Transparent conductive oxide materials have shown unique optical properties, such as negative refraction, hyperbolic dispersion, and epsilon-near-zero dispersion. In particular, aluminum-doped zinc oxide (Al:ZnO) has shown the most promising results over traditionally used noble metals. Pulsed layer deposition is a popular technique due to its fast and controlled growth rate, as well as the stoichiometric target-to-substrate material transfer. But, since it uses large and inhomogeneous kinetic energy, samples could be prone to macro- and microscopic defects. In this work, we investigate multilayered samples of Al:ZnO/ZnO grown by pulsed laser deposition with the goal of developing a low-loss metamaterial with hyperbolic dispersion. Different fabrication conditions, such as Al:ZnO/ZnO ratio, the thickness of an individual layer, different substrates, and deposition temperatures, were investigated. Results of the ellipsometry analysis, based on fitting spectroscopy data using the Berreman formalism, show that the hyperbolic dispersion transition (Re ε∥>0, Re ε⊥< 0) is achieved at λc=1868 nm wavelength (Im (ε⊥)~0.03) for samples with 1:4 Al:ZnO/ZnO deposition ratio. The fitted dielectric functions for samples with various parameters show that a lower deposition temperature leads to a shorter transition wavelength.
In this work, nano-transducers with a superparamagnetic iron oxide (SPIO) core have been synthesized by preparation of precursor gold nanoseeds loaded on SPIO-embedded silica to form a gold nanoshell. The goal is for such nanotansducers to be used in theranostics to detect brain tumors by using MRI imaging and then assist in their treatment by using photothermal ablation. The iron oxide core provides for the use of a magnetic-field to guide the particles to the target (tumor) site. The gold nanoshell can be then readily heated using incident light and/or an alternating magneticfield. After synthesis of nano-transducer samples, Transmission Electron Microscopy was employed to analyze the formation of each layer. Then UV spectroscopy experiments were conducted to examine the light absorbance of the synthesized samples. The UV-visible absorption spectra shows a clear surface plasmon resonance (SPR) band around 530 nm, verifying the presence of gold coating nanoshells. Finally photothermal experiments using a high-power laser beam with a wavelength of 527 nm were performed to heat the samples. It was found that the temperature reaches 45° C in 12 minutes.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods, Au/Ag core/shell
nanorods, and Au bipyramids in solution, using the longitudinal surface-plasmon resonance to enhance optical forces.
Laser light that is detuned slightly to the long-wavelength side of the resonance traps individual and multiple particles
for up to 20 minutes; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under
stable-trapping conditions, the trapping time of individual particles depends exponentially on laser power, in agreement
with a Kramers escape process. Trapped particles have their long axes aligned with the trapping-laser polarization, as
evidenced by a suppression of rotational diffusion about the short axis. When multiple particles are trapped
simultaneously, evidence of interparticle interactions is observed, including a nonlinearly increasing two-photon
fluorescence intensity, increasing fluorescence fluctuations, and changing fluorescence profiles as the trapped particle
number increases.
Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm
scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated
growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with
gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields
either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The
nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are
also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far.
Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon
resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the
Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor
composite materials may present interesting nonlinear optical properties which are being currently investigated.
Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast
laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be
attributed entirely to the heating of conduction electrons and does not exhibit any response associated with
coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.
We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic
activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena
related to electron confinement. Surprisingly, the coherent plasmon response is suppressed relative to the hot electron
response indicating enhanced plasmon dephasing. In a parallel set of studies we demonstrate nanometer scale
localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their
morphology. Position markers are fabricated on an optical and electron-transparent substrate (Si3N4 thin film) that
allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or
aggregates. The second harmonic (SH) activity optical image of individual Ag nanostructures is registered with the
TEM image. Centroid localization of the optical signals allows correlation with better than 25 nm precision. This is
sufficient to determine the origin of optical "hot spots" within multi-particle aggregates.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.
Excitation of plasmons in a metal nanoparticle leads to localization of electromagnetic fields within the particle, which is expected to result in strong optical nonlinearities. We study ultrafast nonlinearities in optical scattering from single gold nanorods under resonant excitation at the plasmon frequency, and observe changes of as much as 20% in the scattering cross section over the 20-fs laser pulse duration. Unexpectedly, the magnitude of the ultrafast nonlinearity is the same as that due to heating of conduction electrons in the metal.
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