Nematic kink states in a laser field

Physical Review E, 2004

We report on the influence of a finite beam size on the molecular reorientation dynamics when a nematic liquid crystal film is excited by a laser beam. We present experimental evidence of a new class of nonlinear dynamics when the excitation is a Gaussian shaped, circularly polarized laser beam at normal incidence. Various nonlinear regimes, periodic, quasiperiodic, intermittent, and possibly chaotic, are observed. A physical interpretation based on walk-off effects is proposed and its implications on current research in the field are discussed.

Physical Review A, 1979

Weak-beam amplification and diffracted-beam production are predicted under a variety of conditions, in which thin nematic-liquid-crystal samples maintained in an external magnetic field near Freedericks transitions are simultaneously illuminated by strong and weak beams derived from the same light source. It is shown that for 4-methoxybenzylidene-4-butylaniline, substantial weak-beam amplification should occur under continuous illumination for modest (-0.1 W/cm) strong-beam intensities, for certain cell and beam geometries. If a small positive frequency difference between the strong and weak incident beams is maintained, weak-beam amplification is further enhanced, and the conditions on the angles of incidence for amplification are considerably relaxed.

Physical Review E, 2009

Theoretical study of dynamical phenomena induced by a linearly polarized plane wave incident perpendicularly on a planar aligned nematic layer with the light intensity as the control parameter is reported. We find the threshold of the Optically Induced Fréedericksz Transition for the planar state as a function of the problem parameters. It occurs that the threshold is substantially lower than that expected before. Above the primary instability the director settles either to a stationary or to an oscillatory states depending on a thickness of the layer. These states become unstable at a secondary threshold through a heteroclinic bifurcation and the director settles to a new stationary distorted state.

Physical Review Letters, 1990

An elliptically polarized cw laser beam propagating in a nematic liquid-crystal film can induce molec- ular reorientation and lead the system through various dynamic regimes: torsional oscillations, precession, nutation with precession, and others.

Physics Letters A, 2002

We report the observation of a new light-induced spatio-temporal transition in homeotropically aligned nematic liquid crystals under the action of circularly polarized light. We believe that the competition of symmetric and asymmetric excitation modes is at the origin of this abrupt transition. A theoretical model explaining the main features of the observation is proposed.

Physical Review A, 1976

The effects of nonlinearity in nematic liquid crystals subject to electric and magnetic fields are emphasized. For the Freedericks transition, the distortion profile is calculated exactly. For the domain modes, the torque equation can be approximated to a high degree by the nonlinear sine-Gordon equation. An exact twodimensional periodic solution is found whose consequences are in good quantitative agreement with all experimental observations. The possibility of a first-order transition to a vortex pattern is predicted.

We studied the typical domain size and configuration character of randomly perturbed nematic liquid crystal system, or in general the system of rod-like objects which interact via a Lebwohl-Lasher-type interaction. We described their local direction with a headless unit director field. We introduced into the system the impurities of concentration p, which impose the random anisotropy field-type disorder to directors. We studied the domain-type pattern of molecules as a function of p, anchoring strength W between a neighboring director and impurity, temperature, and history of samples. In simulations, we quenched the directors either from the random or from homogeneous initial configuration. Our results show that a history of system strongly influences: i) the average domain coherence length; and ii) the range of orientational ordering in the system. In the random case, the obtained order is always short-range (SRO). On the contrary, in the homogeneous case, SRO is obtained only for strong enough anchoring W and large enough concentration p. In other cases, the ordering is either quasi long range (QLRO) or long range (LRO). We further studied field-induced memory effects for the random initial configuration. With increasing external ordering field, either QLRO or LRO is realized. This ordering is preserved even if the field is switched off, and its degree saturates for a large enough value of the field. Therefore, one can control the degree of global ordering and average domain coherence size by temporarily exposing the system to an external ordering field. Such systems could be exploited as soft matter based phase-memory devices.

Discrete and Continuous Dynamical Systems - Series S, 2010

The paper derives the evolution equations for a nematic liquid crystal, under the action of an electromagnetic field, and characterizes the transition between the isotropic and the nematic state. The non-simple character of the continuum is described by means of the director, of the degree of orientation and their space and time derivatives. Both the degree of orientation and the director are regarded as internal variables and their evolution is established by requiring compatibility with the second law of thermodynamics. As a result, admissible forms of the evolution equations are found in terms of appropriate terms arising from a free-enthalpy potential. For definiteness a free-enthalpy is then considered which provides directly the dielectric and magnetic anisotropies. A characterization is given of thermally-induced transitions with the degree of orientation as a phase parameter.

Journal of The Optical Society of America B-optical Physics, 2005

We present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by a circularly polarized beam at normal incidence with the light intensity as the control parameter. The secondary bifurcation above the optical Fréedericksz transition threshold is identified as a supercritical Hopf bifurcation leading to quasi-periodicity, and the subsequent discontinuous transition from quasi-periodicity to periodicity at higher intensity is identified as a homoclinic bifurcation. The bifurcation scenario is compared with the one obtained in the case of an ordinary light wave at small oblique incidence. Despite an analogous sequence of transitions, there are substantial differences.

Journal of the Optical Society of America B, 2005

We present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by a circularly polarized beam at normal incidence with the light intensity as the control parameter. The secondary bifurcation above the optical Fréedericksz transition threshold is identified as a supercritical Hopf bifurcation leading to quasi-periodicity, and the subsequent discontinuous transition from quasi-periodicity to periodicity at higher intensity is identified as a homoclinic bifurcation. The bifurcation scenario is compared with the one obtained in the case of an ordinary light wave at small oblique incidence. Despite an analogous sequence of transitions, there are substantial differences.