This work presents recent advances in photorefractive polymer composites towards improved efficiency, speed, persistence of holograms and sensitivity at both visible and near infrared wavelengths. Besides the pure performance characteristics, a thin-device approach is presented to reduce operating voltage of these devices to practical levels and these materials are analyzed in both reflection and transmission geometries.The thin device operating at 1.3kV holds erasable, Bragg holograms with 80% efficiency in addition to its video-rate response time. The transition of hologram state from 'thick' to 'thin' is analyzed in detail. On the near IR portion of spectrum, new photorefractive polymer composites have been developed that enable high performance operation at 845nm and 975nm. Utilizing our novel photorefractive materials we demonstrate large diffraction efficiency in four-wave mixing experiments and video-rate response times. A major step towards achieving submillisecond response times is obtained through recording photorefractive gratings with individual nanosecond pulses at 532nm. At 4 mJ/cm2 illumination, a maximum diffraction efficiency of 56% has been obtained with a build-up time of only 300 microseconds (t1). This fast response enables applications in optical processing requiring frame rates of 100Hz or more. Due to the short duration of the writing pulses, the recording is insensitive to vibrations. Combining molecules that have different frontier orbital energies in a copolymer system and utilizing thermal fixing approach has led to long grating lifetimes of more than several hours. Later, in this dissertation, two low-glass-transition photorefractive polymer composites are investigated in reflection geometry. 60% is diffraction efficiency is observed in 105 micron thick devices of a PVK based composite. The reflection holograms are more sensitive to reading angle and slight birefringence due to the poling of chromophores has proven to cause a Bragg mismatch.

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No description available.

<p>The microbial life of sub-Antarctic islands plays a key role in the islands ecosystem, with microbial activities providing the majority of nutrients available for primary production. Knowledge of microbial diversity is still in its infancy and this is particularly true regarding the diversity of micro-organisms in the Antarctic and sub-Antarctic regions. One particularly important functional group of micro-organisms is the diazotrophs, or nitrogen-fixing bacteria and archaea. This group have not been well studied in the sub-Antarctic region, but play an important role in the nutrient cycling of the island. This thesis explored the diversity of nitrogen-fixing organisms in the soil of different ecological habitats on the sub-Antarctic Marion Island.</p>

Thesis (M.S.)--Humboldt State University, 2009. / Includes bibliographical references (leaves 55-57). Also available via Humboldt Digital Scholar.

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This research tested the hypotheses that (a) suboptimal root-zone temperatures (RZT) limit the soybean-Bradyrhizobium N$ sb2$-fixing symbiosis primarily through an inhibition of symbiosis establishment and (b) this inhibition is modified by the genotype of micro- or macrosymbiont. Controlled environment and field experiments were conducted utilizing two soybean genotypes and six B. japonicum strains. At 19$ sp circ$C RZT fixed nitrogen levels decreased by 30-40%, predominantly due to a restriction in the latter stages of nodule development. Reductions of 10% and 30% in specific nodule activity rates at 19$ sp circ$C and 15$ sp circ$C RZT respectively, indicated nodule function to be comparatively insensitive to low RZT. Soybean genotypes did not differ in seedling nodulation or N$ sb2$-fixation under cool-soil, field or controlled environment, conditions. At all temperatures, commercial B. japonicum strain 532C was more efficient, but not effective, than strains obtained from the cool-soils of Northern Japan. Under cool-soil field conditions, two of the latter strains increased seedling nodulation and N$ sb2$-fixation.

This project was designed to assess the ability of natural sediment microbial communities and single species microbial populations to preferentially utilize inorganic forms of nitrogen (ammonium, NH4-N, and nitrate, NO3-N, specifically). The first chapter addressed two primary questions: 1) Do sediment microbial communities preferentially assimilate NH4-N or NO3-N?; and, 2) Does preferential uptake of nitrogen change with increased NH4-N or NO3-N availability? The second chapter furthered these analyses by assessing shifts in microbial nitrogen assimilation in response to sustained nitrogen enrichments. Primary questions addressed were: 1) Are microbial communities able to adapt to nitrogen enrichment and preferentially utilize a more available source?; and, 2) Are initial microbial responses to nitrogen enrichment different from sustained responses? Questions were addressed with in vitro laboratory experiments quantifying microbial activity. Overall, microbial community activity changed in response to the form of nitrogen available, enrichment type, and duration of exposure. Data demonstrate sediment microbial communities in the Midwestern US may prefer NO3-N over other forms of nitrogen. However, microbial communities became saturated with NO3-N with increases in concentrations >0.75 mg NO3-N/L. Microbial communities were able to adapt to higher nitrogen concentration and increase rates of assimilation for both NH4-N and NO3-N. Thus, microbial communities are robust in response to nitrogen increases in and ecosystem, even in high nitrogen environments like the Midwestern US. / Preferential uptake of available nitrogen forms -- Adaptive uptake in microbial communities. / Department of Biology

<p>The microbial life of sub-Antarctic islands plays a key role in the islands ecosystem, with microbial activities providing the majority of nutrients available for primary production. Knowledge of microbial diversity is still in its infancy and this is particularly true regarding the diversity of micro-organisms in the Antarctic and sub-Antarctic regions. One particularly important functional group of micro-organisms is the diazotrophs, or nitrogen-fixing bacteria and archaea. This group have not been well studied in the sub-Antarctic region, but play an important role in the nutrient cycling of the island. This thesis explored the diversity of nitrogen-fixing organisms in the soil of different ecological habitats on the sub-Antarctic Marion Island.</p>