Transition from saturable to reverse saturable absorption in multi-walled carbon nanotube-doped sol-gel hybrid glasses

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 super(-3) and 2 x 10 super(-2) J/cm super(2) exhibited SA. Exposure to 1.5 ns long, 11 kHz repetitive pulses of energies between 2 x 10 super(-2) and 1 J/cm super(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] sub(gs) = (1.5 + or - 0.1) x 10 super(-14) cm super(2) for a (2.0 + or - 0.13) x 10 super(15) cm super(-3) density of states. The first excited state absorption cross section was [sigma] sub(es1) = (9.3 + or - 0.6) x 10 super(-16) cm super(2) for a much higher, (2.3 + or - 0.15) x 10 super(16) cm super(-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 + or - 0.5) x 10 super(22) to (2 + or - 0.1) x 10 super(21) cm super(-3); ground-state absorption cross sections increasing from (2 + or - 0.1) x 10 super(-22) to (9.0 + or - 0.5) x 10 super(-12) cm super(2); and excited state absorption cross sections increasing from (7.8 + or - 0.4) X 10 super(-22) to (3.3 + or - 0.15) x 10 super(-20) cm super(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.