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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Reverse saturable absorption and saturable absorption in doped xerogels

Marc Brunel, Michael Canva, Alain Brun, Fréderic Chaput, J.P. Boilot

Published Jan 1, 1994• 0 citations
Publisher

Abstract

Organic molecules often present, at given wavelengths, reverse saturable absorption behavior:1–5 at low fluences, the absorption is weak; at high fluences, some molecules are excited to the first excited state whose absorption is greater. Aluminophthalocyanines encaged in solid inorganic xerogel silica matrices obtained from tetraethoxysilane precursors present this behavior.5 Similar results are also obtained with mixed organic/inorganic doped matrices.

DOI
10.1364/cleo_europe.1994.ctuq4

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Reverse saturable absorption and saturable absorption in doped xerogels

Marc Brunei, Michael Canva, Alain Brun +2 more

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Aug 24, 2005•0 citations
Organic molecules often present, at given wavelengths, reverse saturable absorption behavior:1–5 at low fluences, the absorption is weak; at high fluences, some molecules are excited to the first excited state whose absorption is greater. Aluminophthalocyanines encaged in solid inorganic xerogel silica matrices obtained from tetraethoxysilane precursors present this behavior.5 Similar results are also obtained with mixed organic/inorganic doped matrices.
We investigate the nonlinear absorption of C60-doped porous sol-gel glasses by single- and double-pulse pump–probe experiments. We find that the reverse saturable absorption (RSA) of these samples can be explained in the frame of a five level system as it is commonly used for C60 solutions. We observe a strong saturation of the RSA at high fluences, especially if the molecules are prepared in the triplet state. In a double-pulse pump–probe experiment we measure the triplet quantum yield for C60 solutions (0.8) and doped glasses (0.25) and the singlet and triplet absorption cross sections. In the first excited singlet state (S1) lifetimes of 1 ns and 65 ps are determined in solutions and glasses, respectively. We find that the dynamics of both depopulation processes, direct S1–S0 relaxation, and intersystem crossing is faster in glasses than in the case where C60 is in solution in a liquid aromatic solvent. We tentatively explain these findings by the absence of a stabilizing solvent and a perturbation of the molecular energy levels due to interaction with the solid glass matrix. We finally determine the lifetime of the triplet states in our glass samples to 2 μs.
The main achievements in our research on the physical phenomena of the excited-state nonlinear absorption in a molecular system and its applications in photonic technology are described. In the first part of this paper, some energy-level models and rate-equations are used to explain various mechanisms of the excited-state nonlinear absorption under different conditions, such as reverse saturable absorption caused by the triplet and/or the singlet excited-state absorption; saturable absorption due to the first and/or the second singlet excited-state absorption; and the excited-state nonlinear absorption induced by two-photon absorption. The experimental results for metal-organic and C 60 materials irradiated by ps and ns laser pulses are consistent with the simulated curves of the transmittance versus fluences. In the second part, the applications of excited-state nonlinear absorption in photonic techniques including optical bistability, optical switching, optical limiting, optical modulation, optical logic and optical storage are introduced. The working principles of the photonic devices based on the excited-state nonlinear absorption are presented. The experimental characteristic curves are found in good agreement with the theoretical simulations for these devices.
Three typical excited-state nonlinear absorption effects in molecular systems with weak ground-state absorption (at 532 nm) are experimentally demonstrated: (a) reverse saturable absorption (RSA) caused by strong triplet first-to-second excited-state absorption in a copper phthalocyanine solution with 15-ns laser pulses; (b) RSA caused by strong singlet first-to-second excited-state absorption in a C60{\mathrm{C}}_{60}C60​ solution with 21-ps laser pulses; (c) the RSA-SA turnover caused by strong singlet first-to-second excited-state absorption and weak singlet second-to-third excited-state absorption in a metallo-porphyrin-like solution with 21-ps laser pulses. To simulate these excited-state nonlinear absorption behaviors we propose a general ten-level density-matrix model and its simplified models for the three cases. All simulations are in good agreement with the experimental results.
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.