Date of Graduation

12-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Gregory Salamo

Committee Member

Morgan Ware

Second Committee Member

Roy Mccann

Third Committee Member

Jingyi Chen

Fourth Committee Member

Rick Wise

Keywords

Pure sciences; Gold nanorods; Graphene; Nonlinear optics

Abstract

The nonlinear studies of two-dimensional (2D) nanomaterials, specifically graphene, are very significant since graphene is finding its usefulness in handling the enormous heat in nanoscale high-density power electronics. Graphene has emerged to be a promising nanomaterial as an excellent heat spreader due to its high thermal conductivity. However, the experimental nonlinear study of graphene materials and their application in developing future optoelectronic devices demands for more developed research.

The research objective is first to build a precise, and sensitive technique to investigate and understand the thermal nonlinear properties, including nonlinear refractive index (n2), nonlinear absorption coefficient (β), and thermo-optic coefficient (dn/dT), which are dependent on the laser power and temperature of graphene thin film materials. Many techniques have been used for measuring the nonlinear parameters, and the Z-scan method has been considered the standard approach due to its simplicity and sensitivity. The main goal of this research was to investigate the thermal nonlinearities of nonfunctionlaized and functionalized graphene thin films without and with gold nanorods (NFG, AuNFG, FG, and AuFG) on glass substrates as well as gold nanorods (AuNRs) in deionized water through using the built Z-scan system. Then finally, the effects of gold nanorods on enhancing the thermal nonlinearity of graphene were studied.

In the Z-scan system, a continuous wave (cw) of an argon ion (Ar+) laser beam was used as the excitation source at wavelengths of 457 and 514 nm. The sample was fixed on a micrometer translation stage and moved in the propagation direction (z) of a narrowly focused Gaussian beam. After that, the transmitted signal, passing through the material, was recorded using a photo detector in the far field. In this dissertation work, the beta-barium borate (BBO) crystal was chosen as an excellent candidate to calibrate and test the accuracy of the built Z-scan system and to investigate the thermal nonlinearities, which was important due to the fact that the BBO crystal has remarkably high thermal and optical properties. In the linear regime, the morphology and physical properties of all the research graphene samples were investigated using different optical and structural analytical techniques, including Raman spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), and environmental scanning electron microscopy (ESEM), and UV-Visible spectrophotometer. The Z-scan experimental study indicated that graphene had a negative value of the nonlinear refractive index with a self-defocusing performance. These results also concluded that gold nanorods enhance the nonlinear thermal properties of graphene materials. Gold nanorods were proved to enhance the thermal nonlinear absorption of graphene by 50%. Also, there was a large enhancement on the thermal refraction and the change of refractive index with temperature (dn/dT) due to the presence of gold nanorods.

Therefore, using a variety of nanostructures for the ability to control the thermal-optical behaviors not only increases but also opens the doors for new application areas, including biomolecular and chemical sensing, cooling systems, photo-thermal therapy, thermal storage in solar cells, and other thermal nonlinear devices. This research contributes to increasing not only the physics, but also chemistry, knowledge about the relation between the graphene nonlinear properties, functionalization- gold nanorods and oxygen groups which, as a sequence, helps scientists and engineers to understand and improve the optical technologies in general.

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