Abstract

Transition dynamics from optical saturable absorption (SA) to reverse saturable absorption (RSA) in multi-walled carbon nanotube (MWCNT)-doped fast sol-gel hybrid organic/inorganic glasses was studied by optical transmission of 532 nm laser pulses. Exposure to 6 ns long, temporally isolated single pulses of energies between ${10}^{-3}$ and $2\times {10}^{-2}\mathrm{J}/{\mathrm{cm}}^2$ exhibited SA. Exposure to 1.5 ns long, 11 kHz repetitive pulses of energies between $2\times {10}^{-2}$ and $1\mathrm{J}/{\mathrm{cm}}^2$ exhibited a gradual development of RSA at a rate that increases with the pulse energy. SA results were analyzed by the slow saturable absorber theory, yielding a ground-state absorption cross section ${\sigma }_{\mathrm{gs}}=(1.5\pm 0.1)\times {10}^{-14}{\mathrm{cm}}^2$ for a $(2.0\pm 0.13)\times {10}^{15}{\mathrm{cm}}^{-3}$ density of states. The first excited state absorption cross section was ${\sigma }_{\mathrm{es}1}=(9.3\pm 0.6)\times {10}^{-16}{\mathrm{cm}}^2$ for a much higher, $(2.3\pm 0.15)\times {10}^{16}{\mathrm{cm}}^{-3}$, density of states. The SA/RSA transition temporal evolution effects were modeled by a five-level energy scheme. The numerical simulation yielded light-intensity dependent parameters: ground-state densities decreasing from $(9\pm 0.5)\times {10}^{22}$ to $(2\pm 0.1)\times {10}^{21}{\mathrm{cm}}^{-3}$; ground-state absorption cross sections increasing from $(2\pm 0.1)\times {10}^{-22}$ to $(9.0\pm 0.5)\times {10}^{-21}{\mathrm{cm}}^2$; and excited state absorption cross sections increasing from $(7.8\pm 0.4)\times {10}^{-22}$ to $(3.3\pm 0.15)\times {10}^{-20}{\mathrm{cm}}^2$. The occurrence of such huge state densities is consistent with observation by others on the formation of ionized carbon-black particles as plasma states under illumination.

© 2015 Optical Society of America

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