RECENT DEVELOPMENTS IN HYPER-RAYLEIGH SCATTERING

摘要

At the macroscopic level, nonlinear optical (NLO) effects are described by the nonlinear bulk susceptibility of the arrangement of molecules. At the fundamental level, the second-order NLO properties of molecules are governed by their molecular first hyperpolarizability (or second-order nonlinear polarizability). Therefore, in the quest for ever better NLO materials, it is essential to have a reliable, fast and widely applicable measurement technique for this molecular parameter. Hyper-Rayleigh scattering (HRS) has proven to be just that. The first version of the HRS set-up made use of a nanosecond laser and concomitant gated integrators to pick up the signal. Ionic species and non-dipolar molecules could join the neutral and dipolar ones as potential second-order nonlinear optical materials. With the advent of femtosecond lasers, higher temporal resolution enabled the hyperpolarizability of fluorescent chromophores to be measured also. A cross-correlation technique was developed to effectively suppress the multi-photon fluorescence contribution to the HRS signal. With a femtosecond optical parametric oscillator, this suppression technique has now been implemented in the near-infrared, enabling the measurement of fluorescent molecules with red-shifted absorption. These newly designed chromophores offer large potential for NLO applications.