Anti-Symmetric Coherent Anti-Stokes Raman Scattering from Micron Sized Water Droplets

This work presents the computations of the coherent anti-Stokes Raman scattering (CARS) from water droplets. The nonsymmetric 3430 cm('-1) line of water was chosen for the Raman resonance mode. To determine the CARS field, two plane waves are allowed to fall on the water droplet. As a result two internal Mie fields arise. The source for the CARS field is the third-order electric polarization vector arising from the third-order interactions of the internal fields. In general for isotropic media the third -order polarization vector retains only the components X(,1122) and X(,1221) of the third-order nonlinear optical suceptibility tensor. For a symmetric line, which has no depolarization ratio, only the X(,1122) term is needed. Water has a finite depolarization ratio of 0.2, so that both the terms are now important for the computations of the CARs field. Our results show that for water, the CARS scattered fields for a nonsymmetric line are significantly different from the CARS scattered fields for a symmetric line of a material in droplet form. This difference is brought about in part by polarization scrambling. For a symmetric line polarization scrambling of the internal fields comes about solely through the agency of morphological scrambling. That is the change of polarization of the internal fields relative to that of the input fields comes about because of the need for the fields to obey the boundary conditions over the surface of the droplet. For the 3430 cm('-1) line of water, the line anistropy itself contributes further to the polarization scrambling. Thus the difference in the polarization of the field scattered by a water droplet without and with the depolarized components of the line are polarization scramblings not due to droplet morphology alone, but arise from the instrinsic nonsymmetric properties of the particular line.