When a transparent liquid composed of anisotropically polarizable molecules is
exposed to an intense ultrafast laser pulse (the “pump”), a dipole moment is induced
along the axis of maximum polarizability of each molecule. The induced dipole
moment interacts with the instantaneous electric field of the laser pulse, providing an
impulsive torque that drives the axis of maximum polarizability of the molecules
toward alignment with the polarization of the pump pulse. Since it is the polarizability
of the molecules in a liquid that determines its index of refraction, the pump pulse
creates a slight birefringence in the liquid. The induced birefringence can be
monitored by a low-intensity “probe” pulse. The time dependence of the
birefringence reveals information about microscopic dynamics.