Adiabatic Passage in Multilevel Systems with Frequency - Optical Pulses: Efficiency and Selectivity of Population Transfer.

Coherent optical excitation using the pi-pulse has been a common tool to manipulate populations in multilevel systems. In some applications, the degree, selectivity and specificity of the population transfer are pivotal to the outcome of the experiment. Where the dynamics need to be studied, the temporal window of opportunity created by the excitation is often limited to the ultrafast timescale due to finite lifetimes and intramolecular vibrational redistribution rates. In this thesis, we demonstrate in a variety of chemical systems that some of these problems can be addressed when adiabatic rapid passage using frequency-swept pulses is used to effect population transfer. Optical pulse shapes, readily generated in the laboratory using well-known techniques, are shown to provide selectivity, efficiency and robustness to the population transfer process. Various techniques employed in the generation of high-peak power chirped pulses are presented. Fiber - and grating-chirping techniques are characterized, as are the effects of the pulsed source and the succeeding amplifier system upon the resulting chirped fields. The latitude in the available pulse quality is underscored, despite which we demonstrate efficiency and/or selectivity in the population transfers effected. In increasing order of complexity, we demonstrate almost twice the population transfer in mixed crystals of pentacene in p-terphenyl (representing a basic two-level system) compared to transform-limited pulses. In the D -line system of sodium vapor, selective excitation of one of the D-lines occurs in preference to the other; we observe that the direction of the chirp determines the final state of the population transfer. The process is surprisingly immune to variations in the Rabi frequency (beyond a limit), contributing to robustness; this is understood in terms of numerical simulation of the D-line system of sodium with the dressed-state picture. Stimulated emission pumping is carried out between the 3s-5s-3p transitions of the same system to demonstrate that a chirped "dump" pulse depletes more fluorescence from the excited state than a transform-limited pulse. The production of programmable optical pulse shapes can lead to the control of dynamics in a chemical reaction, which is of great interest to chemical physics. In a part of this thesis, our efforts to generate shaped pulses by time-domain modulation are described. A pulse-forming scheme is used that involves splitting a trigger pulse into eight components and passing each through a delay line, attenuating each individual component, and recombining them. A variety of pulse shapes are demonstrated, but the final assembly is compromised by reflections in the splitter/combiner sections. A detailed design of a multisegment power splitter as a building block is offered as a solution. Finally, we describe the fabrication and characterization of a Ti:LiNbO _3 waveguide phase modulator with traveling-wave electrodes using the conventional technique. (Abstract shortened by UMI.).