ThirdOrder Nonlinear Optical Properties of Composite Materials.
Abstract
Thirdorder 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 Zscan measurement technique has been used, along with open aperture Zscan and other techniques, to measure the nonlinearities. We present experimental results which demonstrate that the effective thirdorder 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, subwavelengththick layers of titanium dioxide and the conjugated polymer poly (p phenylenebenzobisthiazole), 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 nearIR 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 thirdorder nonlinear susceptibility. In the case of gold colloid the small intensitydependent absorption exhibited by pure water can be removed in the composite with proper gold concentration. The real part of the thirdorder susceptibility of this composite is unchanged with respect to that of the pure water. Binary alloys of poly (pphenylenebenzobisthiazole) and poly (benzimidazophenthroline) are observed to also exhibit enhanced ultrafast, nonresonant, thirdorder 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.
 Publication:

Ph.D. Thesis
 Pub Date:
 1996
 Bibcode:
 1996PhDT........21F
 Keywords:

 Physics: Optics; Chemistry: Polymer; Engineering: Materials Science