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Herein we present experimental evidence of the strong surface plasmon enhancement of two- and three-photon absorption of Hoechst 33 258 (2'-(4-hydrophenyl)-5-(4-methyl-1-piperazinyl)-2,5'-bis(1H-benzimidazole)) pentahydrate in an aqueous solution containing activated gold colloid. The 480-fold enhancement in two-photon absorption and 30-fold in three-photon absorption are predominantly attributed to the electric-field enhancement via surface plasmon resonance. The greater enhancement observed in two-photon absorption is due to the characteristic two- and three-photon absorption cross-sections of the dye.

High-order multiphoton excitation processes are becoming a reality for fluorescence imaging and phototherapy treatment because they afford minimization of scattered light losses and a reduction of unwanted linear absorption in the living organism transparency window, making them less susceptible to photodamage, while improving the irradiation penetration depth and spatial resolution. We report the four-photon-excited fluorescence emission of (7-benzothiazol-2-yl-9,-didecylfluoren-2-yl)diphenylamine in hexane and its four-photon absorption cross section sigma4' = 8.1 x 10(-109) cm8 s3 photon(-3) for the transition S0 --> S1 when excited at 1600 nm with a tunable optical parametric generator (OPG) pumped by picosecond laser pulses. When pumped at 1200 nm, three-photon absorption was observed, corresponding to the same transition.

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We measure the optical limiting behavior of carbon black suspensions in various viscosity solvents by using a 10-Hz repetition rate, 532-nm, 5-ns pulsed laser. We found that, for common solvents used in the past such as water and ethanol, the limiting behavior ceases after a few laser firings and a turnover in the limiting curve appears. This can be explained by depletion of the carbon black within the focal volume. This turnover shifts to lower energies as the viscosity of the solvent becomes greater. However, for low viscosity liquids, such as carbon disulfide or pentane, the limiting is unaffected by the repetition rate, at least for frequencies up to 10 Hz, because of diffusion of the carbon black particles. This diffusion allows fresh material to replace the irradiated volume within the time between pulses.