Dispersion of Optical Nonlinearities in Solids

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This project will pursue a basic research program aimed toward developing a fundamental understanding of the nonlinear response of optical materials. Our past research was initially aimed at two-photon absorption (2PA) in semiconductors. This goal led to a predictive capability for 2PA coefficients not only for semiconductors but also for wide gap dielectrics. A difficulty encountered in these experiments is that nonlinear refraction always accompanies nonlinear absorption so that an understanding of the origins of this nonlinear refraction is also needed. Therefore, we broadened the scope of the program. We have developed a new, accurate and sensitive measurement technique, the Z-scan, which can give the sign and magnitude of nonlinear refraction as well as nonlinear absorption. This technique is rapidly gaining wide acceptance as a standard measurement and gives complementary information to other methods that we use such as four-wave mixing. This technique is being used to rapidly increase the data base of nonlinear materials constants. Our measurements to date show both bound and photogenerated carrier nonlinearities. The free-carrier nonlinearities are well predicted by band blocking models. The fast bound electronic refraction n2 is given by a Kramers-Kronig (KK) integral of the nonlinear absorption spectrum including 2PA, electronic Raman and AC-Stark contributions. We established this theory and demonstrated agreement with experiment over a four orders of magnitude change in n2 including a sign change for semiconductors and dielectrics. We propose to study the nondegenerate nonlinearities (eg. n2 at w1 due to w2) which are also predicted by the theory. The outcome of these studies, besides the data base, will be the establishment of a predictive capability for use by optoelectronic device researchers to determine nonlinear coefficients of other materials at other wavelengths including dual wavelength applications. These materials constants are important, for example, for applications in all optical switching, optical limiting, and laser-induced damage. In addition, we have recently observed an effective X(3) caused by the cascading of X(2):X(2) in a second harmonic generating crystal (KTP) that looks quite promising for obtaining large nonlinear refraction for the fundamental. As several organic materials have been shown to have very high X(2)'s, we are initiating studies in organic materials of the cascaded as well as other nonlinearities (eg. in phthalocyanine dyes). Questions of the applicability of the KK analysis to classes of organic materials will be examined.