Nonlinear Optical Properties of Tetrabenzporphyrins
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Abstract
Optical limiting in a solution of tetrabenzporphyrin (TBP) dissolved in tetrahydrofuran (THF) was measured in an f/5 optical focusing arrangement using 532 nm wavelength laser light with a 9 ns pulse duration. The performance of this solution was compared to a variety of other materials. In order to better understand the nonlinear behavior of this material, apertured and unapertured Z-scans were performed. From these measurements it was determined that the excited-state cross section and its ratio to ground-state cross section were large. The nonlinear refraction of this material is made up of contributions from thermal density change and population redistribution. Limiter performance was modeled in an f/64 limiter. The limiter behavior of a slightly modified compound and an additional solvent were measured. F/5 Limiting
Optical materials with large third-order nonlinearities can have an intensity-dependent index of refraction and/or absorption and exhibit nonlinear transmission. Such materials are of interest for optical modulation and switching applications such as optical limiting of lasers. Tetrabenzporphyrin (TBP) is a highly 7t-conjugated organic molecule that interacts strongly with light over most of the visible spectrum. We have shown a solution of TBP in tetrahydrofuran (THF) to outperform similar highly conjugated
molecules and suspended carbon particles, all at the same path length, L = 2mm, and approximately the same transmission, T - 50%, using an f/5 optical limiter configuration, at 532 nm with 9 ns pulses, see Fig. I. The input beam was an expanded Gaussian clipped by the input f/5 aperture at the l/e intensity to emulate a beam arriving from a long distance. Since the sample is much greater in length than the Rayleigh range, it is considered thick and the position of focusing within the sample is important. The data of Fig. I was obtained at the best limiter position for each individual sample tested. We found the best position for limiting occurred somewhere between focusing at the center and focusing just inside the front face of the glass cuvette depending on the sample type. Damage to the glass interface is a concern when the focus is near the glass. Both CBS and TBP limit at - 2 pJ for a 200 pJ input energy. That is, both reduce the transmission by a factor of- 100x. This is significant because both clamp the output below the 50 % probability of damage level or ED 50 level of 2.8 g.J. Flashblinding with impairment of fine visual tasks up to 20 minutes can occur at this level of irradiation.2 On an expanded scale the TBP performs better than the CBS in limiting transmitted energy. TBP can limit the output energy to 0.2 I.J, the maximum permissible exposure as defined by the ANSI standards, at about 4 liJ input, or a factor of 20x, whereas CBS limits to 0.2 10 at about 1 pJ input energy. Therefore, TBP limits the transmission about 4x better than CBS in this range. In addition, at all ranges of energy, the CBS bleaches for repeated shots and needs to be flowed, replenishing the foca
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