Continuous-wave phase-sensitive parametric image amplification

We study experimentally parametric amplification in the continuous regime using a transverse-degenerate type-II Optical Parametric Oscillator operated below threshold. We demonstrate that this device is able to amplify either in the phase insensitive or phase sensitive way first a single mode beam, then a multimode image. Furthermore the total intensities of the amplified image projected on the signal and idler polarizations are shown to be correlated at the quantum level.Comment: 14 pages, 7 figures, submitted to Journal of Modern Optics, Special Issue on Quantum Imagin

Pulsed squeezed light: simultaneous squeezing of multiple modes

We analyze the spectral properties of squeezed light produced by means of pulsed, single-pass degenerate parametric down-conversion. The multimode output of this process can be decomposed into characteristic modes undergoing independent squeezing evolution akin to the Schmidt decomposition of the biphoton spectrum. The main features of this decomposition can be understood using a simple analytical model developed in the perturbative regime. In the strong pumping regime, for which the perturbative approach is not valid, we present a numerical analysis, specializing to the case of one-dimensional propagation in a beta-barium borate waveguide. Characterization of the squeezing modes provides us with an insight necessary for optimizing homodyne detection of squeezing. For a weak parametric process, efficient squeezing is found in a broad range of local oscillator modes, whereas the intense generation regime places much more stringent conditions on the local oscillator. We point out that without meeting these conditions, the detected squeezing can actually diminish with the increasing pumping strength, and we expose physical reasons behind this inefficiency

Multimode Squeezing Properties of a Confocal Opo: Beyond the Thin Crystal Approximation

Up to now, transverse quantum effects (usually labelled as "quantum imaging" effects) which are generated by nonlinear devices inserted in resonant optical cavities have been calculated using the "thin crystal approximation", i.e. taking into account the effect of diffraction only inside the empty part of the cavity, and neglecting its effect in the nonlinear propagation inside the nonlinear crystal. We introduce in the present paper a theoretical method which is not restricted by this approximation. It allows us in particular to treat configurations closer to the actual experimental ones, where the crystal length is comparable to the Rayleigh length of the cavity mode. We use this method in the case of the confocal OPO, where the thin crystal approximation predicts perfect squeezing on any area of the transverse plane, whatever its size and shape. We find that there exists in this case a "coherence length" which gives the minimum size of a detector on which perfect squeezing can be observed, and which gives therefore a limit to the improvement of optical resolution that can be obtained using such devices.Comment: soumis le 04.03.2005 a PR

Disrupted Resting-State Functional Connectivity in Progressive Supranuclear Palsy

BACKGROUND AND PURPOSE: Studies on functional connectivity in progressive supranuclear palsy have been restricted to the thalamus and midbrain tegmentum. The present study aims to evaluate functional connectivity abnormalities of the subcortical structures in these patients. Functional connectivity will be correlated with motor and nonmotor symptoms of the disease. MATERIALS AND METHODS: Nineteen patients with progressive supranuclear palsy (mean age, 70.93 ± 5.19 years) and 12 age-matched healthy subjects (mean age, 69.17 ± 5.20 years) underwent multimodal MR imaging, including fMRI at rest, 3D T1-weighted imaging, and DTI. fMRI data were processed with fMRI of the Brain Software Library tools by using the dorsal midbrain tegmentum, thalamus, caudate nucleus, putamen, and pallidum as seed regions. RESULTS: Patients had lower functional connectivity than healthy subjects in all 5 resting-state networks, mainly involving the basal ganglia, thalamus, anterior cingulate, dorsolateral prefrontal and temporo-occipital cortices, supramarginal gyrus, supplementary motor area, and cerebellum. Compared with healthy subjects, patients also displayed subcortical atrophy and DTI abnormalities. Decreased thalamic functional connectivity correlated with clinical scores, as assessed by the Hoehn and Yahr Scale and by the bulbar and mentation subitems of the Progressive Supranuclear Palsy Rating Scale. Decreased pallidum functional connectivity correlated with lower Mini-Mental State Examination scores; decreased functional connectivity in the dorsal midbrain tegmentum network correlated with lower scores in the Frontal Assessment Battery. CONCLUSIONS: The present study demonstrates a widespread disruption of cortical-subcortical connectivity in progressive supranuclear palsy and provides further insight into the pathophysiologic mechanisms of motor and cognitive impairment in this condition

Spatial patterns in optical parametric oscillators with spherical mirrors: classical and quantum effects: errata

We investigate the formation of transverse patterns in a doubly resonant degenerate optical parametric oscillator. Extending previous work, we treat the more realistic case of a spherical mirror cavity with a finite-sized input pump field. Using numerical simulations in real space, we determine the conditions on the cavity geometry, pump size and detunings for which pattern formation occurs; we find multistability of different types of optical patterns. Below threshold, we analyze the dependence of the quantum image on the width of the input field, in the near and in the far field

Altered speech-related cortical network in frontotemporal dementia

Background: In healthy subjects (HS), transcranial magnetic stimulation (TMS) demonstrated an increase in motor-evoked potential (MEP) amplitudes during specific linguistic tasks. This finding indicates functional connections between speech-related cortical areas and the dominant primary motor cortex (M1). Objective: To investigate M1 function with TMS and the speech-related cortical network with neuroimaging measures in frontotemporal dementia (FTD), including the non-fluent variant of primary progressive aphasia (nfv-PPA) and the behavioral variant of FTD (bv-FTD). Methods: M1 excitability changes during specific linguistc tasks were examined using TMS in 24 patients (15 with nfv-PPA and 9 with bv-FTD) and in 18 age-matched HS. In the same patients neuroimaging was used to assess changes in specific white matter (WM) bundles and grey matter (GM) regions involved in language processing, with diffusion tensor imaging (DTI) and voxel-based morphometry (VBM). Results: During the linguistic task, M1 excitability increased in HS, whereas in FTD patients it did not. M1 excitability changes were comparable in nfv-PPA and bv-FTD. DTI revealed decreased fractional anisotropy in the superior and inferior longitudinal and uncinate fasciculi. Moreover, VBM disclosed GM volume loss in the left frontal operculum though not in the parietal operculum or precentral gyrus. Furthermore, WM and GM changes were comparable in nfv-PPA and bv-FTD. There was no correlation between neurophysiological and neuroimaging changes in FTD. Atrophy in the left frontal operculum correlated with linguistic dysfunction, assessed by semantic and phonemic fluency tests. Conclusion: We provide converging neurophysiological and neuroimaging evidence of abnormal speech-related cortical network activation in FTD

Disrupted Resting-State Functional Connectivity in Progressive Supranuclear Palsy

BACKGROUND AND PURPOSE: Studies on functional connectivity in progressive supranuclear palsy have been restricted to the thalamus and midbrain tegmentum. The present study aims to evaluate functional connectivity abnormalities of the subcortical structures in these patients. Functional connectivity will be correlated with motor and nonmotor symptoms of the disease. MATERIALS AND METHODS: Nineteen patients with progressive supranuclear palsy (mean age, 70.93 ± 5.19 years) and 12 age-matched healthy subjects (mean age, 69.17 ± 5.20 years) underwent multimodal MR imaging, including fMRI at rest, 3D T1-weighted imaging, and DTI. fMRI data were processed with fMRI of the Brain Software Library tools by using the dorsal midbrain tegmentum, thalamus, caudate nucleus, putamen, and pallidum as seed regions. RESULTS: Patients had lower functional connectivity than healthy subjects in all 5 resting-state networks, mainly involving the basal ganglia, thalamus, anterior cingulate, dorsolateral prefrontal and temporo-occipital cortices, supramarginal gyrus, supplementary motor area, and cerebellum. Compared with healthy subjects, patients also displayed subcortical atrophy and DTI abnormalities. Decreased thalamic functional connectivity correlated with clinical scores, as assessed by the Hoehn and Yahr Scale and by the bulbar and mentation subitems of the Progressive Supranuclear Palsy Rating Scale. Decreased pallidum functional connectivity correlated with lower Mini-Mental State Examination scores; decreased functional connectivity in the dorsal midbrain tegmentum network correlated with lower scores in the Frontal Assessment Battery. CONCLUSIONS: The present study demonstrates a widespread disruption of cortical-subcortical connectivity in progressive supranuclear palsy and provides further insight into the pathophysiologic mechanisms of motor and cognitive impairment in this condition

Climate change on sea currents is not expected to alter contemporary migration routes of loggerhead sea turtles

For marine species, traveling with the current potentially reduces energetic costs. Still, the extent to which organisms adjust routes to follow current flow remains an open question. Moreover, the extent to which climate change is altering sea currents, and in turn species migration routes, remains unknown, representing a major challenge to spatial ecology and conservation efforts.We developed an approach to assess the extent to which projected optimal paths and corridors overlap with the observed migration routes of loggerhead sea turtles (Caretta caretta), minimizing exposure to opposing sea currents. To illustrate this approach, we used migratory tracks of the species traveling between breeding and foraging areas in the Mediterranean Sea. We calculated the energetic costs to sea turtles based on actual tracks and corresponding optimal paths. We also explored whether projected changes in ocean currents, driven by climate change, would alter the spatial patterns of optimal routes.The energetic cost of observed tracks was, on average, 1.25 times higher than that of corresponding optimal paths. While optimal corridors differed spatially to observed corridors used by loggerheads, some positive correlations still existed for three cases ( 0.43, 0.42, 0.30). Climate change projections showed no significant change to the migratory movement of sea turtles, as corridors for different climatic conditions overlapped by at least 70%.Our results show that loggerheads do not explicitly take advantage of ocean currents to facilitate long distance migrations and reduce energetic demands. The contemporary and future migration routes are characterized by similar energetic demands and together with their strong spatial overlap suggest that climate change is expected to minimally alter the species migration routes in the future. The approach presented here could be applied to different spatial scales and marine taxa, allowing possible mechanisms between sea currents (or other environmental characteristics) and species movements to be elucidated

Synchronization of Hamiltonian motion and dissipative effects in optical lattices: Evidence for a stochastic resonance

We theoretically study the influence of the noise strength on the excitation of the Brillouin propagation modes in a dissipative optical lattice. We show that the excitation has a resonant behavior for a specific amount of noise corresponding to the precise synchronization of the Hamiltonian motion on the optical potential surfaces and the dissipative effects associated with optical pumping in the lattice. This corresponds to the phenomenon of stochastic resonance. Our results are obtained by numerical simulations and correspond to the analysis of microscopic quantities (atomic spatial distributions) as well as macroscopic quantities (enhancement of spatial diffusion and pump-probe spectra). We also present a simple analytical model in excellent agreement with the simulations