Optical nanofabrication of photopolymer based photonic bandgap structures: Materials and applications
Hsiao, Vincent K.S.
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The development of functional 1D and 2D polymeric photonic bandgap (PBG) structures is presented. This work focused on the holographic writing of transmission and reflection gratings consisting of (i) active structures where the optical properties can be modified externally and (ii) passive structures where the optical properties can be modified by infiltration into the structure. Specifically, holographic written polymer dispersed liquid crystal (H-PDLC) materials served as the initial system of interest in this dissertation. The 1D photonic bandgap structures (either transmission or reflection gratings) fabricated from these materials could have the optical properties modified (switched) electrically, thermally, or optically. This system provided significant variability and the choice of the constituent materials was essential for the application of interest. In this dissertation, we demonstrated different morphology and electro-optical switching behavior using two different LC (E7, TL202) in H-PDLC system. This dissertation presents also two photonic applications of these gratings: (a) a H-PDLC reflection grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution with an overall lasing efficiency measured to be 10%; and (b) an optically pumped and electrically switched distributed-feedback (DFB) laser consisting of a Pyrromethene 580 lasing dye-doped H-PDLC transmission grating structure. In addition to the development of active structures, this dissertation presents first demonstration of highly reflective wide-bandwidth Bragg reflectors using a photopolymer/LC/non-reactive solvent system, similar to the initial system used for active structures. The key to this fabrication method that distinguishes it from a traditional H-PDLC system is the use of a non-reactive solvent to dissolve the photoinitiator and coinitiator in the acrylate monomer/liquid crystal (LC) mixture. The addition of the non-reactive solvent results in the creation of controllable periodic voids inside the thin film. Peak reflectivity as high as 80% and a broad reflection bandwidth of 80 nm were observed in the reflection gratings. Moreover, this dissertation presents the maturation of this process whereby the use of the liquid crystals could be eliminated. Specifically, a photopolymer/formamide system is demonstrated in which the high polarity of formamide (hydrophilic) generates a micro-emulsion in the monomer (hydrophobic). The use of the resulting periodic porous polymer structures for optical detection of organic solvent vapors and for fabrication of muiti-wavelength reflectors is presented. Finally this dissertation presents some initial future directions in the optimization of these structures and applications that can be pursued using these structures.