Young's double slit interference pattern from a twisted beam

The interference pattern of a Laguerre Gaussian beam in a double slit experiment is reported. Whereas a typical laser beam phase front is planar, a Laguerre Gaussian beam exhibits a wave front that is twisting along the direction of propagation. This leads to a distorted interference pattern. The topological charge also called the order of the twisted beam can be then readily and simply determined. More precisely, the naked eye resolution of the distortion shift of the interference pattern directly informs about the number of twists made as well as on the sign of the twist. These results are in very good agreement with theoretical calculations that offer a general description of the double slit interference with twisted beams.Comment: 11 pages, 4 figure

Rotation of millimeter-size objects using ordinary light

International audienceThe ability to optically rotate bodies offers new degrees of control of micro-objects with applications in various domains including micro-electromechanical-systems (MEMS), biomanipulations or optofluidics. Here we demonstrate the optically induced rotation of simple asymmetric two-dimensional objects using plane waves originating either from ordinary laser sources or from black body radiation. The objects are floating on an air/water interface. We observe a steady state rotation depending on the light intensity and on the asymmetry of the object. We interpret this rotation in terms of light diffraction by the edges of the object. Such systems could be easily implemented in optofluidic devices to induce liquid flow without the need for special light sources

Soap lms as 1D waveguides

International audienceLaser light is injected in a free standing hori-zontal draining soap lm through the glass frame sustain-ing the lm. Two propagation regimes are clearly identi-ed depending on the lm thickness. At the beginning of the drainage, the soap lm behaves as a multimode-one dimensional optoouidic waveguide. In particular, we ob-serve that the injected light creates a bottleneck in the lm and part of the injected light is refracted leading to whiskers. At the end of the drainage where the lm thick-ness is below µm, there is a strong selection among the various possible optical modes in the lm, and part of the light is deeected. This leads to a self selection of the mode propagation inside the lm

Determination of the topological charge of a twisted beam with a Fresnel bi-prism

International audienceThe self-interference pattern of a Laguerre Gaussian beam using a Fresnel bi-prism is shown to be very different from what could be expected from a usual laser beam. Actually it resembles the interference pattern that could be obtained using a double slit experiment. The interferences are shifted and the topological charge and its sign can be readily determined considering the shift order of the pattern only. However, since there is no diffraction nor absorption loses unlike in a double slit interference, such a set up could be used even for low power twisted beams or beams with high topological charge. Even fractional topological charges could be determined with an absolute precision of 0.05

Light angular momentum of a plane wave diffracted by a two-dimensional simple object.

International audienceThe question of the transfer of angular momentum from light to objects is a key issue in optical tweezers and thus raises issues in physics, chemistry, and biology. Most of the attempts have been performed either with spin angular momentum using light's polarization or using orbital angular momentum with twisted beams. We experimentally demonstrate here the exchange of angular momentum from a linearly polarized plane wave to a two dimensional asymmetric object. More precisely, we show that the diffracted light from a snail-shaped mask carries local orbital angular momentum whose topological charge depends on the symmetry of the object. This opens the way to a means to produce micrometer-sized twisted beams from a high power laser or even pulsed beams

Profile of the liquid film wetting a channel

International audienceWe have developed a simple optical technique to investigate the characteristics of liquid films wetting solid surfaces. To validate this technique, we have studied the wetting film that separates a train of lamellas moving through a channel. Total reflection of the laser beam on the wetting film/air interface is used to extract the profile and the thickness of the wetting film. For quasistatic movement of lamellas, we show that the thickness is well described by a capillary number power law

Numerical investigation of ductile damage parameters identification: benefit of local measurements

International audienceIdentification of material parameters is an important issue to improve the accuracy of finite element computations. Identification of these parameters by inverse analysis is based on experimental observables coming from mechanical experiments. In this paper, a simple tensile test is used. Two types of observables are investigated to identify ductile damage law parameters. The first one is a global measurement, such as the load-displacement curve. The second is a local observable based on full field measurements. Our approach, based on response surfaces, allows an efficient analysis of identification issues. Ill conditioned problems and multi-minima can be obtained using only a global observable. Full field measurements are a good way to improve the identification of plastic hardening and damage law parameters. In fact, local measurements combined with global ones, lead to a better formulation of the inverse problem

Microclimate Data Improve Predictions of Insect Abundance Models Based on Calibrated Spatiotemporal Temperatures

International audienceSpecialty section: This article was submitted to Invertebrate Physiology, a section of the journal Frontiers in Physiology A large body of literature has recently recognized the role of microclimates in controlling the physiology and ecology of species, yet the relevance of fine-scale climatic data for modeling species performance and distribution remains a matter of debate. Using a 6-year monitoring of three potato moth species, major crop pests in the tropical Andes, we asked whether the spatiotemporal resolution of temperature data affect the predictions of models of moth performance and distribution. For this, we used three different climatic data sets: (i) the WorldClim dataset (global dataset), (ii) air temperature recorded using data loggers (weather station dataset), and (iii) air crop canopy temperature (microclimate dataset). We developed a statistical procedure to calibrate all datasets to monthly and yearly variation in temperatures, while keeping both spatial and temporal variances (air monthly temperature at 1 km² for the WorldClim dataset, air hourly temperature for the weather station, and air minute temperature over 250 m radius disks for the microclimate dataset). Then, we computed pest performances based on these three datasets. Results for temperature ranging from 9 to 11 • C revealed discrepancies in the simulation outputs in both survival and development rates depending on the spatiotemporal resolution of the temperature dataset. Temperature and simulated pest performances were then combined into multiple linear regression models to compare predicted vs. field data. We used an additional set of study sites to test the ability of the results of our model to be extrapolated over larger scales. Results showed that the model implemented with microclimatic data best predicted observed pest abundances for our study sites, but was less accurate than the global dataset model when performed at larger scales. Our simulations therefore stress the importance to consider different temperature datasets depending on the issue to be solved in order to accurately predict species abundances. In conclusion, keeping in mind that the mismatch between the size of organisms and the scale at which climate data are collected and modeled remains a key issue, temperature dataset selection should be balanced by the desired output spatiotemporal scale for better predicting pest dynamics and developing efficient pest management strategies

Finite Element Modeling of Void Nucleation, Growth and Coalescence for Large Plastic Strain and Complex Loading Paths

Proceedings http://mech.fsv.cvut.cz/cfrac/proceedings.pdfInternational audienceDuctile damage prediction is still an important issue in the mechanical industry. Ductile failure criteria as well as coupled damage models were enhanced in the last decade by accounting for the influence of stress triaxiality ratio and Lode angle. For the Gurson-Tvergaard-Needleman (GTN) model, these improvements were essentially made for the growth and coalescence stages based on unit-cell models in which the behavior of a single ellipsoidal void is studied under various loading conditions. In the present work, the stage of nucleation is also addressed in addition to void growth and coalescence for large plastic strain and different particle/void morphologies

Time-resolved torsional relaxation of spider draglines by an optical technique.

International audienceThe sensitivity of the torsional pendulum demonstrates the self-shape-memory effect in different types of spider draglines. Here we report the time-resolved noncovalent bonds recovery in the protein structure. The torsional dynamics of such multilevel structure governed by reversible interactions are described in the frame of a nested model. Measurement of three different relaxation times confirms the existence of three energy storage levels in such two protein spidroin systems. Torsion opens the way to further investigations towards unraveling the tiny torque effects in biological molecules