Laser emission and coherent Raman scattering from individual flowing droplets

It may be possible to use the coherent but nondirectional laser emission from individual particles as active markers for flow field visualization and remote illumination. In particular, new applications may be viable with coherent emission droplets as velocimetry markers in otherwise optically dense two phase flows and in overhwelmingly high elastic scattering environments such as near a solid boundary. Furthermore, determination of the chemical species of individual droplets appears to be possible with coherent Raman emission. New research was initiated on the nonlinear optical emission from water droplets and ethanol droplets with and without Rhodamine 6G or Courmarin dyes. Laser emission from individual liquid ethanol droplets containing Rhodamine 6G at wavelengths commensurate with morphology dependent resonances (MDRs) was observed. For a conventional dye laser, optical feedback at selected wavelengths is provided by an external wavelength dispersive element. For a droplet dye laser, higher Q at selected wavelengths is provided by the MDRs associated with the liquid-air interface of the droplet. At these resonances, standing waves result from the total internal reflection at the liquid-air boundary of counterpropagating waves close to the circumference of the droplet. Stimulated Raman scattering (SRS) from individual H2O or ethanol droplets at wavelengths commensurate with MDRs and the spontaneous Raman-Stokes radiation was observed.