Summary of Research Interests
My research interests focus upon the development and application of diode laser-based spectroscopic techniques to a variety of fundamental and applied problems in gas phase chemistry. The research seeks to employ cutting edge optical material technology to produce continuous wave, narrow band, high power, single mode laser sources operating in the uv and mid-IR spectral regions and to use such radiation in conjunction with sensitive time-resolved absorption methods for novel experiments in reaction dynamics, plasmas diagnostics and aeronomy.
Frequency up and down conversion in periodically poled materials
High performance, low noise single mode diode lasers are commercially available in the wavelength range 635nm to 1.7µm; this region however is not very interesting for diagnostic purposes as most electronic transitions lie at shorter wavelengths (uv), and fundamental vibrational transitions are at considerably longer wavelengths (mid-IR). Such wavelengths can however be generated by nonlinear optical methods. These methods are generally inefficient when using standard birefringent crystals in conjunction with continuous wave (cw) diode lasers and my research circumvents this by employing an alternative methodology which relies upon nonlinear interactions in novel periodically poled crystals and waveguides. These ferroelectric materials offer a sufficiently enhanced nonlinearity compared to conventional birefringent crystals such that enhancement cavities are not required and thus they are robust sources of high power uv or mid-IR radiation that has (inherently) low amplitude noise. Currently, uv generation is carried out using periodically poled magnesium oxide doped lithium tantalate, (PPMgOLT), to produce radiation down to 325nm while production of mid-IR radiation (approx. =3.3µm) is achieved by difference frequency generation with a YAG laser and a selection of diode lasers (operating in the telecommunications band(s)) in periodically poled lithium niobate (PPLN).
(i) Reaction dynamics
The objective of this research area is to study the fundamental vector properties of chemical processes by developing and applying sensitive diode laser based absorption techniques. An example of a vector property is the relationship between the velocity vector of a reaction product and its angular momentum vector and such properties have been found to be very sensitive to the topography of the potential energy surface. Consequently the measurement of vector correlations constitute a stringent test for ab-initio calculations and provide information on the photodynamics, the multiple potential energy surfaces accessed, and non-adiabatic processes during dissociation.
(ii) Plasma diagnostics
Quantitative measurements of the concentrations of trace gas species in plasmas are vital for understanding and directing research in this applied area. Diode lasers in combination with modulation spectroscopies and/or cavity enhanced methods are sensitive enough to measure physico-chemical properties such as the absolute number densities, translational, internal temperatures, field effects and the electron energy distribution as a function of the plasma operating conditions.
Many trace species play important roles in key processes that determine the equilibrium and composition of the atmosphere, OH for example (whose concentration is typically 4 – 6 parts per trillion) essentially controls the daytime oxidative capacity of the atmosphere. In order to fully understand the chemistry occurring in our atmosphere it is necessary to be able to measure the concentrations of these trace species both accurately and quickly. Cavity enhanced spectroscopies in conjunction with diode lasers provide a robust, sensitive and accurate method for making quantitative measurements of such species in both clean and polluted atmospheres.