Electrostatic Confinement, Patterning, and Manipulation of Charged Nanoparticles by Combining Nanostructured Surfaces and Ionic Charge Regulation.

Electrostatic forces are amongst the most versatile when applied to mediate the interactions between nanostructured interfaces. Depending on the experimental conditions, these forces can be either attractive or repulsive, and their directionality can be controlled dynamically. In this dissertation, we employ these forces to confine and manipulate charged nanoparticles using nanostructured interfaces. The various methodologies discussed herein inform and complement each other while opening pathways for diversified applications. Electrostatic confinement of nanoscale species in solution has far-reaching effects in fields as diverse as biophysics, gene therapy, single-particle motion monitoring studies and bottom-up fabrication of nanostructures. We present a methodology to uniaxially confine charged nanoparticles on one-dimensional electrodes without the usage of geometrical barriers. An actively-tunable, engineered model system for electrostatic binding interactions is demonstrated and interaction characteristics are discussed in relation to mimicking the natural biological interaction between charged species. We further investigate the electrostatic interactions between nanoparticles and patterned sinusoidal-void structures. A size-selective nanoparticle confinement and patterning technique is demonstrated. In addition, ionic charge regulation in the electrical double layer, its ramifications and its applications are discussed. In many particle-fractionation applications, complementary geometries are critical for understanding confinement characteristics and so a novel methodology is introduced to detect and visualize relative size variations in pre-characterized nanoparticle ensembles. This capped particle optical-sizing methodology is easily accessible, has high-throughput, and is relatively facile when compared to existing size-characterization techniques. Finally, a nanoparticle-manipulation-based transparent display concept is demonstrated that has been supplemented by our enhanced understanding of the above mentioned confinement methodologies of electrostatically confining charged nanoparticles in solution.PhDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120879/1/ashwinp_1.pd

Surface scattering contribution to the plasmon width in embedded Ag nanospheres

Nanometer-sized metal particles exhibit broadening of the localized surface plasmon resonance (LSPR) in comparison to its value predicted by the classical Mie theory. Using our model for the LSPR dependence on non-local surface screening and size quantization, we quantitatively relate the observed plasmon width to the nanoparticle radius $R$ and the permittivity of the surrounding medium $\epsilon_m$. For Ag nanospheres larger than 8 nm only the non-local dynamical effects occurring at the surface are important and, up to a diameter of 25 nm, dominate over the bulk scattering mechanism. Qualitatively, the LSPR width is inversely proportional to the particle size and has a nonmonotonic dependence on the permittivity of the host medium, exhibiting for Ag a maximum at $\epsilon_m\approx2.5$. Our calculated LSPR width is compared with recent experimental data.Comment: 11 pages, 4 figures. Accepted for publication in Optics Expres

Theoretical approach to atomic-scale nanoplasmonics as probed by light and swift electrons

223 P.This thesis tackles the theoretical description of atomic-scale features in plasmonic nanostructures asprobed by light and swift electrons. Plasmonic nanostuctures are known to localize and enhanceelectromagnetic fields in their proximity, and thus serve as building blocks to perform improved andenhanced molecular spectroscopy on them. We focus on the analysis of the effect of atomic-scale featuresin the overall response of plasmonic nanoparticles and nanocavities. We apply ab initio atomisticquantum time-dependent density functional theory (TDDFT) to unveil the near-field distribution aroundmetallic antennas, and describe "classically" various atomic-scale features such as continuous protrusionson the surfaces of the metal using a Boundary Element Method (BEM), providing an extra localization ofthe field. Moreover, we propose an analytical model to address the signal increase observed in surfaceenhancedRaman scattering (SERS) spectra related to local variations of the electron density associated toatomic-scale defects. Last, we identify the excitation of confined bulk plasmons (CBP) within theTDDFT calculations for the electron energy loss (EEL) probability of atomistic clusters, and provide asemi-analytical expression within a Hydrodynamic Model (HDM) to address such excitation

Quantum Plasmonics

Quantum plasmonics is an exciting subbranch of nanoplasmonics where the laws of quantum theory are used to describe light–matter interactions on the nanoscale. Plasmonic materials allow extreme subdiffraction confinement of (quantum or classical) light to regions so small that the quantization of both light and matter may be necessary for an accurate description. State-of-the-art experiments now allow us to probe these regimes and push existing theories to the limits which opens up the possibilities of exploring the nature of many-body collective oscillations as well as developing new plasmonic devices, which use the particle quality of light and the wave quality of matter, and have a wealth of potential applications in sensing, lasing, and quantum computing. This merging of fundamental condensed matter theory with application-rich electromagnetism (and a splash of quantum optics thrown in) gives rise to a fascinating area of modern physics that is still very much in its infancy. In this review, we discuss and compare the key models and experiments used to explore how the quantum nature of electrons impacts plasmonics in the context of quantum size corrections of localized plasmons and quantum tunneling between nanoparticle dimers. We also look at some of the remarkable experiments that are revealing the quantum nature of surface plasmon polaritons

Theory of self-induced back-action optical trapping in nanophotonic systems

Optical trapping is an indispensable tool in physics and the life sciences. However, there is a clear trade off between the size of a particle to be trapped, its spatial confinement, and the intensities required. This is due to the decrease in optical response of smaller particles and the diffraction limit that governs the spatial variation of optical fields. It is thus highly desirable to find techniques that surpass these bounds. Recently, a number of experiments using nanophotonic cavities have observed a qualitatively different trapping mechanism described as "self-induced back-action trapping" (SIBA). In these systems, the particle motion couples to the resonance frequency of the cavity, which results in a strong interplay between the intra-cavity field intensity and the forces exerted. Here, we provide a theoretical description that for the first time captures the remarkable range of consequences. In particular, we show that SIBA can be exploited to yield dynamic reshaping of trap potentials, strongly sub-wavelength trap features, and significant reduction of intensities seen by the particle, which should have important implications for future trapping technologiesComment: 7 pages, 5 figure

Theoretical approach to atomic-scale nanoplasmonics as probed by light and swift electrons

223 P.This thesis tackles the theoretical description of atomic-scale features in plasmonic nanostructures asprobed by light and swift electrons. Plasmonic nanostuctures are known to localize and enhanceelectromagnetic fields in their proximity, and thus serve as building blocks to perform improved andenhanced molecular spectroscopy on them. We focus on the analysis of the effect of atomic-scale featuresin the overall response of plasmonic nanoparticles and nanocavities. We apply ab initio atomisticquantum time-dependent density functional theory (TDDFT) to unveil the near-field distribution aroundmetallic antennas, and describe "classically" various atomic-scale features such as continuous protrusionson the surfaces of the metal using a Boundary Element Method (BEM), providing an extra localization ofthe field. Moreover, we propose an analytical model to address the signal increase observed in surfaceenhancedRaman scattering (SERS) spectra related to local variations of the electron density associated toatomic-scale defects. Last, we identify the excitation of confined bulk plasmons (CBP) within theTDDFT calculations for the electron energy loss (EEL) probability of atomistic clusters, and provide asemi-analytical expression within a Hydrodynamic Model (HDM) to address such excitation

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-ofstates engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of optical antennas based on the background of both well-developed radiowave antenna engineering and the emerging field of plasmonics. In particular, we address the plasmonic behavior that emerges due to the very high optical frequencies involved and the limitations in the choice of antenna materials and geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.Comment: Review article with 76 pages, 21 figure