Personne : Dubé, Louis J.
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Dubé
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Louis J.
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Université Laval. Département de physique, de génie physique et d'optique
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ncf11850600
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Publication Accès libre Light-induced chaotic rotations in nematic liquid crystals(American Physical Society through the American Institute of Physics, 2006-02-23) Brasselet, Étienne.; Dubé, Louis J.Various nonlinear rotation regimes are observed in an optically excited nematic liquid-crystal film under boundary conditions for the light and material that are invariant by rotation. The excitation light is circularly polarized, the intensity profile is circularly symmetric, and the beam diameter at the sample location is a few times smaller than the cell thickness. A transition to chaos via quasiperiodicity is identified when the light intensity is taken as the control parameter. Transverse nonlocal effects are suggested to be the cause of the observed dynamics, and a simple model consisting of a collection of coupled rotators is developed to provide a qualitative explanation.Publication Restreint Bifurcation analysis of optically induced dynamics in nematic liquid crystals : circular polarization at normal incidence(Optical Society of America, 2005-08-01) Brasselet, Étienne.; Galstian, Tigran; Dubé, Louis J.; Dmitry, Krimer; Kramer, LorenzWe present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by an elliptically polarized beam at normal incidence with the intensity and the polarization state of light as the control parameters. The asymmetry arising from the elliptical polarization of the excitation lightwave is shown to affect dramatically the dynamics, and various new dynamical behaviors are reported: (i) quasi-periodic rotations for almost circular polarization; (ii) a discontinuous transition, identified as a homoclinic bifurcation, to a largely reoriented state over a large range of ellipticity values; (iii) oscillations associated with large reorientation; and (iv) optical multistability between several distinct dynamical regimes.Publication Accès libre Time evolution of epidemic disease on finite and infinite networks(American Physical Society through the American Institute of Physics, 2009-02-02) Noël, Pierre-André; Davoud, Bahman; Dubé, Louis J.; Brunham, Robert C.; Pourbohloul, BabakMathematical models of infectious diseases, which are in principle analytically tractable, use two general approaches. The first approach, generally known as compartmental modeling, addresses the time evolution of disease propagation at the expense of simplifying the pattern of transmission. The second approach uses network theory to incorporate detailed information pertaining to the underlying contact structure among individuals while disregarding the progression of time during outbreaks. So far, the only alternative that enables the integration of both aspects of disease propagation simultaneously while preserving the variety of outcomes has been to abandon the analytical approach and rely on computer simulations. We offer an analytical framework, that incorporates both the complexity of contact network structure and the time progression of disease spread. Furthermore, we demonstrate that this framework is equally effective on finite- and “infinite”-size networks. This formalism can be equally applied to similar percolation phenomena on networks in other areas of science and technology.Publication Restreint Transverse nonlocal effects in optical reorientation of nematic liquid crystals(Optical Society of America, 2006-01-01) Brasselet, Étienne.; Lherbier, Aurélien; Dubé, Louis J.We reexamine the optical reorientation of a homeotropic nematic-liquid-crystal film under finite beam-size excitation and linear polarization. We study the case of normal and oblique incidence (extraordinary wave). Under the most basic assumptions (geometrical optics approximation, elastic isotropy) we systematically analyze the dependence of transverse nonlocal effects as a function of the angle of incidence, the beam size, and the intensity. Besides recovering some well-known results (Fréedericksz transition threshold and spatial extension of the molecular reorientation as a function of the beam size), we identify a series of nonmonotonous behaviors of the material response as well as shape variations of the reorientation profiles.Publication Accès libre Optically induced dynamics in nematic liquid crystals : the role of finite beam size(American Physical Society, 2004-02-10) Brasselet, Étienne.; Galstian, Tigran; Dubé, Louis J.; Doyon, BernardWe 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 discussedPublication Restreint The control of hamiltonian chaos(Canadian association of physicists, 2001-03-01) Pourbohloul, Babak; Després, Philippe; Dubé, Louis J.; Doyon, BernardWe demonstrate the control of the chaotic dynamics of Hamiltonian systems. This ability offers the possibility to select (stabilize) at will regular behavior(s) within the chaotic regime and to make efficient use of the richness and diversity of chaos.Publication Restreint Chaotic rotations generated by light in nematic liquid crystals(Gordon and Breach, 2006-09-22) Brasselet, Étienne.; Dubé, Louis J.Various nonlinear rotation regimes are observed in an optically excited nematic liquid crystal film under boundary conditions (for the light and the material) that are invariant by rotation. Th excitation light is circularly polarized, the intensity profile is circularly symmetric and the beam diameter at the sample location is a few times smaller than the cell thickness. A transition to chaos via quasiperiodicity is identified when the light intensity is taken as the control parameter. Transverse nonlocal effects are suggested to be the cause of the observed dynamics and a simple model consisting of a collection of coupled rotators is developed to provide a qualitative explanation.Publication Restreint On stochastic dynamics of hydrogenic ion transport through solids(North-Holland, 2000-09-07) Beuve, Michael; Dubé, Louis J.; Gervais, Benoît; Lamour, Emily; Rozet, Jean-Pierre; Vernhet, DominiqueWe investigate the problem of hydrogenic ion transport through solids. To this end, we consider a system made of a single electron in a given potential and driven by a stochastic force uniformly distributed in time [D.G. Arbò et al., Phys. Rev. A 60 (1999) 1091], and we derive the time evolution equation of the ensemble average density matrix. We show that the random phase approximation of this equation leads to the rate equation model used to describe ion transport [P. Nicolai et al., J. Phys. B 23 (1990) 3609; J.P. Rozet et al., J. Phys. B 22 (1988) 33]. With the help of the Wigner transform, we also demonstrate that both quantum and classical dynamics of an electron under a train of kicks obeys the same equation within the semiclassical approximation. We conclude that seemingly different approaches may be regarded as different approximations of the same quantum problem and we show with a numerical example where differences arise.Publication Accès libre Des ponts d'Euler à la grippe aviaire(Institut des sciences mathématiques, 2009-02-22) Allard, Antoine; Noël, Pierre-André; Dubé, Louis J.Publication Restreint Bifurcation analysis of optically induced dynamics in nematic liquid crystals : elliptical polarization at normal incidence(Optical Society of America, 2005-08-01) Krimer, Dmitry; Brasselet, Étienne.; Kramer, Lorenz; Galstian, Tigran; Dubé, Louis J.We present a detailed bifurcation analysis of the nonlinear reorientation dynamics of a homeotropically aligned nematic liquid-crystal film excited by an elliptically polarized beam at normal incidence with the intensity and the polarization state of light as the control parameters. The asymmetry arising from the elliptical polarization of the excitation lightwave is shown to affect dramatically the dynamics, and various new dynamical behaviors are reported: (i) quasi-periodic rotations for almost circular polarization; (ii) a discontinuous transition, identified as a homoclinic bifurcation, to a largely reoriented state over a large range of ellipticity values; (iii) oscillations associated with large reorientation; and (iv) optical multistability between several distinct dynamical regimes.