One-dimensional and three-dimensional photonic crystals created using atomic layer deposition

Zachary Aspen Sechrist
Abstract & Cover

Photonic crystals (PCs) manipulate the flow of light via Bragg reflections. Potential applications for such crystals include low threshold lasers, low loss waveguides, highly efficient light bulb filaments, and many other exciting products. While predictions for PC applications grow very rapidly, the realization of these structures has been relatively slow. This thesis explores ways in which atomic layer deposition (ALD) may be used to create, or modify PCs. One-dimensional PCs can be constructed entirely from ALD films. Thin alternating layers of tungsten (W) and alumina (Al2O3) were deposited in a viscous flow reactor and studied using X-ray reflectivity, X-ray diffraction, quartz crystal microbalance, secondary ion mass spectrometry, and transmission electron microscopy. The optimization of thin film growth and nucleation presented in this thesis led to thin film stacks that displayed ultrahigh reflectivity in the hard X-ray regime, and very low thermal conductivity. Three-dimensional PCs were modified with ALD. The first in depth investigation of intensity and position of a Bragg reflection as a function of high index fill fraction will be shown. This study investigated Al2O3 ALD growth rates inside of PCs, and allows for predictions of how all other ALD systems are expected to behave in a similar system. The extent of red-shift for the Bragg peak of this system also revealed the degree of disorder present in the PC prior to deposition. Three-dimensional PCs were also coated with W metal. This system created a photonic band gap (PBG), which is a section photon energy that cannot propagate through the PC. The location of the PBG was adjusted by varying the lattice constant of the PC. This system was tuned to interact with infrared (IR) and ultraviolet (UV) light. This was the first demonstration of a 3-dimensional metal PBG in the UV region. Modified 3-dimensional PCs were studied with scanning electron microscopy to examine the structure of the crystals. UV-visible-IR spectroscopy was used to track optical response changes of the crystal as it was coated with ALD. The reflectance spectra were compared with transfer matrix method numerical simulations to understand the rate and uniformity of ALD on the crystal.

Source of Information
University of Colorado Boulder
(Boulder, USA)
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