Measuring the electric charge in cloud droplets by use of second-harmonic generation.

We report on the observation of second-harmonic generation (SHG) from electrically charged microdroplets while they are excited with femtosecond laser pulses. The intense SHG emission results from chi(3) coupling between two photons of the incident laser pulse and the electrostatic field induced by the surface charges. For I-ine =2 X 10(12) W/cm(2), we estimate the second-harmonic emission to be I-SHG = 2.5 X 10(3) photons per droplet and per pulse. The possibility of using SHG to measure remotely the electric charge of water droplets in thunderclouds is discussed. (c) 2005 Optical Society of America.