The increase in the magnitude of local electric fields through resonances of plasmonic excitations in metallic nanoparticles is a major area of current optical research. This dissertation is focused on plasmon-enhanced second harmonic generation of organic ionic self-assembled films via localized surface plasmon resonance of gold nanorods. By matching the plasmon resonance of the gold nanorods to the wavelength of the fundamental light, it is possible to greatly enhance the SHG efficiency. To demonstrate this, the surface of the gold nanorods was functionalized with a nonlinear-optical (NLO) polymer, PCBS, via the layer-by-layer method and deposited on a polymer thin film created on a glass substrate using the ionic self-assembled multilayer (ISAM) method.
The sample fabrication is divided into two parts: gold nanorod synthesis and functionalization. The gold nanorods were synthesized by the seed-mediated method with varying amounts of silver ions to control their LSPR wavelengths. The functionalization started by replacing the original thick CTAB bilayer on the surface of the gold nanorods by a thin PAH-DTC layer via dialysis. The nanorods were then alternately coated with PAH (polycation) and PCBS (NLO polyanion) up to three bilayers of PAH/PCBS. The number of polymer layers on the nanorods was chosen in consideration of the LSPR decay length (a few nm). The functionalized gold nanorods were then deposited on either PAH/PCBS or PAH/PSS ISAM films.
Characterization was performed via optical spectral measurement, zeta potential measurement, and field-emission scanning electron microscopy (FESEM). The LSPR wavelength shifted when the surrounding medium changed. It was red-shifted for each added polymer layer on the nanorod surface. However, when the functionalized nanorods were deposited on the ISAM film, the resonance peak blue-shifted. The zeta potential confirmed the proper electric charge of each polymer layer coated on the nanorods. Finally, FESEM was performed on the samples for visual inspection of the nanorod deposition and distribution after the SHG measurement was complete.
The SHG from the functionalized gold nanorods was measured using a Maker-like fringe method. In this method, second harmonic waves generated from the front and rear sides of the substrate interfere constructively and destructively when the sample is rotated with respect to the incoming pump wave. Electrical noise reduction techniques were implemented to improve the SHG signal readings. Signal processing was implemented using LabVIEW software in order to read a reliable SHG signal from the setup. The maximum tolerable fluence of the gold nanorods was determined in order to prevent optical damage. The interference fringe pattern was observed from the functionalized gold nanorods and compared with that from the conventional ISAM film. The enhancement from the gold nanorods was as high as 600 times compared to the bare films. Polarization dependent SHG measurements were conducted to ascertain the effect of coupling between p- or s-polarized fundamental incident light to the SH light. To further improve the SHG enhancement, the self-assembly method herein can be extended from a monolayer to multilayers of functionalized gold nanorods. / Ph. D. / The field of optics examines the interactions of light and matter. The most commonly observed optical phenomena are the reflection and refraction of light where the frequency of light remains unchanged. However, when light becomes intense, interesting optical phenomena occur where the frequency of the outgoing light differs from that of the incoming light. With the invention of the first working laser in 1960, many interesting nonlinear phenomena were experimentally confirmed, including second harmonic generation (SHG) which was the first nonlinear optical process to be observed. In the original SHG experiment, a visible ruby laser was illuminated into a quartz crystal which produced UV light. This demonstrated light frequency doubling corresponding to wavelength halving from 694 nm (ruby laser) to 347 nm (UV light).
Following progress in molecular engineering, many organic materials and polymers have been employed to study nonlinear optics for applications such as optical frequency conversion, electro-optic modulation, and second harmonic generation imaging microscopy. Nonetheless, the SHG conversion efficiency is very low due to phase-mismatch. This stems from frequency dispersion in a medium, where the incoming light and the generated light travel at different velocities. In the past, efforts toward enhancing the SHG conversion efficiency was focused on selecting specific crystals in which the incoming light and the second harmonic light pass through different effective path lengths in the medium.
Although the phase-matching method is the most effective method to achieve high conversion efficiency it is also important to increase the intrinsic nonlinearity of a material. A new multidisciplinary approach using the surface plasmon resonance has become an important technique for improving the conversion efficiency. Plasmons are the collective oscillation of electrons on a metal surface. At the resonant optical frequency, the amplitude of the plasmon oscillation becomes maximized. When metallic nanoparticles are resonantly illuminated with light, the electric field can be locally intensified at the sharp boundaries of the nanoparticle. Since the intensity of SHG increases by the square of the incoming light intensity, the SHG efficiency can be greatly enhanced via surface plasmons on the metal nanoparticles.
In this dissertation, the fabrication of new optical materials incorporating gold plasmonic nanoparticles for SHG enhancement was demonstrated. The plasmonic nanomaterials were fabricated by coating the surface of gold nanorods with nonlinear polymer films and depositing them on another polymer thin film on a flat glass substrate. The enhanced SHG intensity was measured and compared with that of the conventional nonlinear polymer films alone. It was observed that the enhancement from the gold nanorods was as high as 600 times. To further improve the SHG enhancement, the surface modified gold nanorods can be extended from a single layer to multilayers on the polymer film substrate.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/74048 |
| Date | 09 January 2017 |
| Creators | Lee, Jeong-Ah |
| Contributors | Physics, Heflin, James R., Robinson, Hans D., Park, Kyungwha, Heremans, Jean J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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