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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- PublicationRestreintDynamics of light induced reorientation of nematic liquid crystals in spatially confined beams(Gordon and Breach, 2010-10-18) Brasselet, Étienne.; Galstian, Tigran; Dubé, Louis J.Optically induced reorientation dynamics in a nematic liquid crystal is investigated for circularly polarized laser beams with spot sizes smaller than the sample thickness. Various dynamical regimes, such as periodic, quasi-periodic, intermittent, self-organized and possibly chaotic regimes are observed. The role finite beam size is identified and a qualitative interpretation based on the spatial walk-off of the ordinary and extraordinary beams arising from double refraction phenomenon is proposed.
- PublicationRestreintBond percolation on a class of correlated and clustered random graphs(IOP Pub., 2012-08-15) Hébert-Dufresne, Laurent; Allard, Antoine; Noël, Pierre-André; Dubé, Louis J.; Marceau, Vincent.We introduce a formalism for computing bond percolation properties of a class of correlated and clustered random graphs. This class of graphs is a generalization of the configuration model where nodes of different types are connected via different types of hyperedges, edges that can link more than two nodes. We argue that the multitype approach coupled with the use of clustered hyperedges can reproduce a wide spectrum of complex patterns, and thus enhances our capability to model real complex networks. As an illustration of this claim, we use our formalism to highlight unusual behaviours of the size and composition of the components (small and giant) in a synthetic, albeit realistic, social network.
- PublicationRestreintPhase space engineering in optical microcavities I : preserving near-field uniformity while inducing far-field directionality(2010-08-16) Gagnon, Denis; Dubé, Louis J.; Painchaud-April, Guillaume; Poirier, JulienOptical microcavities have received much attention over the last decade from different research fields ranging from fundamental issues of cavity QED to specific applications such as microlasers and bio-sensors. A major issue in the latter applications is the difficulty to obtain directional emission of light in the far-field while keeping high energy densities inside the cavity (i.e. high quality factor). To improve our understanding of these systems, we have studied the annular cavity (a dielectric disk with a circular hole), where the distance cavity-hole centers d is used as a parameter to alter the properties of cavity resonances. We present results showing how one can affect the directionality of the far-field while preserving the uniformity (hence the quality factor) of the near-field simply by increasing the value of d. Interestingly, the transition between a uniform near- and far-field to a uniform near- and directional far-field is rather abrupt. We can explain this behavior quite nicely with a simple model, supported by full numerical calculations, and we predict that the effect will also be found in a large class of eigenmodes of the cavity
- PublicationAccès libreTargeting unknown and unstable periodic orbits(American Institute of Physics, 2002-03-07) Dubé, Louis J.; Doyon, BernardWe present a method to target and subsequently control (if necessary) orbits of specified period but otherwise unknown stability and position. For complex systems where the dynamics is often mixed [e.g., coexistence of regular and chaotic regions in area-preserving (Hamiltonian) systems], this targeting algorithm offers a way to not only gently bring the system from the chaotic domain to an unstable periodic orbit (where control is applied), but also to access stable regions of phase space (where control is not necessary) from within the stochastic regions. The technique is quite general and applies equally well to dissipative or conservative discrete maps and continuous flows.
- PublicationRestreintTransport of Kr35+ inner-shells through solid carbon foils(Royal Swedish Academy of Sciences, 2001-01-01) Vernhet, Dominique; Dubé, Louis J.; Fourment, Claude; Lamour, Emily; Rozet, Jean-Pierre; Gervais, Benoit; Martín García, Fernando; Minami, Tatsuya; Reinhold, Carlos O.; Seliger, Marek; Burgdörfer, JoachimNew experimental data on the transport of Kr35+ inner-shells initially populated either by capture or by excitation processes are presented. Absolute Lyman (np → 1s) intensities, directly related to the np state populations, as well as 3lj substate populations have been determined over a range of carbon target thickness allowing to study the transport from single collision conditions to equilibrium. Results are compared with predictions of different transport simulations which take into account multiple collisions, the strong target polarization induced by the incoming HCI (the wake field), and radiative decay. Very good agreement is found between theory and experiment for the np populations up to n = 5 where induced wake mixing becomes visible. The simulations also explain the behavior of the 3lj populations of Kr35+ which exhibit a strong sensitivity to the presence of radiative decay during transport and the effective value of the wake field.
- PublicationRestreintEvaluation of a general three-denominator Lewis integral(ScienceDirect, 1995-12-01) Roy, Utpal; Dubé, Louis J.; Mandal, Puspajit; Sil, N. C.An integral of the type ∫dq(q2+μ20)1+1(|q—q1|2+μ21)m+1(|q—q2|2+μ22)n+1 is expressed by contour integration as a sum of two finite series for any finite values of 1, m, n, thus avoiding parametric differentiation of a complicated closed form expression with respect to μ0, μ1, μ2. This integral is frequently encountered in studies of atomic, molecular, nuclear and plasma physics.
- PublicationRestreintBeam shaping using genetically optimized two-dimensional photonic crystals(Optical Society of America, 2012-11-29) Gagnon, Denis; Dubé, Louis J.; Dumont, JoeyWe propose the use of two-dimensional (2D) photonic crystals (PhCs) with engineered defects for the generation of an arbitrary-profile beam from a focused input beam. The cylindrical harmonics expansion of complex-source beams is derived and used to compute the scattered wave function of a 2D PhC via the multiple scattering method. The beam shaping problem is then solved using a genetic algorithm. We illustrate our procedure by generating different orders of Hermite-Gauss profiles, while maintaining reasonable losses and tolerance to variations in the input beam and the slab refractive index.
- PublicationRestreintAb initio investigation of lasing thresholds in photonic molecules(Optical Society of America, 2014-07-17) Gagnon, Denis; Déziel, Jean-Luc; Dubé, Louis J.; Dumont, JoeyWe investigate lasing thresholds in a representative photonic molecule composed of two coupled active cylinders of slightly different radii. Specifically, we use the recently formulated steady-state ab initio laser theory (SALT) to assess the effect of the underlying gain transition on lasing frequencies and thresholds. We find that the order in which modes lase can be modified by choosing suitable combinations of the gain center frequency and linewidth, a result that cannot be obtained using the conventional approach of quasi-bound modes. The impact of the gain transition center on the lasing frequencies, the frequency pulling effect, is also quantified
- PublicationAccès libreMaster equation analysis of mesoscopic localization in contagion dynamics on higher-order networks(American Physical Society through the American Institute of Physics, 2021-03-01) Hébert-Dufresne, Laurent; Allard, Antoine; St-Onge, Guillaume; Dubé, Louis J.; Thibeault, VincentSimple models of infectious diseases tend to assume random mixing of individuals, but real interactions are not random pairwise encounters: they occur within various types of gatherings such as workplaces, households, schools, and concerts, best described by a higher-order network structure. We model contagions on higher-order networks using group-based approximate master equations, in which we track all states and interactions within a group of nodes and assume a mean-field coupling between them. Using the susceptible-infected-susceptible dynamics, our approach reveals the existence of a mesoscopic localization regime, where a disease can concentrate and self-sustain only around large groups in the network overall organization. In this regime, the phase transition is smeared, characterized by an inhomogeneous activation of the groups. At the mesoscopic level, we observe that the distribution of infected nodes within groups of the same size can be very dispersed, even bimodal. When considering heterogeneous networks, both at the level of nodes and at the level of groups, we characterize analytically the region associated with mesoscopic localization in the structural parameter space. We put in perspective this phenomenon with eigenvector localization and discuss how a focus on higher-order structures is needed to discern the more subtle localization at the mesoscopic level. Finally, we discuss how mesoscopic localization affects the response to structural interventions and how this framework could provide important insights for a broad range of dynamics.
- PublicationAccès libreHeterogeneous bond percolation on multitype networks with an application to epidemic dynamics(Published by the American Physical Society through the American Institute of Physics, 2009-03-26) Allard, Antoine; Noël, Pierre-André; Dubé, Louis J.; Pourbohloul, BabakConsiderable attention has been paid, in recent years, to the use of networks in modeling complex real-world systems. Among the many dynamical processes involving networks, propagation processes—in which the final state can be obtained by studying the underlying network percolation properties—have raised formidable interest. In this paper, we present a bond percolation model of multitype networks with an arbitrary joint degree distribution that allows heterogeneity in the edge occupation probability. As previously demonstrated, the multitype approach allows many nontrivial mixing patterns such as assortativity and clustering between nodes. We derive a number of useful statistical properties of multitype networks as well as a general phase transition criterion. We also demonstrate that a number of previous models based on probability generating functions are special cases of the proposed formalism. We further show that the multitype approach, by naturally allowing heterogeneity in the bond occupation probability, overcomes some of the correlation issues encountered by previous models. We illustrate this point in the context of contact network epidemiology.