Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Author "Attie, Hendriks"

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    Control of CO2 vibrational dynamics via shaped-pulse coherent anti-Stokes Raman spectroscopy
    (Stellenbosch : Stellenbosch University, 2017-03) Attie, Hendriks; Hermann, Uys; Stellenbosch University. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT : In this work we investigate the coherent control of carbon dioxide (CO2) vibrational dynamics using Coherent anti-Stokes Raman Scattering (CARS). During CARS, vibrational modes are excited via stimulated Raman scattering (SRS). Subsequently a narrowband probe field interacts with the molecular ensemble providing not only information about the modes populated, but also on the evolution of the wave-packet created during excitation. By spectrally shaping one of the SRS pump fields the vibrational dynamics can be controlled. In this work it was assumed that the pump pulse structure which will lead to a desired dynamics is unknown. To find that structure, a learning algorithm was developed which utilizes a spatial light modulator (SLM) in a 4f-optical con guration to spectrally shape the pump. Both a time-frequency representation of the shaped pulse (called the von Neumann basis) and a standard Fourier domain representation were bench-marked during optimization of a second harmonic generation (SHG) signal in a BBO crystal to ascertain which will suit the optimization problem best in terms of convergence rate and parameter space size. It was found that the von Neumann basis converged faster than the standard Fourier domain representation while still operating on a larger parameter space and therefore it was used in all subsequent work. In addition, we developed a quantum mechanical theoretical model of the CARS process to ensure proper understanding of our measurements. We demonstrated experimentally that mode excitation selectivity can be achieved using the pump fields extracted by the learning algorithm, and we explore the underlying selectivity mechanisms. Control of the relative phase of oscillation of different vibrational modes is also observed. Our work demonstrates coherent quantum control of all relevant aspects of the molecular vibrational dynamics of CO2.