Shadowgraphic Imaging of Dilute Carbon and Mo 2 Ag 4 S 8 ,[PPh 3 ] 4 Suspensions
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US Army Tank-automotive & Armaments Command, MS:263, Warren, M1 48397-5000 *University of Dayton Research Institute, Electrical & Computer Engineering Division 300 College Park, Dayton, OH 45469-0150 ABSTRACT
Time-resolved imagery is presented showing the changes that occur in the focal volume of dilute liquid/particle suspensions following the arrival of single, Q-switched, frequency-doubled, Nd:YAG laser pulses. Limiting data and corresponding imagery at 21, 84, 244, 790, and 2900 nanoseconds following the laser pulse are presented for a carbon particle suspension consisting of used (carbonized) 1OW-15 motor oil and for a suspension of the inorganic metallic cluster molecule Mo2Ag 4S8 [PPh3] 4. The images in conjunction with the accompanying limiting data show that the reduction in transmission, observed as the input pulse energy is increased, results from scattering from bubbles augmented by plasma absorption. Keywords: laser induced breakdown, plasma, suspension, bubble, nonlinear materials, shadowgraph, imagery INTRODUCTION
Limiting from carbon suspensions has been actively pursued for several years."' Recently, the response of the inorganic metal cluster Mo 2Ag4S,[PPh3]4 has also sparked interest.4"" The molecular structure of Mo 2Ag4S,[PPh3]4 is shown in Fig. 3 of reference 4. To date, very little imaging of these suspensions has been attempted. Conversely, the imaging of plasma, bubbles and shock waves resulting from laser-induced breakdown in aqueous media (tap water, distilled water, saline, vitreous) has been widely reported due to the increasing use of lasers for ophthalmic surgery as well as the growing study of laser-matter interaction.6 7 Similar imaging techniques have been employed in our group for imaging the response of nonlinear materials proposed for laser power limiting as well as breakdown in the neat solvents used to host these materials. The materials discussed in this paper are a suspension of used motor oil (carbon) in toluene and a suspension of the inorganic metallic cluster Mo2Ag4 S,[PPh3]4 in acetonitrile. The Mo 2Ag 4S,[PPh3 ]4 was synthesized by Shu Shi (currently at SS-ALL Technologies in Singapore), and optical limiting has been reported in similar cluster molecules by S. Shi et al. 5 High-resolution, time-resolved imagery of the events which occur within the focal volume allow us to infer the physical processes which yield nonlinear transmission of the incident laser pulse. For each material, two shadowgraphic and one emitted light image are captured for each data point on the limiting curve. The time delay
between the capture of the first shadowgraph and the second is varied and complete limiting curves and image sets are obtained. The resulting data set shows the change that occurs within the focal volume over time following the arrival of the laser pulse in the cell. The relative locations and scales of various features observed in the image data are combined with limiting data to infer the nature of the material's response. A knowledge of thermophysical data is then used t
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