Transient laser-induced molecular reorientation

Transient laser-induced molecular reorientation and laser heating in a nematic liquid crystal... 

Quasielectric Light Scattering and Order Fluctuations in the Isotropic Phase 174 Nuclear Magnetic Resonance and Order Fluctuations in the Isotropic Phase. 175 Quasielastic Light Scattering and Orientational Fluctuations below Tc. . . 177 Nuclear Magnetic Resonance and Orientational Fluctuations below Tc.. .. 177 Optical Kerr Effect and Transient Laser-Induced Molecular Reorientation.. 181... 

Optical Kerr Effect and Transient Laser-Induced Molecular Reorientation... 

H. Hsiung. L. P. Shi, Y. R. Shen, Transient laser induced molecular reorientation and laser heating in a nematic liquid crystal, Phys. Rev A 1984,30,1453-1460. [163]... 

The superiority of using lasers for material studies often lies in its spatial and temporal flexibilities, that is, the material can selectively excited and probed in space and time. These qualities may allow us to elucidate fundamental material properties not accessible to conventional techniques. The location, dimension, direction, and duration of the material excitation can be readily controlled through adjustment of the beam spot, direction, polarization, and pulse width of the exciting laser field. The flexibilities can be further enhanced when two or more light waves are used to induce excitations. Such a technique, however, has not yet been fully explored in liquid-crystal research. Although the recent studies of optical-field-induced molecular reorientation in nematic liquid-crystal films have demonstrated the ability of the technique to resolve spatial variation of excitations, corresponding transient phenomena induced by pulsed optical fields have not yet been reported in the literature. Because of the possibility of using lasers to induce excitations on a very short time scale, such studies could provide rare opportunities to test the applicability of the continuum theory in the extreme cases. 

Generally speaking, excitation of a medium by short laser pulses can be used to study dynamic properties of the medium over a very wide time range. Here, we have shown that nanosecond-pulse excitation can yield information about the dynamics of molecular reorientation on the -10-sec time scale, and thermal effect on the 10—l(X)-msec time scale. The power of this technique lies in the fact that a single 6-function-like laser pulse may induce a number of fundamental excitation modes of vastly different time constants. Consider, for example, molecular reorientation coupled with flow induct by a picosecond laser pulse in a liquid crystal. It can be shown that, aside from the thermal effect, the transient behavior will manifest itself with three characteristic time constants ... 

We have presented here the first observation of transient molecular reorientation induced in a liquid crystal by a -switched laser pulse. The response time of molecular reorientation in the nematic phase is of the order of 10—100 psec. Although this is 10 —10 times longer than the duration of the laser pulse, transient molecular reorientation is still strong enough to yield an easily detectable phase shift in the probe beam. Residual al> sorption and subsequent very rapid radiationless conversion into heat can result in a temperature rise in the medium which decays via heat diffusion with relaxation times in the 10—200 msec range. The temperature rise also induces a refractive-index change in the medium and hence a phase shift in the probe beam. This thermal effect and the molecular reorientation are initiated simultaneously by the pulsed laser excitation. They are in general coupled... 

Wong, G. K. L., and Y. R. Shen. 1974. Study of pretransitional behavior of laser-field-induced molecular alignment in isotropic nematic substances. Phys. Rev. A. 10 1277 see, however, Deeg, F. W., and M. D. Payer. 1989. Analysis of complex molecular dynamics in an organic hquid by polarization selective subpicosecond transient grating experiments. Chem. Phys. 91 2269, and references therein, where the observed individual molecular reorientation effect is associated with nuclear reorientation whose dynamics is more complex than the Debye relaxation process described by Wong and Shen. 

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