TRANSIENT GRATING EFFECTS IN ABSORBING COLLOIDAL SUSPENSIONS

The time dependence of the refractive index change induced in an absorbing colloidal suspension by a nanosecond laser pulse is studied by the four-wave mixing method. A very large transient is observed even in very dilute suspensions whose amplitude oscillates on a time scale much shorter than the laser pulse duration; the effect is found in a variety of systems. The behavior of the transient suggests that it is associated with an acoustic wave in the suspending fluid excited by an explosive event at the surface of the particles. By comparing the predictions of various hydrodynamic models with the observations and by studying the effect of changing the experimental conditions on the form of the transient, it is proposed that the behavior arises from the very rapid growth of vapor bubbles around those particles which are heated sufficiently strongly; the four-wave mixing signal monitors the coherent superposition of the resulting pressure pulses in the fluid.