Density-induced heterodyne-amplified rotational dynamics: a new technique for studying orientational relaxation

A new time-domain method, density-induced heterodyne-amplified rotational dynamics (DIHARD), for studying the orientational relaxation of pure or mixed viscous liquids is described. Extremely weak absorption of an excitation pulse produces a density perturbation like that in a thermal lensing experiment. Through translational–rotational coupling, an orientational anisotropy is produced, which is heterodyne detected with a weak probe field. The DIHARD signal appears with the time for the velocity of sound to cross the excitation spot size and decays with the reorientational dynamics of the sample. DIHARD can provide data in situations where laser-excited optical Kerr experiments are difficult to perform.