Optical-heterodyne-detected induced phase modulation for the study of femtosecond molecular dynamics

A new experimental technique for the study of ultrafast optical response of nonlinear materials, which detects nonrelaxational response in the media with high sensitivity, is introduced. Polarization-selective optical-heterodyne- detection scheme is applied to the measurement of the spectral shift of a probe pulse which is caused by induced phase modulation (IPM) brought about by a pump pulse. This technique, being sensitive to the high-frequency component of the nonlinear response, is especially applicable to the study of intermolecular dynamics in disordered media. The temporal responses of the optical Kerr effect in liquid carbon tetrachloride, benzene, and carbon disulfide are studied using this technique with femtosecond optical pulses. In the response of benzene, damped oscillations are clearly observed which are due to intermolecular vibrational motions of the molecules. The data obtained from benzene and carbon disulfide are analyzed using a Fourier-transform method. The response functions for the IPM measurements are reconstructed, and the Fourier spectra are also obtained. They are discussed in terms of inertial molecular motions and the local structures in these liquids.