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

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 2×10−2 J/cm2 exhibited SA. Exposure to 1.5 ns long, 11 kHz repetitive pulses of energies between 2×10−2 and 1 J/cm2 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 σgs=(1.5±0.1)×10−14 cm2 for a (2.0±0.13)×1015 cm−3 density of states. The first excited state absorption cross section was σes1=(9.3±0.6)×10−16 cm2 for a much higher, (2.3±0.15)×1016 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±0.5)×1022 to (2±0.1)×1021 cm−3; ground-state absorption cross sections increasing from (2±0.1)×10−22 to (9.0±0.5)×10−21 cm2; and excited state absorption cross sections increasing from (7.8±0.4)×10−22 to (3.3±0.15)×10−20 cm2. 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.