Femtosecond and picosecond nonlinear optical studies of liquids, liquid crystals, and glasses

The ultrafast dynamics of a variety of condensed phase systems have been studied with time-resolved four wave mixing techniques. Transient grating optical Kerr effect (TG-OKE) measurements have been made on liquids and liquid crystals, illuminating the rotational dynamics of those systems. Photon echo experiments have been done on a near infrared dye imbedded in a polymer matrix, revealing information about both the rate of and mechanisms for optical dephasing.; The results of TG-OKE experiments on the nematic liquid crystal pentylcyanobiphenyl are presented. The experiments are performed in the isotropic phase. The rotational dynamics are monitored from femtoseconds to tens of nanoseconds. The nanosecond dynamics are due to the microscopic pseudonematic domains found in the isotropic phase of liquid crystals, and are well described by Landau-deGennes theory up to 30-35{dollar}spcirc{dollar}C above the nematic-isotropic phase transition. The subnanosecond rotational dynamics, corresponding to molecular reorientation within the pseudonematic domains, are temperature independent in this temperature range. Normal hydrodynamic behavior is recovered when the liquid crystal is put in solution.; The rotational dynamics of the simple liquid 2-ethyl naphthalene are presented. After the ultrafast librational dynamics and collision induced effects have decayed, there remains ({dollar}>{dollar}3-4 ps) a triple-exponential decay of the orientational anisotropy. The slower two components exhibit hydrodynamic temperature dependencies, and correspond to rigid rotor rotational diffusion. The fast decay is temperature independent from 2{dollar}spcirc{dollar}C to 40{dollar}spcirc{dollar}C, and is attributed to the relaxational dynamics of local structures in the liquid. Studies of 2-ethyl naphthalene in carbon tetrachloride solutions show a normal hydrodynamic biexponential decay.; The temperature dependence of the optical dephasing of the near infrared dye 1,1{dollar}spprime{dollar},3,3,3{dollar}spprime{dollar},3{dollar}spprime{dollar} -hexamethylindotricarbocyanine iodide (HITCI) in poly(methyl methacrylate) (PMMA) is measured from 1.5 K to 10 K. The optical dephasing is characteristic of that of a chromophore in a low temperature glass, and is dominated by two-level system dynamics at the lowest temperatures. At higher temperatures ({dollar}>{dollar}5 K), the dephasing is exponentially activated. The 15 cm{dollar}sp{lcub}-1{rcub}{dollar} activation energy is the same as that reported for rhodamine B in PMMA, demonstrating that the dephasing mechanism is a mode of the bulk glass.