Joan K. Frisoli

Non-invasive determination of shock wave pressure generated by optical breakdown

Shock waves generated by a laser-induced plasma were investigated using a pump-and-probe technique. Both 7-ns and 40-ps laser pulses at 1.06 μm were employed to initiate breakdown in water. Two He-Ne laser beams were used as a velocity probe, allowing the accurate measurement of the shock velocity around the plasma. The maximum shock pressure was determined from the measured shock velocities, the jump condition and the equation of state for water. The conservation of the total momentum of the shock front was used to derive expressions for the shock velocity, particle velocity and shock pressure vs. the distance (r) from the center of the plasma. For a shock wave of spherical symmetry, the shock pressure is proportional to 1/r2. Our work shows that the expanding plasma initially induces a shock wave; the shock wave dissipates rapidly becoming an acoustic wave within 300–500 μm.

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm2 excitation and recovers with a fast time constant ≈34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS1 and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S1) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.

RAPID SERUM CAROTENE LOADING WITH HIGH-DOSE BETA -CAROTENE : CLINICAL IMPLICATIONS

The kinetics of serum β-carotene loading were evaluated to help determine the minimum time required to load fatty tissues, including atherosclerotic plaque and skin, with β-carotene. Loading atherosclerotic plaque with yellow β-carotene pigment increases absorption of blue laser radiation, potentially enhancing the selectivity and quality of laser angioplasty. Five healthy volunteers received 300 mg β-carotene daily (the maximum FDA recommended dose) for 21 days in three divided doses with meals. Serum total carotenoid levels increased exponentially with a 10-day time constant from an average of 1.7–6.5 μg/ml (p < 10-4). The skin began to yellow visibly at 10 days and became increasingly yellow for several weeks thereafter, suggesting that augmenting the yellow color of fatty tissues and atherosclerotic plaque with oral β-carotene may require a minimum of several weeks to attain the maximum effect.

Medical applications of laser-induced fluorescence: pharmacokinetics of photosensitizers

Laser-induced fluorescence has been used to measure tissue levels of chloroaluminum phthalocyanine tetrasulfonate versus time in order to determine its pharmacokinetics. A hamster cheek pouch carcinoma model was used in vivo. The data have been modeled using a four compartment pharmacokinetic model, yielding rate constants which describe the transport. A minimum of 13 rate constants was needed to achieve acceptable fits to the tumor and normal tissue as well as plasma data. The model gives insight into the role of binding and unbinding processes that are not otherwise evident.