Modeling nonlinear optics in microdroplets

Abstract

Nonlinear mixing processes such as coherent anti-Stokes Raman scattering (CARS),1 and third-order sum-frequency generation (TSFG)2 have been modeled in microdroplets. The radiation from the third-order nonlinear polarization is modified by the spherical cavity. When the input waves are Gaussian beams tightly-focused near the center of the droplet, the morphology-dependent resonances (MDRs) are not excited, and the phase matching analysis is similar to that in bulk materials (the bending of the beams at the droplet surface must be considered). CARS generated by such focused beams may be useful for probing the spatially-dependent internal species concentrations. When the generating waves are focused at the edge of the droplet, or when the generating waves have been generated inside the droplet by a nonlinear process (e.g., stimulated Raman scattering in the case of TSFG), then the output intensity is very sensitive to droplet size, and to the spatial and frequency overlaps between the MDRs. Intensity-dependent shifts in MDR frequencies, and effects of two-photon on time-dependent scattering have also been modeled.3