37 research outputs found
The Lorenz model for single-mode homogeneously broadened laser: analytical determination of the unpredictible zone
We have applied harmonic expansion to derive an analytical solution for the
Lorenz-Haken equations. This method is used to describe the regular and
periodic self-pulsing regime of the single mode homogeneously broadened laser.
These periodic solutions emerge when the ratio of the population decay rates is
smaller than 0.11. We have also demonstrated the tendency of the Lorenz-Haken
dissipative system to behave periodic for a characteristic pumping rate "2CP"
[4], close to the second laser threshold "2C2th" (threshold of instability).
When the pumping parameter "2C" increases, the laser undergoes a
period-doubling sequence. This cascade of period doubling leads towards chaos.
We study this type of solutions and indicate the zone of the control parameters
for which the system undergoes irregular pulsing solutions. We had previously
applied this analytical procedure to derive the amplitude of the first, third
and the fifth order harmonics for the laser-field expansion [4, 14]. In this
work, we extend this method in the aim of obtaining the higher harmonics. We
show that this iterative method is indeed limited to the fifth order, and that
above, the obtained analytical solution diverges from the numerical direct
resolution of the equations.Comment: 20 pages, 4 figures, 1 anne
Kindling molecules: a new way to âbreakâ the Abbe limit
International audienceFluorescence microscopy is a key tool for biological investigations. However, compared to other techniques like electron microscopy, the achievable resolution is still limited. Tremendous efforts have been devoted to improve the resolution of far-field optical microscopy. Several techniques do exist; however their adoption by biologists has been slowed by several technical limitations. We propose a new method based on a recently discovered family of optically switchable fluorescent molecules. Kindling proteins open the way to very high resolution in far-field fluorescence 3-D microscopy with relatively simple techniques
Polarized confocal theta microscopy
International audienceWe propose a comprehensive treatment of theta microscopy based on dipole emission, which better describes fluorescence emission than the isotropic emission model, as fluorescence emission is often polarized. Formulas describing the point spread function for polarized confocal fluorescence theta microscopy are given. Examples are given and some advantages of polarized theta fluorescence microscopy are presented
Isotropic-Resolution Tomographic Diffractive Microscopy
International audienceMicroscopy techniques based on recording of the optical field diffracted by the specimen, in amplitude and phase, like Digital Holographic Microscopy (DHM) have been a growing research topic in recent years. Tomographic acquisitions are possible if one is able to record information, while controlling variations of the specimen illumination. Classical approaches consider either illumination variation, simple to implement, but suffering fro the classical "missing cone" problem, or sample rotation, delivering images with quasi-isotropic, but lower resolution. We have developed an original-, combined tomographic diffractive microscope setup, making use of specimen rotation as well as illumination rotation, which is able to deliver images with an almost isotropic resolution better than 200 nm
MĂ©canismes d'usure de revĂȘtements de nitrure de titane et d'aluminium en microtribologie
Le comportement Ă lâusure de films minces de Ti1-xAlxN (0 †x †1) dĂ©posĂ©s par PVD est caractĂ©risĂ© et analysĂ© en fonction de la micro et nano-structure des couches. La structure de ces films est Ă lâorigine de leurs propriĂ©tĂ©s fonctionnelles. Les directions de croissance des films nanostructurĂ©s ont Ă©tĂ© mesurĂ©es par diffraction des rayons X. Les modes dâendommagement des films ont Ă©tĂ© obtenus par des essais de microtribologie Ă tempĂ©rature ambiante contre une bille dâalumine pour pallier lâusure du pion, Ă faible chargement et faible vitesse de glissement, pour Ă©viter la formation de couches dâoxyde en surface. Pour comprendre les mĂ©canismes dâusure engendrĂ©s par le frottement de la bille, plusieurs essais ont Ă©tĂ© mis en Ćuvre : dĂ©termination de la tĂ©nacitĂ© par des essais de rayure, observation de la trace par microscopie Ă©lectronique Ă balayage et mesure du volume dâusure aprĂšs plusieurs allers-retours par profilomĂ©trie interfĂ©romĂ©trique par holographie. Lorsque les dĂ©bris dâusure restent piĂ©gĂ©s dans le sillon, leur quantitĂ©, directement liĂ©e Ă la rĂ©sistance Ă lâinitiation de fissures du revĂȘtement, et leur nature, ductiles ou fragiles, ont une grande influence sur lâendommagement final. Les films Ă forte teneur en aluminium texturĂ©s selon la direction [002] du rĂ©seau hexagonal, prĂ©sentent un comportement fragile. Lorsque la teneur en aluminium augmente, le nombre de cycles au bout duquel le coefficient de frottement subit un accroissement considĂ©rable diminue. Le nombre de cycles Ă partir duquel le coefficient de frottement augmente considĂ©rablement diminue lorsque la concentration en aluminium augmente. Les films riches en titane dont la direction principale de croissance est [200] du rĂ©seau cubique, prĂ©sentent un meilleur comportement Ă lâusure. Pour ces films ductiles, lâexistence de domaines orientĂ©s suivant la direction [111] semble jouer un rĂŽle important sur la tĂ©nacitĂ©. En effet, la quantitĂ© de dĂ©bris gĂ©nĂ©rĂ©s est liĂ©e Ă une plus ou moins grande proportion de domaines cristallisant dans cette direction
Validation of image processing tools for 3-D fluorescence microscopy
International audience3-D optical fluorescent microscopy becomes nowadays an efficient tool for volumic investigation of living biological samples. Using optical sectioning technique, a stack of 2-D images is obtained. However, due to the nature of the system optical transfer function and non-optimal experimental conditions, acquired raw data usually suffer from some distortions. In order to carry out biological analysis, raw data have to be restored by deconvolution. The system identification by the point-spread function is useful to obtain the knowledge of the actual system and experimental parameters, which is necessary to restore raw data. It is furthermore helpful to precise the experimental protocol. In order to facilitate the use of image processing techniques, a multi-platform-compatible software package called VIEW3D has been developed. It integrates a set of tools for the analysis of fluorescence images from 3-D wide-field or confocal microscopy. A number of regularisation parameters for data restoration are determined automatically. Common geometrical measurements and morphological descriptors of fluorescent sites are also implemented to facilitate the characterisation of biological samples. An example of this method concerning cytogenetics is presented