Two-Photon Absorbing Organic Chromophores for Optical Limiting
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amino)stilbene by using direct nonlinear transmission measurements with 5 ns, 600 nm laser pulses (4). Stimulated by this result, we have begun a program to develop an understanding of the origin of the large effective two-photon absorptivity in this compound, to establish molecular structure/property relationships for two-photon absorption, and to investigate the utility of this and related materials for optical limiting. In this paper, we report on studies of the two-photon absorption and optical limiting properties of a series of bis-donor diphenylpolyene derivatives with varying lengths of the polyene bridge and, in the case of the stilbene derivative, the effect of dialkylamino versus diphenylamino substitution. We have used nonlinear transmission measurements with nanosecond and picosecond laser pulses to investigate the magnitude and mechanism of the observed nonlinear absorption and have determined the dispersion of the nonlinear absorptivity using nanosecond pulses. Finally, we have utilized these materials to demonstrate strong optical limiting in the visible and established their potential for broadband optical limiting with high transparency. EXPERIMENTAL
The chromophores studied in this work were bis(dialkylamino)diphenylpolyenes with varying lengths of the polyene bridge and bis(diphenylamino)stilbene, whose structures are shown in Fig. 1. The molecules were synthesized using standard Wittig chemistry; the synthetic details and characterizing data will be reported elsewhere. The molecules were studied in toluene or acetone solution. Concentrations of 0.01 M were used for nonlinear transmission measurements, while concentrations up to 0.5 M were employed for optical limiting studies. For all measurements, optical cells of 1 cm pathlength were used. Two laser systems were utilized for nonlinear transmission measurements. For nanosecond measurements a Nd:YAG-pumped Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
dye laser with -5 ns pulse duration and a relatively uniform "top-hat" spatial profile was used. For picosecond measurements, an amplified synchronously pumped cavity dumped dye laser with a 4 ps pulse duration was used. A two-lens telescope was used to collimate the beam and to reduce the beam diameter to give a cross sectional area of 0.0025 cm 2 for the nanosecond measurements, while various lens combinations were used to produce areas between 0.009 - 0.11 cm 2 for the picosecond measurements. Nanosecond optical limiting measurements were done in an f/5 optical system, with a calculated beam waist diameter and area of about 3.5 gam and 4 x 10-7 cm2, respectively. The beam waist was located -2 mm from the exit window of the sample cell. The incident laser intensity was controlled using neutral density filters and a half-wave plate/polarizer combination. A beam splitter was inserted into the beam path to provide an incident intensity reference. The beams in the signal and reference arms were detected with silicon photodiodes, the outputs of which were integrated, then digit
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