Browsing by Author "Su, Man-Nung"

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    Optical Properties and Ultrafast Electron Dynamics in Gold, Aluminum and Hybrid Nanomaterials
    (2018-08-08) Su, Man-Nung; Link, Stephan
    Novel nanomaterials have been attracting numerous attention for their enhanced properties which lead to many potential applications. Plasmonic nanostructures, supported by surface plasmon resonances, possess efficient hot carrier generation and manipulatable optical properties and are great candidates for applications such as solar cells, photocatalysis, etc. To efficiently utilize these properties in real-life applications, a fundamental understanding of the optical properties is necessary. In this dissertation, I study the optical properties and ultrafast electron dynamics of novel nanomaterials, including gold nanostructures fabricated by lithography, aluminum nanostructures as emerging plasmonic nanomaterials, and hybrid nanostructures including gold nanoblock dimers and “hedgehog” particles. I utilize single-particle spectroscopy combined with pump-probe transient extinction spectroscopy as a powerful tool to resolve the structural-optical relationship for nanomaterials. In the first part of the dissertation, the optomechanics of lithographically fabricated nanostructures are investigated for their advantages of better control on the size, shape, and material composition. I reveal the roles of adhesion layers and polycrystallinity in lithographically fabricated gold nanostructures on their acoustic vibrations. The thickness of the very thin adhesion layers greatly impacts the acoustic vibration frequencies. The vibration damping channel is dominated by the polycrystallinity. Lithography also supports nanoparticle clusters with precise interparticle geometry control. Taking advantage of this, I discover a mechanical coupling though substrates in such gold nanoparticle cluster. This mechanical coupling is a breakdown of classical continuum elastic theory. In the second part of the dissertation, aluminum nanostructures are studied as a great alternative to noble metals for their wider spectral tunability and lower cost. I investigate their ultrafast dynamics and reveal the effects of their native oxide layers and unique Drude-like electron structure on their optomechanical and optical response. The native oxide layer could be a key for longer-lived hot electrons through trapping at the core/shell interface. In the third part of the thesis, I studied the steady-state optical properties of composite nanostructures. Gold nanoblock dimers with edge-to-edge configurations possess strong localized electric field enhancement. I study the effect of interparticle distance over a wide range on their optical properties. Hedgehog particles, composed of a polystyrene core with ZnO spikes, possess special wettability that makes them dispersed in both hydrophilic and hydrophobic solvents. I reveal their optical properties that are very insensitive to spike geometry and environment. The findings presented in this dissertation bring deeper and more detailed understanding to the optical properties and ultrafast dynamics for such nanostructures.
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    Single-Particle Absorption Spectroscopy by Photothermal Contrast
    (American Chemical Society, 2015) Yorulmaz, Mustafa; Nizzero, Sara; Hoggard, Anneli; Wang, Lin-Yung; Cai, Yiyu; Su, Man-Nung; Chang, Wei-Shun; Link, Stephan; Laboratory for Nanophotonics
    Removing effects of sample heterogeneity through single-molecule and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here we present an approach capable of recording pure absorption spectra of individual nanostructures. We demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, we are furthermore able to record absorption spectra of single gold nanorods with different aspect ratios. We find that the spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.
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    Tuning the acoustic frequency of a gold nanodisk through its adhesion layer
    (Nature Publishing Group, 2015) Chang, Wei-Shun; Wen, Fangfang; Chakraborty, Debadi; Su, Man-Nung; Zhang, Yue; Shuang, Bo; Nordlander, Peter; Sader, John E.; Halas, Naomi J.; Link, Stephan; Laboratory for Nanophotonics
    To fabricate robust metallic nanostructures with top-down patterning methods such as electron-beam lithography, an initial nanometer-scale layer of a second metal is deposited to promote adhesion of the metal of interest. However, how this nanoscale layer affects the mechanical properties of the nanostructure and how adhesion layer thickness controls the binding strength to the substrate are still open questions. Here we use ultrafast laser pulses to impulsively launch acoustic phonons in single gold nanodisks with variable titanium layer thicknesses, and observe an increase in phonon frequencies as a thicker adhesion layer facilitates stronger binding to the glass substrate. In addition to an all-optical interrogation of nanoscale mechanical properties, our results show that the adhesion layer can be used to controllably modify the acoustic phonon modes of a gold nanodisk. This direct coupling between optically excited plasmon modes and phonon modes can be exploited for a variety of emerging optomechanical applications.