Laser-Induced Optical Kerr Effect and the Dynamics of Orientational Order in the Isotropic Phase of a Nematogen

Prost, Jacques; Lalanne, J. R.

doi:10.1103/physreva.8.2090

Cited by 91 publications

(19 citation statements)

References 5 publications

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“…at This equation is identical to the evolution equation forming the basis of the analysis of the laser-induced Kerr effect (or optical-field induced ordering) for a nematogen in its isotropic phase [22,23], the modelling of which was initially due to the de Gennes [24]. The optical Kerr effect follows if we assume that K is directly proportional to a tensor which is representative of the deviation from isotropy of some electromagnetic property such as the magnetic susceptibility (the proportionality relation necessarily follows as the only general relation linking any couple of traceless secondorder tensors for isotropic behaviour).…”

Section: Kerr Effectmentioning

confidence: 97%

Continuum modelling of polyelectrolytes in solution

Drouot

Maugin

1985

Rheol Acta

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A phenomenological model for solutions of polyelectrolytes accounting for electromechanical interactions is proposed within the framework of continuum thermodynamics. The modelling involves the conformation of macromolecules in the form of a tensorial "internal variable" and the electric polarization of the solution. The paper aims to demonstrate the possible competition between flow and electricity effects on rigorous phenomenological grounds. Three kinds of dissipative mechanisms are accounted for: viscosity of the solution, electric relaxation and the relaxation of the conformation of macromolecules. Anisotropic effects induced by the applied electric field and flow-induced polarization through changes in conformation are exhibited. These effects are small. By way of application, the problem of the orientation and conformation taken by macromolecules of polyelectrolytes under the combined influence of a simple shear flow and a longitudinal or orthogonal electric bias field is treated in detail and illustrated graphically. Some optical properties are mentioned.

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Section: Kerr Effectmentioning

confidence: 97%

Continuum modelling of polyelectrolytes in solution

Drouot

Maugin

1985

Rheol Acta

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“…This level of truncation provides a minimal description of the qualitative features of the nematic-isotropic transition. Applying the condition, (6) to (5) where B S t = 2c (9) The actual nematic-isotropic phase transition occurs at a temperature, T, between T * and TI-given by, 2 B 2 9 aC T = T * + --which is the temperature at which B goes to zero for the nematic branch.…”

Section: Thermal Properties Of Nematic Liquid Crystalsmentioning

confidence: 99%

Molecular Orientational Relaxation and the Thermal Nonlinear Refraction of Nematic Liquid Crystals

Greeff

1994

Molecular Crystals and Liquid Crystals Science and Technology.

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Laser heating is known to make an important contribution to the nonlinear refraction of liquid crystals due to the large rate of change of the refractive index with temperature, dnldT. There is a strong enhancement of dnldT near the nematic-isotropic phase transition which is associated with the rapid variation of the nematic order parameter, S, with temperature. This has been seen to lead to a strong enhancement of the nonlinear refraction. However, if the nonlinear refraction is probed with laser pulses on the nanosecond time scale, incomplete relaxation of the order parameter during the pulse results in a reduction of the observed nonlinear refraction. The time scale for relaxation of the order parameter also shows a large peak near the phase transition temperature, so that for short laser pulses, the temperature dependence of the effective nonlinear refractive index, n2, will be strongly influenced by the relaxation dynamics of the order parameter. This paper presents a simple model for the time evolution of the orientational order of a nematic liquid crystal subjected to heating by a gaussian laser pulse. By treating the orientational degrees of freedom with a phenomenological Landau-deGennes theory. and assuming that the other degrees of freedom equilibrate quickly compared to the pulse duration, it is possible to solve for the time dependence of the order parameter, and thus the indices of refraction. By defining an intensity weighted average of the change in index of refraction, an effective nonlinear refractive index can be extracted, such as would be measured in an experiment of the z-scan type. Using parameters which have been measured for the material 5CB, it is found that the slow relaxation completely cancels the enhancement of dnldT near the phase transition, resulting in an effective n2 which is nearly independent of temperature, for pulse durations less than a few tens of nanoseconds.

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“…Orientational fluctuation of these domains, or equivalently, randomization of their directors with respect to each other after a reorienting torque, gives rise to the well-known collective response, with exponential decay time in the range of nanoseconds and above. This collective response of LC is described well by the Landau-de Gennes theory treating the LC intermolecular interactions as a mean-field [34,40,41,42,43,44,45,46,51,52]. As the temperature is lowered towards the isotropic-nematic transition, the size of the pseudo-nematic domains, and thus the collective response decay time, tend to diverge, exhibiting characteristic pretransitional behavior.…”

Section: Pure Lc Responsementioning

confidence: 99%

“…The strong host-host interaction makes the mean-field approximation inadequate for describing the rotational dynamics of liquid crystals in the isotropic phase. This topic in fact has received considerable interests, both theoretically and experimentally, over the last three decades [34,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60]. Though a complete understanding of the subject seems still missing [58,59,60], many salient features have been understood.…”

Section: Pure Lc Responsementioning

confidence: 99%

Dynamical Study of Guest-Host Orientational Interaction in LiquidCrystalline Materials

Truong

2005

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Guest-host interaction has long been a subject of interest in many disciplines.Emphasis is often on how a small amount of guest substance could significantly affect the properties of a host material. This thesis describe our work in studying a guest-host effect where dye-doping of liquid crystalline materials greatly enhances the optical Kerr nonlinearity of the material. The dye molecules, upon excitation and via intermolecular interaction, provides an extra torque to reorient the host molecules, leading to the enhanced optical Kerr nonlinearity.We carried out a comprehensive study on the dynamics of the photoexcited dyedoped liquid crystalline medium. Using various experimental techniques, we separately characterized the dynamical responses of the relevant molecular species present in the 2 medium following photo-excitation, and thus were able to follow the transient process in which photo-excitation of the dye molecules exert through guest-host interaction a net torque on the host LC material, leading to the observed enhanced molecular reorientation.We also observed for the first time the enhanced reorientation in a pure liquid crystal system, where the guest population is created through photoexcitation of the host molecules them-

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Laser-Induced Optical Kerr Effect and the Dynamics of Orientational Order in the Isotropic Phase of a Nematogen

Cited by 91 publications

References 5 publications

Continuum modelling of polyelectrolytes in solution

Continuum modelling of polyelectrolytes in solution

Molecular Orientational Relaxation and the Thermal Nonlinear Refraction of Nematic Liquid Crystals

Dynamical Study of Guest-Host Orientational Interaction in LiquidCrystalline Materials

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