when the magnitude of the charge is large.

Bruggeman approach for isotropic chiral mixtures revisited

Two interpretations of the Bruggeman approach for the homogenization of isotropic chiral mixtures are shown to lead to different results. Whereas the standard interpretation is shown to yield the average polarizability density approach, a recent interpretation turns out to deliver a null excess polarization approach. The difference between the two interpretations arises from differing treatments of the local field.

Plasmon resonance differences between the near- and far-field and implications for molecular detection

The localized surface plasmon resonance (LSPR) of a nanoplasmonic particle is often considered to occur at a single resonant wavelength. However, the physical measures of plasmon resonance, namely the far-field measures of scattering, absorption, and extinction, and the near-field measures of surface-average or maximum electric field intensity, depend differently on the particle polarizability, and hence may be maximized at different wavelengths. We show using analytic Mie theory that the differences in peak wavelength between the near- and far-fields can reach over 200 nm for nanoparticle sizes commonly used in spectroscopy applications. Using finite element analysis, we also consider the effect of varying particle shape to spheroidal geometries, and consider polarization dependence. Using the quasi-static and extended quasi-static approximation, we show that the differences between the near- and far- field measures of plasmon resonance can be largely explained by radiation damping effects. We suggest that accounting for these differences is relevant both for optimizing device design, and for improving fundamental understanding of surface-enhanced mechanisms such as surface-enhanced Raman spectroscopy (SERS).

Plasmonic nanoflowers: bioinspired manipulation of plasmonic architectures via active polymers

While technology relies on components defined in a fixed position on a rigid substrate, nature prefers soft substrates, and allows components to move significantly during morphogenesis. Taking inspiration from biological fabrication, we have developed a technique, called active polymer nanofabrication, which utilizes thermally active polymers to create complex nanoplasmonic substrates designed for molecular detection. We demonstrate the ability of active polymer nanofabrication to create ultra-dense nanoplasmonic prism arrays (plasmonic nanoflowers), and correlate changes in array morphology with optical properties. We investigate the associated changes in local electromagnetic fields with finite element analysis. Finally, we demonstrate the ability of active polymers to deform macroscopically while retaining nanostructure morphology. We expect these properties will make active polymer nanofabrication useful for a wide range of nanoplasmonic devices.