Manipulation of electromagnetic fields with plasmonic nanostructures: Nonlinear frequency mixing, optical manipulation, enhancement and suppression of photocurrent in a silicon photodiode, and surface-enhanced spectroscopy

Metallic nanostructures are one of the most versatile tools available for manipulating light at the nanoscale. These nanostructures support surface plasmons, which are collective excitations of the conduction electrons that can exist as propagating waves at a metallic interface or as localized excitations of a nanoparticle or nanostructure. Plasmonic structures can efficiently couple energy from freely propagating electromagnetic waves to localized electromagnetic fields and vice-versa, essentially acting as an optical antenna. As a result, the intensity of the local fields around and inside the nanostructure are strongly enhanced compared to the incident radiation. In this thesis, this ability to manipulate electromagnetic fields on the nanoscale is employed to control a wide range of optical phenomena. These studies are performed using structures based on metallic nanoshells, which consist of a thin Au shell coating a silica nanosphere. To investigate the parameters controlling the plasmonic response of metallic nanoshells, two changes to the nanoshell composition are studied: (1) the Au shell is replaced with Cu which has interband transitions that strongly influence the plasmon resonance, and (2) the silica core is replaced by a semiconducting Cu 2O core which has a significantly higher dielectric constant and non-trivial absorbance. The focusing of electromagnetic energy into intense local fields by plasmonic nanostructures is then directly investigated by profiling the nanoshell near field using a Raman-based molecular ruler. Next, plasmons supported by Au nanoshells are used to control the fluorescence of near-infrared fluorophores placed at controlled distances from the nanoshell surface. In this context, the analogy of an optical antenna is very relevant: the enhanced field at the surface of the nanoshell increases the absorption of light by the fluorophore, or equivalently couples propagating electromagnetic waves into a localized receiver, while the large scattering cross section enhances the coupling of energy from a localized source, the fluorophore, to far-field radiation. Excellent agreement with models based on Mie theory is achieved for both Raman and fluorescence. Experimentally measured enhancements of the radiative decay rate for fluorophores on Au nanoshells and Au nanorods are also consistent with this model. Plasmonic nanostructures can also control the flow of light into larger structures. This is observed by measuring the nanoparticle-induced enhancement and suppression of photocurrent in a silicon photodiode is at the single particle level for silica nanospheres, Au nanospheres, and two types of Au nanoshell Finally, the simultaneous physical manipulation of an individual plasmonic nanostructure on the few-nanometer scale using light and detection of the local electromagnetic field during this ongoing process with the same incident beam is performed. For this experiment, a Au nanoshell is separated from a metallic surface by a few-nanometer thick polymer layer to form a nanoscale junction, or nanogap Illuminating this structure with ultrashort optical pulses, exciting the plasmon resonance, results in a continuous, monitorable collapse of the nanogap. An easily detectable four-wave mixing (FWM) signal is simultaneously generated by this illumination of the nanogap, providing a continuous, highly sensitive optical monitor of the nanogap spacing while it is being optically reduced. The dramatic increase in this signal upon contact provides a clear, unambiguous signal of the gap closing

Nonlinear high-temperature superconducting terahertz metamaterials

We report the observation of a nonlinear terahertz response of split-ring resonator arrays made of high-temperature superconducting films. Intensity-dependent transmission measurements indicate that the resonance strength decreases dramatically (i.e. transient bleaching) and the resonance frequency shifts as the intensity is increased. Pump–probe measurements confirm this behaviour and reveal dynamics on the few-picosecond timescale.Los Alamos National Laboratory. Laboratory Directed Research and Development ProgramUnited States. Office of Naval Research (Grant N00014-09-1-1103)National Science Foundation (U.S.) (American Competitiveness in Chemistry Fellowship 1041979

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

SARS-CoV-2 Omicron is an immune escape variant with an altered cell entry pathway

Vaccines based on the spike protein of SARS-CoV-2 are a cornerstone of the public health response to COVID-19. The emergence of hypermutated, increasingly transmissible variants of concern (VOCs) threaten this strategy. Omicron (B.1.1.529), the fifth VOC to be described, harbours multiple amino acid mutations in spike, half of which lie within the receptor-binding domain. Here we demonstrate substantial evasion of neutralization by Omicron BA.1 and BA.2 variants in vitro using sera from individuals vaccinated with ChAdOx1, BNT162b2 and mRNA-1273. These data were mirrored by a substantial reduction in real-world vaccine effectiveness that was partially restored by booster vaccination. The Omicron variants BA.1 and BA.2 did not induce cell syncytia in vitro and favoured a TMPRSS2-independent endosomal entry pathway, these phenotypes mapping to distinct regions of the spike protein. Impaired cell fusion was determined by the receptor-binding domain, while endosomal entry mapped to the S2 domain. Such marked changes in antigenicity and replicative biology may underlie the rapid global spread and altered pathogenicity of the Omicron variant

Influence of dielectric function properties on the optical response of plasmon resonant metallic nanoparticles

The optical properties of plasmon resonant metallic nanoparticles are of great interest because of their ability both to control optical fields on the nanometer scale and to function as sensitive indicators of their local environment. I investigate the relationship between the dielectric function of a metal and the optical properties of the constituent metallic nanoparticle. Using a Drude shell - silica core nanoshell geometry, I examine how systematic changes in the parameters of the Drude dielectric function affect the near and far field properties of the nanoparticle. The nanoshell geometry allows separation of intrinsic properties and extrinsic phase retardation, or finite size, effects