Metal Organic Complexes for Optical Power Limiting
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We have recently been examining metal-organic compounds in the liquid state as candidate materials for NLO applications such as power limiting.14'15 The self-healing nature of NLO liquids makes them attractive for use in optical power limiters that operate over a wide dynamic range of pulse energies. These compounds are very easy to process and are potentially very stable. In addition, these materials can exhibit large non-resonant third-order susceptibilities because of their highly conjugated structures and because the transition metal atoms can have an increased number of low-lying states.16' 1718 We have recently found that y values of six molybdenum-phosphine complexes appeared to increase rapidly with the number of phenyl substituents in the ligands. We report in this paper the results of degenerate fourwave mixing (DFWM) measurements on an extended series of transition nfetal based complexes with different types of phosphorus-donor ligands. We have shown that the y values
increase as the number of nt conjugated structures in the ligands. Hence, by further increasing the number of n conjugated structures, these metal-organics may ultimately provide selfhealing, stable, liquids which have a large laser-induced nonlinear refraction but with a very low linear transmission loss. EXPERIMENTS A number of transition metal complexes have been synthesized' 9 and characterized by FTIR, mass spectroscopy, 'H NMR spectroscopy, and by elemental analyses. The complexes were dissolved in either dry tetrahydrofuran (THF) or methylene chloride (CH 2C12) to form a series of solutions with concentrations ranging from 10-4 to 10-2 mol/lit. As described earlier, an ideal NLO liquid for an optical power limiter should have a low linear absorptance. All sample solutions are colorless and transparent to the eye. The linear absorptance of the samples was measured on a Gilford model Response I UV-VIS spectrophotometer. The only significant absorption peaks observed for the samples were at wavelengths below 350 nm, which was well removed from the operating laser wavelength of 530 nm. At this operating wavelength, the linear absorption coefficient, cx, of each of the sample solutions was found to be negligible (below 0.1 cml1.). Hence, it appears that linear absorption processes do not appear to play a major role in these samples. This result is consistent with previous measurements we have 20 performed with z-scan techniques. The NLO properties of each solution were measured using a degenerate four-wave mixing (DFWM) apparatus. The layout for the DFWM experiments, discussed in detail in a previous publication,21 is shown in Figure 1. The light source is a pulsed, frequency doubled Nd:YAG laser with a 20 nanosecond pulse width. The intensity of the phase conjugate signal, 'sample, for the NLO sample is compared to the phase conjugate intensity, I(. from carbon disulfide (CS 2), which is a reference NLO material with a well-characterized Y(3). The 22 magnitude of the third order susceptibility for the NLO sample is given by
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