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Journal of the Optical Society of America B

Theoretical analysis of the picosecond, induced absorption exhibited by chloroaluminum phthalocyanine

Steve Hughes, G. Spruce, J. M. Burzler, R. Rangel-Rojo, Brian S. Wherrett

Published Feb 1, 1997• 17 citations
Publisher

Abstract

Picosecond measurements of the optical response of chloroaluminum phthalocyanine in solutions are presented. The observed nonlinearities are modeled and interpreted theoretically by rate-equation and density-matrix techniques within a three-level model. Single-pulse power limiting is observed at 532 nm, and the absorption cross section of the first excited state is determined to be 15 times that of the ground state. From time-resolved excitation–probe measurements we determine an excited-state recombination time of 7 ns and a reorientational time of 7 ps. Further, the observation of the associated optical coherence effects, which are manifested as a negative coherent artifact, allow us to obtain an 8ps coherence time.

DOI
10.1364/josab.14.000400

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Picosecond measurements of the optical response of chloroaluminum phthalocyanine in solutions are presented. The observed nonlinearities are modeled and interpreted theoretically by rate-equation and density-matrix techniques within a three-level model. Single-pulse power limiting is observed at 532 nm, and the absorption cross section of the first excited state is determined to be 15 times that of the ground state. From time-resolved excitation–probe measurements we determine an excited-state recombination time of 7 ns and a reorientational time of 7 ps. Further, the observation of the associated optical coherence effects, which are manifested as a negative coherent artifact, allow us to obtain an 8ps coherence time.
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.
The theory of dual-beam excitation-probe measurements for induced absorption three-level systems is presented, including third-order and fifth-order (excited-state absorption saturation) responses, molecular reorientational effects, and optical coherence effects. Negative coherent artifacts are predicted for the self-diffraction configurations and verified experimentally for the induced-absorptive dye 1, 3, 3, 1′, 3′, 3′-hexamethylindotricarbocyanine iodide. We obtain the reorientational times and the recombination lifetimes of the first and the second excited states of the material. These lifetimes are important to the design and exploitation of induced absorbers for optical limiting. The ratio between off-diagonal and diagonal χ(3) tensor elements is also obtained from the modeling.
Based on five-level rate-equation theory, we develop the laser-pulse-duration dependence of two-photon-absorption-induced singlet and triplet excited-state absorptions (ESAs). We present analytical expressions for the effective three-photon absorption coefficients caused by both singlet and triplet ESAs under the pulsed excitation on time scales from femtoseconds to microseconds. We demonstrate that the triplet ESA is predominant with longer laser pulses (microseconds to tens of nanoseconds) and that the resultant nonlinear absorption (NLA) can be adequately interpreted by a simplified four-level model. Under the excitation of picosecond laser pulses, generally speaking, the competition between singlet and triplet ESAs is observable. In this instance, the photodynamics of the system can be understood by a five-level model. In the femtosecond regime, however, a three-level model is validated in the prediction of NLA, because the triplet ESA becomes negligible.
The dynamic processes of reverse saturable absorption (RSA) in a solution of C60 are studied in detail by use of rate equations and the light-propagation equation for a five-energy-level model. The RSA experimental results for C60 in toluene solution with a nanosecond–picosecond Nd:YAG laser at 532 nm are consistent with the theoretical simulations. Effects of sample concentration and thickness on RSA and contributions of the singlet and triplet excited states to RSA for various laser-pulse widths and wavelengths are also analyzed. Finally, an all-optical switch based on excited-state nonlinear absorption and the pump-probe technique is implemented.
We theoretically and experimentally study absorption of picosecond laser pulses in materials described by a four-level system that exhibit reverse saturable absorption (RSA). Using an approximate solution to the rate equations, we derive, analyze, and verify, numerically and experimentally, a single dynamical equation for the spatial evolution of the pulse fluence that includes both the rate equations and the propagation equation. This analytical approach considerably simplifies the study of optical limiting with picosecond pulses and helps to predict the behavior of the nonlinear transmittance, the level of output signal clamping, and a possible turnover from RSA to saturable absorption that restricts the performance of optical limiters based on RSA. The results obtained can also be used to characterize RSA materials by the pump–probe technique.