Transient grating experiments on liquid crystal MBBA film

We report time profiles of transient diffraction signals created by crossed beam excitation of nematic liquid crystal MBBA. Slightly above its solid - nematic transition temperature (21 .deg. C), the signals decays biexponentially; the fast decay and the slow decay show characteristics of the thermal diffusion and mass diffusion, respectively. Above its nematic - isotropic transition temperature (43 .deg. C), only the thermal decay is observed. Between 32 .deg. C and 42 .deg. C,a new feature in the intermediate time range develops and accompanies multiple order diffractions. Considering the phase relations between the multiple orders, we produce the grating shape by synthesizing with the Fourier components. It shows that the grating shape evolves along the grating vector. We interpret the observed phenomena with the propagation of the torque induced by ordering (and disordering) orientation along the grating vector.