Third-harmonic and sum-frequency generation by time-ordered ultrashort laser pulses: Time- and polarization-dependent nonlinear-optical susceptibilities

The influence of the time ordering of ultrashort laser pump pulses on nonlinear-optical phenomena is studied. We discuss nonstationary regimes of noncollinear nonlinear-optical interactions where femtosecond laser pulses induce transitions of a quantum system to an electronically excited state and produce vibrational wave packets, leading to an asymmetry in the dependence of the efficiency of second- and third-order nonlinear-optical processes on the delay time between the pump pulses. The methods of polarization nonlinear optics are shown to be ideally suited for probing the structure of oriented molecular dipoles in poled organic materials and studying the motion of collectivized pi-electrons in quadratic-nonlinear organic molecules. Due to the existence of a distinguished direction in systems of oriented molecular dipoles, methods of polarization femtosecond nonlinear optics can selectively control the content and dynamics of vibrational wave packets in poled materials.