Third-Order Nonlinear Optical Properties of Composite Materials.

Third-order nonlinear optical properties of four types of composite structures are studied both theoretically and experimentally. Each of the four composites possess nonlinear optical qualities not found in any of the starting materials. A new Z-scan measurement technique has been used, along with open aperture Z-scan and other techniques, to measure the nonlinearities. We present experimental results which demonstrate that the effective third-order susceptibility of a composite optical material can exceed those of the materials from which it is constructed. In particular, we have formed a composite of alternating, sub-wavelength-thick layers of titanium dioxide and the conjugated polymer poly (p -phenylene-benzobisthiazole), and find that its nonlinear susceptibility exceeds that of its more nonlinear constituent by 35%. The enhancement of the nonlinear susceptibility, which under more ideal but still realistic conditions can be as large as a factor of ten, can be understood as a consequence of local field corrections. Using a liquid composite containing gold particles and the near-IR laser dye 1,1^' 3,3,3^'3 ^' hexamethylindotricarbocyanine iodide we have shown that nonlinear absorption can be removed from a material made of two components, each of which has a large imaginary third-order nonlinear susceptibility. In the case of gold colloid the small intensity-dependent absorption exhibited by pure water can be removed in the composite with proper gold concentration. The real part of the third-order susceptibility of this composite is unchanged with respect to that of the pure water. Binary alloys of poly (p-phenylene-benzobisthiazole) and poly (benzimidazo-phenthroline) are observed to also exhibit enhanced ultrafast, nonresonant, third-order nonlinear optical properties compared to the components. This novel mechanism of enhancement of nonlinear optical response originates from the coupling of the component electronic structures in the supramolecular materials. Alloys and blends thus represent a novel, simple approach to developing more efficient nonlinear optical materials. In concluding the work the design of a new optical power limiter is presented. We have constructed such a device based on nonlinear induced scattering in a cell containing crushed glass and a mixture of acetone and carbon disulfide. For 30 ps long laser pulses, the transmitted energy saturates at a value of 6 muJ. We also present the results of a theoretical modeling study that shows how the operating characteristics of such a device can be optimized.