Third-order sum-frequency generation in droplets: experimental results

Abstract

New experimental third-order sum-frequency generation (TSFG) spectra are presented for CCl4 droplets. The droplet radii are discretely tuned over a size range in increments of 1 part in 6000. For each droplet radius only one or a few of the many possible TSFG peaks are dominant. Both low- and high-resolution TSFG spectra of D2O droplets are measured. The possible generating waves are the stimulated Raman radiation [known to occur at the morphology-dependent resonance (MDR) frequencies] and the internal fields at the incident laser frequency, which is usually not on a MDR. The experimental findings suggest that the resultant TSFG frequency must be near an output MDR. A physical description of the third-harmonic generation (THG) process in droplets is presented. MDR’s are treated as traveling waves that rotate in the azimuthal direction. Their azimuthal phase velocities are derived, and the concept of phase matching among the MDR’s is presented. The relation between phase matching and spatial overlap of MDR’s is discussed. Reasons are presented for the low probability of having the THG or TSFG frequency on or near a MDR.