Aerosol-induced laser breakdown thresholds: wavelength dependence

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Abstract
Aerosol-induced laser breakdown thresholds have been measured for liquid droplets at wavelengths λ = 1.064, 0.532, 0.355, 0.266 μm using a Nd:YAG laser with 5–10-ns pulse duration. Breakdown thresholds are 2–3 orders of magnitude below those for clean air and range from 4 × 107 to 3 × 109 W cm−2 for nominal 50-μm diam droplets, depending on laser wavelength and droplet composition. Thresholds decrease with decreasing wavelength; they also decrease for droplets having a higher real refractive index. For water droplets the breakdown threshold intensity varies approximately as λ0.5. The wavelength dependence of breakdown thresholds can be qualitatively explained by considering (1) the effect of enhancement of internal fields and energy density within and near droplets and (2) the increasing importance of multiphoton absorption processes at shorter wavelengths. Laser transmission losses through the breakdown plasma and observations of the suppression of stimulated Raman scattering by the addition of small quantitites of absorbing material to water and carbon tetrachloride droplets are also reported.