Light-induced chaotic rotations in nematic liquid crystals.

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.

On Jacobian matrices for flows.

We present a general method for constructing numerical Jacobian matrices for flows discretized on a Poincaré surface of section. Special attention is given to Hamiltonian flows where the additional constraint of energy conservation is explicitly taken into account. We demonstrate the approach for a conservative dynamical flow and apply the technique for the general detection of periodic orbits.

Optically induced dynamics in nematic liquid crystals: The role of finite beam size.

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.

Optically induced dynamics in nematic liquid crystals: the role of twist deformation and asymmetry.

We present a theoretical study of optically induced dynamics in a homeotropic nematic liquid crystal excited at normal incidence. By retaining the first symmetric and antisymmetric reorientation modes, the dynamical equations are reduced to a four-dimensional problem. The main advantage of this minimal approach is to emphasize the role of twisted mode and asymmetry of the light-induced molecular reorientation in a manner suitable for a clear physical interpretation. Theoretical results are compared with experiments in the particular case of circularly polarized light beams to show the physical origin of mode competition and of the breakdown of chiral and longitudinal symmetry. The model successfully describes previous experimental studies such as time-dependent three-dimensional molecular dynamics, light-induced stabilized helical reorientation, and in-plane precession regime in achiral nematics. While a recent experiment has revealed a new spatiotemporal transition, the model succeeds to describe all the features of such a bifurcation pointing out anew the importance of asymmetry. Finally, the first quantitative description of the appearance of a giant mirrorless optical bistability when twisted reorientation modes are excited is demonstrated. A qualitative physical interpretation is suggested for all these phenomena.

Targeting unknown and unstable periodic orbits.

We 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.

Intramedullary epidermoid cysts of the spinal cord. Case report.

Epidermoid cysts are tumors familiar to neurosurgeons, but intramedullary epidermoid cysts are rare. The authors report the case of a 6-year-old girl presenting with progressive paraparesis. A midthoracic intramedullary mass was revealed on myelography and magnetic resonance (MR) imaging and confirmed as an intramedullary epidermoid cyst at surgery, at which time the cyst was removed. This is the fourth report documenting a purely intramedullary epidermoid cyst occurring in a child. The pathology and etiology, epidemiology, clinical features, radiology (including MR image characteristics), and surgical treatment of such rare intramedullary benign tumors are discussed. Magnetic resonance imaging reduces the delay in diagnosis of spinal cord tumors but should be guided by clinical judgment.

Paediatric myelography with iohexol.

32 paediatric patients requiring myelography were studied with iohexol 210 mgI/ml (n = 15) and iohexol 180 mgI/ml (n = 17). Visualization was good or excellent in over 90% of the group receiving the higher concentration and in over 83% of the lower. There were no side effects in either group and vital signs were not significantly altered during the procedure. In the majority of patients (14/18) who had CT, additional diagnostic information was obtained. Iohexol 180 mgI/ml is preferred for routine use, but 210 mgI/ml may give superior visualization with no apparent added risk. Procedural technique and good communication with the patients and their families may have contributed to the absence of side effects.

Management of thyroid nodules in children: a 20-year experience.

The medical records and surgical slides of 58 patients with the diagnosis of thyroid nodules (solitary nodule in 50 patients) are reviewed. The most common cause of thyroid nodules in this series is follicular adenoma (27 patients or 46%). A nuclear scan (technetium or radioactive iodine) was performed in 55 patients, of which 40 showed a cold nodule. Twelve of the 40 cold nodules were malignant (30%). However, for solitary nodules the incidence of cancer is 27%. This last figure is significantly greater than the one recently reported by Hung et al (18.5%). Available diagnostic methods are reviewed and the clinical management as derived from our experience is presented.

[Trichopoliodystrophy or Menkes disease]. / Trichopoliodystrophie, ou la maladie de Menkes.

Trichopoliodystrophy (also known as Menkes' kinky or steely hair disease), a recessive sex-linked syndrome, is characterized by severely retarded mental and physical development, convulsions, a particular phenotype and abnormalities of the hair, bones and arteries. Very low levels of copper and ceruloplasmin in the serum confirm the diagnosis. This rare disorder is caused by an inborn error of copper metabolism whose nature is not yet clear. Recent hypotheses favour either an abnormality in the transport of copper across the cell membrane or increased affinity for copper of the intracellular binding protein. Because the metabolic abnormality is expressed autonomously and irregularly in various tissues, the distribution of copper within the body is disordered. Up to now none of the many forms of copper therapy has succeeded in modifying the fatal course of the disease in humans. This article presents a new case, the first in Canada, and a review of the other 69 cases described in the literature. The new case illustrates, in addition to the classic picture, less well known features, such as diverticula of the bladder mucosa and serosa, as well as cortical atrophy and malformed cerebral vessels demonstrated by computer-assisted tomography.