Characterization of Perturbed Droplets by Nonlinear Optical Spectroscopy.

Some of the basic properties of morphology dependent resonance (MDR) associated with the slightly deformed microdroplet are studied in this dissertation work. The perturbations that can affect the droplet shape and size are caused by evaporation, inertial effects, and laser pulses themselves. Nonlinear spectroscopic techniques are developed to determine the droplet evaporation rate and droplet shape distortion of closely spaced droplets. The angular distribution (within the range from 0^circ to 180^ circ) of stimulated Raman scattering (SRS) from the ethanol droplet is observed to be sinusoidal with n peaks, which corresponds to the mode number n of a MDR. The angular distribution measurement can be used to identify the mode number n of a MDR that supports the nonlinear optical process. The experimental result confirms that the counter-propagating SRS in droplet are coherent throughout the entire duration of the pumping laser pulse. In another experiment, the closely spaced SRS spectral peaks are resolved with a high-resolution interferometer. The observed frequency -split SRS peaks are consistent with the predictions of the degeneracy splitting of azimuthal MDR modes as a result of slight-shape deformation caused by inertial effects on the droplets. On the droplet surface, laser-induced bulging is shown to cause the extra leakage of internally generated dye-laser radiation. Both on the surface and within the droplet, laser-induced inhomogeneous index of refraction change is shown to cause the extra elastic scattering of the internal dye-laser radiation. The extra radiation leakage is found to be strongly dependent on the spatial overlap between the MDR's and the inhomogeneities of index of refraction. The evaporation rate variation for the first several trailing droplets is determined in a segmented droplet stream. The drag exerted on the segmented droplet stream and on the lead droplet is compared with that on the continuous stream. By developing a new diagnostic technique based on the variation of the laser wavelength along the droplet rim, both the amplitude and shape of the droplet deformation can be determined from a single-laser-shot measured droplet lasing spectrum.