Ultrafast Nonlinear Optical Measurements Using the Tunable-Laser Grating Technique.

The Tunable-Laser-Induced Grating technique is a nonlinear-optical frequency-domain method for the measurement of atomic and molecular processes which occur on picosecond and femtosecond timescales. This dissertation presents a continuation of the efforts on this technique begun by Trebino and Siegman. A four-wave mixing calculation of the frequency response of the induced grating diffraction efficiency is presented. The resulting mathematical expression describes the general features of Tunable-Laser-Induced Grating spectra and provides a framework for discussing effects observed in transparent media such as the optical Kerr effect in carbon disulfide. Experimental results on several systems of interest are also reported. In particular, we find that the use of parallel polarized excitation beams creates intensity gratings which drive long-lived scalar responses of the sample material. Examples of intensity gratings include thermal gratings in saturable absorber dyes and electrostrictively -driven acoustic waves in transparent liquids, both of which obscure the measurement of ultrafast processes taking place in these systems. By using orthogonally polarized excitation beams, we have eliminated these intensity-grating effects and have measured ultrafast responses of both the saturable absorber dye malachite green and the optical Kerr liquid carbon disulfide. High temporal resolution measurements of the optical Kerr response of carbon disulfide in particular indicate clearly the ultrafast inertial character of the nuclear component of carbon disulfide's optical Kerr response. Finally, the Tunable-Laser-Induced Grating method measures only the square magnitude of the sample's response in the frequency domain, and the resulting loss of phase information can lead to ambiguities in determining the relative strength factors in multi-component responses. We show, both theoretically and experimentally, how the controlled introduction of a complex-valued coherent background can eliminate this so-called "rise-fall" ambiguity.