Multiple scattering of ultrasound in weakly inhomogeneous media: application to human soft tissues

Waves scattered by a weakly inhomogeneous random medium contain a predominant single-scattering contribution as well as a multiple-scattering contribution which is usually neglected, especially for imaging purposes. A method based on random matrix theory is proposed to separate the single- and multiple-scattering contributions. The experimental setup uses an array of sources/receivers placed in front of the medium. The impulse responses between every couple of transducers are measured and form a matrix. Single-scattering contributions are shown to exhibit a deterministic coherence along the antidiagonals of the array response matrix, whatever the distribution of inhomogeneities. This property is taken advantage of to discriminate single- from multiple-scattered waves. This allows one to evaluate the absorption losses and the scattering losses separately, by comparing the multiple-scattering intensity with a radiative transfer model. Moreover, the relative contribution of multiple scattering in the backscattered wave can be estimated, which serves as a validity test for the Born approximation. Experimental results are presented with ultrasonic waves in the megahertz range, on a synthetic sample (agar-gelatine gel) as well as on breast tissues. Interestingly, the multiple-scattering contribution is found to be far from negligible in the breast around 4.3 MHz.     

Numerical vibroacoustic analysis of plates with constrained-layer damping patches

A numerical vibroacoustic model that can manage multilayered plates locally covered with damping patches is presented. All the layers can have an on-axis orthotropic viscoelastic behavior. Continuity of displacements and transverse shear stresses at each interface is enforced, which permits to write the entire displacement field in function of the displacements of the--common--first layer, leading to a two-dimensional plate model. The problem is then discretized by Rayleigh-Ritz's method using a trigonometric basis that includes both sine and cosine functions in order to treat various boundary conditions. The excitation can be of mechanical kind (concentrated or distributed forces) or of acoustic kind (plane wave of any incidence, diffuse field, etc.). The model permits to compute different vibroacoustic indicators: the mean square velocity of the plate, the radiation efficiency, and the transmission loss. Comparisons between the present model and numerical results from literature or finite element computations show that the model gives good results in both mechanical and acoustical aspects. Then, a comparison of the effects of different distributions of patches is presented. The role of the surface covering rate is first discussed, followed by a study involving different geometries for the same surface covering rate.     

Photon-photon gates in Bose-Einstein condensates

It has recently been shown that light can be stored in Bose-Einstein condensates for over a second. Here we propose a method for realizing a controlled phase gate between two stored photons. The photons are both stored in the ground state of the effective trapping potential inside the condensate. The collision-induced interaction is enhanced by adiabatically increasing the trapping frequency and by using a Feshbach resonance. A controlled phase shift of π can be achieved in 1 s or less.     

Correlation between thermal and rheological studies to characterize the behavior of bitumen

The objective of this work is a comprehensive thermo-rheological study of pure bitumen. The bitumen is a complex material consisting of asphaltenes dispersed in a maltene matrix. As a consequence, its flow behavior is characterized by the presence of a yield stress, which depends on temperature below 50°C. Applying the Cox–Merz rule, a master curve of viscosity can be obtained over a wide range of shear rates for temperatures above 50°C. It can be accurately modeled by a Carreau–Yasuda law with a yield stress. This specific rheological behavior can be explained by the changes induced by the temperature on the microstructure, evidenced by modulated differential scanning calorimetry measurements.     

Numerical null controllability of a semi-linear heat equation via a least squares method

This Note deals with the computation of distributed null controls for a semi-linear 1D heat equation, in the sublinear and slightly superlinear cases. Under sharp growth assumptions, the existence of controls has been obtained in [E. Fernández-Cara, E. Zuazua, Null and approximate controllability for weakly blowing up semi-linear heat equation, Ann. Inst. Henri Poincaré Analyse non linéaire 17 (5) (2000) 583] via a fixed point reformulation; see also [V. Barbu, Exact controllability of the superlinear heat equation, Appl. Math. Optim. Optimization, Theory and Applications 42 (1) (2000) 73]. More precisely, Carleman estimates and Kakutani?s theorem together ensure the existence of fixed points for a corresponding linearized control mapping. In practice, the difficulty is to extract from the Picard iterates a convergent (sub)sequence. We introduce and analyze a least squares reformulation of the problem; we show that this strategy leads to an effective and constructive way to compute fixed points.     

Optically-pumped semiconductor squeezed-light generation

The theory presented shows that light emitted by low-temperature semiconductors under intense optical pumping (with fluctuations at the shot-noise level, SNL) should be amplitude-squeezed down to half the SNL at nonzeri frequencies. Amplitude squeezing may also be obtained at zero frequency when spontaneous carrier recombination is significant. It is essential that the optical gain depends on photon emission rate, e.g. as a result of spectral-hole burning. A commuting-number theory that agrees exactly with quantum theory for large particle numbers is employed. Comparison is made with results previously reported for three-level atom lasers.     

Des extensions sextiques de ℚ de clôture galoisienne engendrée par deux corps cubiques linéairement disjoints

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