An investigation into the tropospheric chemistry of acidic aerosols and ammonia in the laboratory

This thesis describes a broad range of experiments based on an aerosol flow-tube system to probe the interactions between atmospherically relevant aerosols with trace gases. This apparatus was used to obtain simultaneous optical and size distribution measurements using FTIR and SMPS measurements respectively as a function of relative humidity and aerosol chemical composition. Heterogeneous reactions between various ratios of ammonia gas and acidic aerosols were studied in aerosol form as opposed to bulk solutions. The apparatus is unique, in that it employed two aerosol generation methods to follow the size evolution of the aerosol while allowing detailed spectroscopic investigation of its chemical content. A novel chemiluminescence apparatus was also used to measure [NH4+]. SO2.H2O is an important species as it represents the first intermediate in the overall atmospheric oxidation process of sulfur dioxide to sulfuric acid. This complex was produced within gaseous, aqueous and aerosol SO2 systems. The addition of ammonia, gave mainly hydrogen sulfite tautomers and disulfite ions. These species were prevalent at high humidities enhancing the aqueous nature of sulfur (IV) species. Their weak acidity is evident due to the low [NH4+] produced. An increasing recognition that dicarboxylic acids may contribute significantly to the total acid burden in polluted urban environments is evident in the literature. It was observed that speciation within the oxalic, malonic and succinic systems shifted towards the most ionised form as the relative humidity was increased due to complete protonisation. The addition of ammonia produced ammonium dicarboxylate ions. Less reaction for ammonia with the malonic and succinic species were observed in comparison to the oxalic acid system. This observation coincides with the decrease in acidity of these organic species. The interaction between dicarboxylic acids and ‘sulfurous’/sulfuric acid has not been previously investigated. Therefore the results presented here are original to the field of tropospheric chemistry. SHO3-; S2O52-; HSO4-; SO42- and H1,3,5C2,3,4O4-;C2,3,4O4 2- were the main components found in the complex inorganic-organic systems investigated here. The introduction of ammonia produced ammonium dicarboxylate as well as ammonium disulfite/sulfate ions and increasing the acid concentrations increased the total amount of [NH4+].

Photonic integrated circuits for the generation of coherent optical signals

The demand for optical bandwidth continues to increase year on year and is being driven primarily by entertainment services and video streaming to the home. Current photonic systems are coping with this demand by increasing data rates through faster modulation techniques, spectrally efficient transmission systems and by increasing the number of modulated optical channels per fibre strand. Such photonic systems are large and power hungry due to the high number of discrete components required in their operation. Photonic integration offers excellent potential for combining otherwise discrete system components together on a single device to provide robust, power efficient and cost effective solutions. In particular, the design of optical modulators has been an area of immense interest in recent times. Not only has research been aimed at developing modulators with faster data rates, but there has also a push towards making modulators as compact as possible. Mach-Zehnder modulators (MZM) have proven to be highly successful in many optical communication applications. However, due to the relatively weak electro-optic effect on which they are based, they remain large with typical device lengths of 4 to 7 mm while requiring a travelling wave structure for high-speed operation. Nested MZMs have been extensively used in the generation of advanced modulation formats, where multi-symbol transmission can be used to increase data rates at a given modulation frequency. Such nested structures have high losses and require both complex fabrication and packaging. In recent times, it has been shown that Electro-absorption modulators (EAMs) can be used in a specific arrangement to generate Quadrature Phase Shift Keying (QPSK) modulation. EAM based QPSK modulators have increased potential for integration and can be made significantly more compact than MZM based modulators. Such modulator designs suffer from losses in excess of 40 dB, which limits their use in practical applications. The work in this thesis has focused on how these losses can be reduced by using photonic integration. In particular, the integration of multiple lasers with the modulator structure was considered as an excellent means of reducing fibre coupling losses while maximising the optical power on chip. A significant difficultly when using multiple integrated lasers in such an arrangement was to ensure coherence between the integrated lasers. The work investigated in this thesis demonstrates for the first time how optical injection locking between discrete lasers on a single photonic integrated circuit (PIC) can be used in the generation of coherent optical signals. This was done by first considering the monolithic integration of lasers and optical couplers to form an on chip optical power splitter, before then examining the behaviour of a mutually coupled system of integrated lasers. By operating the system in a highly asymmetric coupling regime, a stable phase locking region was found between the integrated lasers. It was then shown that in this stable phase locked region the optical outputs of each laser were coherent with each other and phase locked to a common master laser.