Excitons, Photons, and Plasmons in Novel Material Structures

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Characterizing the interaction of light and matter has become increasingly important in recent decades, as devices scale down, data transfer speeds up, and the use of photon-based technology (photonics and optoelectronics) becomes widespread. Copper chloride (CuCl) thin films and zinc oxide (ZnO) inverse opal photonic crystals are the two material systems investigated herein, and they interact with light in very different ways. CuCl is a nonlinear optical material with unique excitonic properties. CuCl thin films were fabricated by thermal evaporation and characterized by low temperature photoluminescence (PL) and angle-dependent second harmonic generation. Angle-dependent PL from CuCl films under resonant two-photon excitation revealed that the generated excitonic molecules have an angular dispersion along the laser direction, implying the creation of bipolaritons rather than biexcitons. A broad non-resonant two-photon excitation regime was discovered in CuCl pellets, but not for films. This regime is likely caused by the presence of surface and bulk defects that create an intermediate state for exciton generation. Measurements of second harmonic generation (SHG) from CuCl films revealed the strength of the SHG response is highly dependent on the stress, orientation, and quality of the growing films. The value of the second order nonlinear coefficient (d14) was determined for a CuCl thin film and found to be lower than the bulk values. Photonic crystals are metamaterials with periodic variations in the refractive index. The spacing and dimensions of the variations control how light interacts with the structure. Zinc oxide (ZnO) inverse opal photonic crystals with high filling-fraction and directional photonic pseudo-gaps in the visible spectrum were fabricated. The effects of fabrication on optical quality were quantified via PL and reflectivity measurements. Angle- and polarization-dependent reflection and PL revealed strong enhancement of spontaneous emission from ZnO due to modification of the density of states by high-order photonic bands (frozen modes). Quantum dots were embedded into the inverse opals to examine how the frozen mode affected their emission. Similar enhancement was not seen, likely due to their position in the inverse opal and variations in the local density of states. Metal nanoparticles infiltrated into the inverse opals created a structure exhibiting both photonic and plasmonic responses. This preliminary work will bring about new and exciting results for inverse opal photonic crystals.

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  • 09/20/2018
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