Abstract

We have observed fluence-dependent nonlinear transmission in a suspension of carbon particles in a water/ethylene glycol solution. The particle sizes are ~300- with ~5000-Å agglomerates. We see the onset of nonlinearity at input energies as low as 0.2 μJ using tightly focused 10-ns (FWHM) 532-nm pulses. Assuming strong absorption by the carbon a straightforward estimation of the temperature rise by linear absorption gives temperatures of a few thousand degrees celsius. Therefore, we associate this nonlinear transmission with localized rapid heating of the particles, which leads to thermionic emission and the creation of an expanding plasma, which both absorbs and scatters subsequent radiation. Evidence for this interpretation is given in experiments where we simultaneously monitor nonlinear transmission, nonlinear scattering, and nonlinear absorption (using the photoacoustic technique). The most striking nonlinearity is seen in side-scattered light which increases drastically with increased input energy. This is consistent with the growth in size and, therefore, increased scattering of rapidly expanding microplasmas. In addition, we find that the nonlinearity disappears rapidly with repeated laser firings indicating material changes. Flowing the suspension returns the nonlinearity as expected.

© 1988 Optical Society of America