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The resonant interaction of light with metal nanoparticles can result in extraordinary optical effects in both the near and far fields. Plasmonics, the study of this interaction, has the potential to enhance performance in a wide range of applications, including sensing, photovoltaics, photocatalysis, biomedical imaging, diagnostics, and treatment. However, the mechanisms of plasmonic enhancement often remain poorly understood, limiting the design and effectiveness of plasmonics for advanced applications. This dissertation focuses on evaluating the mechanisms of plasmonic enhancement and distinguishing between near and far field effects using simulations and experimental results. Thorough characterization of metal nanoparticle colloids shows that electromagnetic simulations can be used to accurately predict the optical response of nanoparticles only if the true shapes and size distributions are taken into account. By coupling these optical interaction calculations with heat transfer models, experimental limits for the maximum optical power before nanoparticle melting can be found. These limits are important for plasmonic multiphoton luminescence imaging applications. Subsequently, we demonstrate ultrafast laser plasmonic nanoablation of silicon substrates using gold nanorods to identify the near-field enhancement and mechanism of plasmon-assisted ablation. The experimentally observed shape of the ablation region and reduction of the ablation threshold are compared with simulations to show the importance of the enhanced electromagnetic fields in near-field nanoablation with plasmonic nanoparticles. The targeted use of plasmonic nanoparticles requires narrow size distribution colloids, because wide size distributions result in a blurring and weakening of the optical response. A new synthesis method is presented for the seeded-growth of nearly monodisperse metal nanoparticles ranging from 10 to 100 nm in diameter, both with and without dielectric shells of controlled thickness. This method is used to acquire fine control over the position and width of the plasmonic peak response. We also demonstrate self-assembled sub-monolayers of these particles with controllable concentrations, which is ideal for looking at plasmonic effects in surface and layered geometries. Finally, we present results for the spatial distribution of absorption around plasmonic nanoparticles. We introduce field-based definitions for distinguishing near-field and far-field regions and develop a new set of equations to determine the point-by-point enhanced absorption in a medium around a plasmonic nanoparticle. This set of equations is used to study plasmon-enhanced optical absorption for thin-film photovoltaic cells. Plasmonic nanoparticle systems are identified using simulations and proof-of-concept experiments are used to demonstrate the potential of this approach.

The resonant interaction of light with metal nanoparticles can result in extraordinary optical effects in both the near and far fields. Plasmonics, the study of this interaction, has the potential to enhance performance in a wide range of applications, including sensing, photovoltaics, photocatalysis, biomedical imaging, diagnostics, and treatment. However, the mechanisms of plasmonic enhancement often remain poorly understood, limiting the design and effectiveness of plasmonics for advanced applications. This dissertation focuses on evaluating the mechanisms of plasmonic enhancement and distinguishing between near and far field effects using simulations and experimental results. Thorough characterization of metal nanoparticle colloids shows that electromagnetic simulations can be used to accurately predict the optical response of nanoparticles only if the true shapes and size distributions are taken into account. By coupling these optical interaction calculations with heat transfer models, experimental limits for the maximum optical power before nanoparticle melting can be found. These limits are important for plasmonic multiphoton luminescence imaging applications. Subsequently, we demonstrate ultrafast laser plasmonic nanoablation of silicon substrates using gold nanorods to identify the near-field enhancement and mechanism of plasmon-assisted ablation. The experimentally observed shape of the ablation region and reduction of the ablation threshold are compared with simulations to show the importance of the enhanced electromagnetic fields in near-field nanoablation with plasmonic nanoparticles. The targeted use of plasmonic nanoparticles requires narrow size distribution colloids, because wide size distributions result in a blurring and weakening of the optical response. A new synthesis method is presented for the seeded-growth of nearly monodisperse metal nanoparticles ranging from 10 to 100 nm in diameter, both with and without dielectric shells of controlled thickness. This method is used to acquire fine control over the position and width of the plasmonic peak response. We also demonstrate self-assembled sub-monolayers of these particles with controllable concentrations, which is ideal for looking at plasmonic effects in surface and layered geometries. Finally, we present results for the spatial distribution of absorption around plasmonic nanoparticles. We introduce field-based definitions for distinguishing near-field and far-field regions and develop a new set of equations to determine the point-by-point enhanced absorption in a medium around a plasmonic nanoparticle. This set of equations is used to study plasmon-enhanced optical absorption for thin-film photovoltaic cells. Plasmonic nanoparticle systems are identified using simulations and proof-of-concept experiments are used to demonstrate the potential of this approach. / text

This Master's thesis presents numerical and experimental results that benchmark a state-of-the-art terahertz time-domain spectrometer. We begin by describing the theory behind the nonlinear optical mechanisms through which we generate and detect short pulses of THz radiation. Based on a coherent electro-optic detection scheme, our measurements trace out the oscillating electric field of the THz pulses generated from the optical mixing process of optical rectification. A numerical simulation based on the theory presented in this work helps present the physical intuition behind our use of these nonlinear optical processes and is furthermore used to complement our measurements with theory. Utilizing the simultaneous amplitude and phase information provided by our detection scheme, we perform terahertz time-domain spectroscopy on sample materials. Our samples of interest are the III-V zinc-blende semiconductors GaP and ZnTe which are nonlinear media popular for their advantageous dispersive and absorptive properties in the terahertz range. Therefore, the thesis culminates in the demonstration of a material parameter extraction procedure which we use to obtain the complex refractive index of a GaP crystal and a ZnTe crystal.

Group III nitrides are efficient light emitters. The modification of internal optoelectronic properties of these materials due to strain, external or internal electric field are an area of interest. Insertion of metal nanoparticles (MNPs) (Ag, Au etc) inside the V-shaped inverted hexagonal pits (IHP) of InGaN/GaN quantum wells (QWs) offers the potential of improving the light emission efficiencies. We have observed redshift and blueshift due to the Au MNPs and Ag MNPs respectively. This shift could be due to the electric field created by the MNPs through electrostatic image charge. We have studied the ultrafast carrier dynamics of carriers in hybrid InGaN/GaN QWs. The change in quantum confinement stark effect due to MNPs plays an important role for slow and fast carrier dynamics. We have also observed the image charge effect on the ultrafast differential transmission measurement due to the MNPs. We have studied the non-linear absorption spectroscopy of these materials. The QWs behave as a discharging of a nanocapacitor for the screening of the piezoelectric field due to the photo-excited carriers. We have separated out screening and excitonic bleaching components from the main differential absorption spectra of InGaN/GaN QWs.

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NR 20140805

The mechanical tools currently used in orthopedic and dental surgery are imprecise and may cause heat damage. Ultrashort pulse lasers are a promising replacement, but their ablation efficiency must be improved. The goal of this thesis was to achieve high ablation efficiency, precision, and minimal collateral damage using an ultrafast laser on bovine hard tissue. This work used two types of lasers: a Ti:Sapphire laser (210 fs, 800 nm, 1 kHz) and a fiber laser (1 ps, 1035 nm, 100 kHz - 1 MHz). This thesis begins with a review of the literature on laser-tissue interactions and the effect of certain laser parameters on the ablation process. The next section uses a Ti:Sapphire laser and bovine bone to explore the properties of laser-tissue interactions, including ablation threshold and incubation coefficient. Results showed that as the number of incident pulses goes up, ablation threshold goes down. The threshold range went from 1.08 ± 0.15 J/cm2 at 25 incident pulses to 0.73 ± 0.12 J/cm2 at 1000 pulses. The incubation coefficient, S, was calculated to be 0.90 ± 0.02. The relationship between ablation depth and fluence, scanning speed, and number of successive passes was characterized as a first step towards preparing large-cavity with high removal efficiency using a Ti:Sapphire and fiber lasers. Depth increased with fluence and number of passes, but it decreased with scanning speed. The influence of environmental conditions including air, compressed air flow, still water and flowing water on cavity ablation depth, and rate was investigated using a Ti:Sapphire laser with aim to enhance ablation efficiency. Findings showed that the deepest cavities and fastest ablation rates were achieved with compressed air flow. Air flow also resulted in the most precise cuts, the smoothest surfaces, and the absence of microcracks. This thesis also used a fiber laser to explore the effect of fluence and repetition rate on removal rate and ablation quality. Results indicated that ablation rate increases with fluence and pulse rate. When the repetition rate exceeded 600 kHz, the laser caused thermal and mechanical damage, indicated by the presence of amorphous carbon. The effect of environmental conditions and laser parameters such as repetition rate provide valuable insights into the ultrafast laser ablation mechanisms for medicine and biology field. / Thesis / Doctor of Philosophy (PhD)

Frequency-resolved optical gating (FROG) is the standard for measuring femtosecond laser pulses. It measures relatively simple pulses on a single-shot and complex pulses using multi-shot scanning and averaging. However, experience from intensity autocorrelation suggests that multi-shot measurements may suffer from a coherent artifact caused by instability in the laser source. In this thesis, the coherent artifacts present in modern pulse measurement techniques are examined and single-shot techniques for measuring complex pulse(s) are proposed and demonstrated. The study of the coherent artifact in this work shows that modern pulse measurement techniques also suffer from coherent artifacts and therefore single-shot measurements should be performed when possible. Here, two single-shot experimental setups are developed for different scenarios. First, an extension of FROG is developed to measure two unknown pulses simultaneously on a single-shot. This setup can measure pulses that have very different center wavelengths, spectral bandwidths, and complexities. Second, pulse-front tilt is incorporated to extend the temporal range of single-shot FROG to tens of picoseconds which traditionally can only be attained by multi-shot scanning. Finally, the pulse-front tilt setup is modified to perform a single-shot measurement of supercontinuum, one of the most difficult pulses to measure due to its long temporal range, broad spectral bandwidth, and low pulse energy.