Source: Dissertation Abstracts International, Volume: 53-10, Section: B, page: 5269.

Adviser: Richard K. Chang.

Stimulated Raman scattering (SRS) is experimentally and numerically investigated in microdroplets. In order to study the characteristics of SRS and to apply SRS as an optical diagnostic technique, aqueous droplets containing ammonium nitrate and hydrocarbon droplets are used. High spectral resolution Stokes SRS spectra consist of several morphology-dependent-resonance related peaks. The wave-number spacing of these Stokes SRS peaks provides droplet size information. The first-order Stokes SRS intensity has an exponential dependence on the external input-laser intensity. The ratio of the first-order Stokes SRS intensities of different species provides relative species concentration information. The fluctuations in the first-order Stokes SRS intensity are smaller when a single-mode input laser is used instead of a multimode input laser. The differential evaporation in a multicomponent hydrocarbon droplet is also deduced from the Stokes SRS spectra. One- and two-photon absorption losses are found to inhibit Stokes SRS in hydrocarbon droplets. By working at longer wavelengths to minimize both losses, it is possible to acquire the Stokes SRS spectra of hydrocarbon droplets. An aerosol spray is imaged with Stokes SRS. Only the large droplets in the spray support Stokes SRS, which is used to selectively detect the presence of the large droplets in the spray.

In order to study the temporal evolution of the multiorder Stokes SRS pulses in droplets a heuristic model is developed. In the model the standard one-dimensional coupled nonlinear-wave equations for multiorder Stokes SRS in an optical cell are modified to describe the growth and coupling of multiorder Stokes SRS waves circulating around the droplet rim. Calculations, made with a slightly enhanced value of the model-dependent Raman gain coefficient, agree with the experimental observations of the delay time of the first-order Stokes SRS. In addition, with enhanced phase-matching values, the numerical simulation of multiorder Stokes SRS temporal profiles compares favorably with the experimental results. Both numerical and experimental results exhibit correlated decay of the pump Stokes SRS waves and the growth of the resultant Stokes SRS waves.