Time-resolved study of smectic A guest host system response to 488-nm pulsed excitation by means of laser speckle

Using a pulsed, pump-probe experimental arrangement, we have investigated the speckle generated by a 100 μm path-length cell of smectic A liquid crystal of 4-cyano-4′- octylbiphenyl doped with 0.1% wt/wt dichroic dye. Upon irradiation, the guest-host system undergoes a phase transition from an initially transparent condition to a scattering state. From the statistical theory of speckle, we are seeking to establish the effective scales of turbulence in the liquid crystal, both transversely and longitudinally. A time-resolved analysis of the observed speckle during the relaxation of the system back to the ordered, homeotropic condition revealed the effective scale lengths of the liquid-crystaldye mixture in the irradiated region. Dynamic response as a function of molecular anchoring conditions was also investigated. The importance of this research to power-limiting applications will be discussed.