Graded Photonic Bandgap Structures and Applications
The development of periodic nanostructured materials with specific optical properties can significantly impact the development of optical based sensors. Moreover, the development of these structures in inexpensive polymer systems can reduce much of the costs in these systems related to wavelength filtering and spectral analysis. For example, highly reflective Bragg filters can be produced by creating a multilayered polymeric structure with a modulated refractive index profile. One mechanism to produce such periodic structures is by using spatially selective photoinitiated polymerization of a pre-polymer solution containing a photoinitiator, coinitiator, and monomer (along with various other components depending on the desired end product). In order to provide a periodic illumination pattern on the order of hundreds of nanometers, optical holography using laser beams can be used. For example, there has been significant research in the use of laser based holography to control the location, and size, of nanoscale droplets of liquid crystals in a polymeric structure - the appropriately named area of holographic polymer dispersed liquid cystals ( H-PDLC ). These structures have been demonstrated to be easily tuned and designed to have desired optical switching properties. The efforts presented in this dissertation build on that prior work and can be categorized into three parts: (1) Tailoring of the H-PDLC material systems to meet application needs in terms of bandwidth, absorption spectrum, resonant wavelength peak, background fluorescence, etc.; (2) Designing new architecture using these different material systems, e.g., graded gratings; and (3) Applying these reflection gratings and graded gratings in optical sensors and spectral analysis devices. Obviously, all these efforts are interconnected with each other. Chapter 1 and 2 describe the properties of the material systems and the fabricated samples. Specifically, we present how we modified the standard H-PDLC material systems to meet specific requirements. Different solvent systems and photoinitiators were compared to determine their effects on the resulting optical performance. In Chapter 2 we present the characterization of the fabricated polymeric gratings in terms of optical transmission and reflection, surface and cross-section morphologies and thickness variations. In addition, the expected (theoretical) optical properties of the grating structures are presented and compared to the experimental results in Chapter 2. Chapter 3 presents the application of the reflection gratings in enhancing the performance of an oxygen sensor in different platforms. A PBG oxygen sensor, integrated sensor and fiber based sensor will be discussed and the enhanced sensitivity and signal to noise ratio are reported. In Chapter 4, we present the first demonstration of a one-step holographic lithography process to fabricate a rainbow colored graded structure. The detailed characterization reveals that the graded structures exist in both the cross-section and on the surface at the nanometer level and micrometer level respectively. A model of the internal structure is proposed and verified experimentally. In Chapter 5 we present and demonstrate the potential applications of graded gratings as a spectrum analyzer and reflection grating for multispectral imaging. Finally our conclusions and some proposed future directions are presented in Chapter 6. Specifically, we present a description of the use of these structures in (1) introducing defect modes into the PBG structures (2) fabricating random surface multilayer structures using Focused ion beam or Reactive ion etching (3) fabrication of transmitted graded grating in a certain wavelength range with double exposures. Other research interests can be pursued including improved fluorescence optical sensors performance and graded grating and its sensing applications in the IR range.