3RD-ORDER NONLINEARITIES STUDIED BY FEMTOSECOND INFRARED METHODS

A novel femtosecond IR method is introduced and used to study the thir d-order response of liquids. This method, based on the gated detection of a CW probe beam, requires only one ultrafast pulse and a CW probe beam. This opens the possibility for the study of the two-color third- order response in a much wider spectral range. It is demonstrated here that this detection method is sensitive to the change in both the amp litude and phase of the probe field. In transparent liquids, the third order susceptibility affects the geometry, polarization, and frequenc y of the probe IR beam, all of which can be measured by this method. E xamples of study with optical femtosecond pump and CW IR probes on CS2 and other liquids are presented. The measured third-order susceptibil ities are compared to those measured in the optical region. At 1843 cm (-1) for CS2 the linear dipole polarizability correlation function ade quately describes the nuclear contribution to the third order suscepti bilities; for benzene and benzene derivatives, the linear polarizabili ty dominates the nuclear response, but contributions from the dipolar and hyperpolarizability may need to be considered.