Nonlinear Optical Interaction of Laser Radiation with Water Droplets

Abstract

In considering high-intensity laser propagation through the atmosphere, nonlinear optical effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), superbroadening, self-focusing, and dielectric breakdown of the optically transparent air become even more important when the air contains water droplets. For transparent water droplets with large size parameter (defined as droplet circumference 2πa divided by wavelength of interest λ), the droplet can be envisioned as a lens to concentrate the incident intensity (10) at three main locations:1 (1) outside the shadow face with ≅ 103 × 10; (2) inside the shadow face with ≅ 102 × 10; and (3) inside the illuminated face with less than 102 × 10. The nonuniform internal-field distribution and internal intensity enhancement significantly affect the nonlinear optical effects. Furthermore, a large transparent droplet can be envisioned as an optical cavity for specific internal wavelengths which satisfy the droplet cavity resonance condition [commonly referred to as morphology-dependent resonances (MDR’s)] associated with a sphere or spheroid.2-4 An analogy to a Fabry-Perot interferometer can be made by associating the liquid-air interface with the reflector (via total internal reflection) and the droplet circumference with the round-trip distance. For spheres5,6 and spheroids,7 the Q-factor of the droplet and the precise wavelengths which satisfy the MDR’s can be predicted by Lorenz-Mie and T-matrix formalism.