Dispersion of Optical Nonlinearities in Solids

该项目将开展一项基础研究计划，旨在对光学材料的非线性响应有一个基本的了解。我们过去的研究最初是针对半导体中的双光子吸收(2PA)。这一目标导致了对2PA系数的预测能力，不仅对半导体，而且对宽禁带介质也是如此。在这些实验中遇到的一个困难是，非线性折射总是伴随着非线性吸收，因此也需要了解这种非线性折射的起源。因此，我们扩大了计划的范围。我们开发了一种新的、准确和灵敏的测量技术--Z扫描，它可以给出非线性折射和非线性吸收的符号和大小。这项技术正在迅速被广泛接受为一种标准测量，并为我们使用的其他方法提供补充信息，如四波混频。这项技术正被用来快速增加非线性材料常数的数据库。到目前为止，我们的测量显示了束缚和光生载流子的非线性。能带阻挡模型很好地预测了自由载流子的非线性。快界电子折射率N_2由非线性吸收光谱的Kramers-Kronig(KK)积分给出，其中包括2pA、电子拉曼和交流-斯塔克贡献。我们建立了这一理论，并证明了氮气中四个数量级的变化，包括半导体和电介质的符号变化，与实验是一致的。我们建议研究非简并的非线性(例如。由于w2)，在w1处的n2，这也是由理论预测的。除了数据库之外，这些研究的结果将是建立一种预测能力，供光电子器件研究人员用来确定其他材料在包括双波长应用在内的其他波长的非线性系数。例如，这些材料常数在全光开关、光限幅和激光诱导损伤中的应用非常重要。此外，我们最近在二次谐波晶体(KTP)中观察到了由X(2)：X(2)级联产生的有效X(3)，它看起来很有希望获得大的基频非线性折射。由于几种有机材料已被证明具有非常高的X(2)‘，S，我们正在着手研究有机材料的级联以及其他非线性(如在酞菁染料中)。KK分析是否适用于各类有机材料的问题将被审查。

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.