Nonlinear effects in acousto-optic imaging

Acousto-optic (AO) imaging is a promising technique that is able to reveal optical properties in the millimeter range inside scattering media by tagging the photon paths with an ultrasonic beam. To increase both the contrast and the resolution of the AO images, we have explored the possibility of using the nonlinear response of the speckle modulation. Variation of the second-harmonic signal as the square of the ultrasonic amplitude has been found, and strong reduction of the tagged zone size has been demonstrated.     

Real-time reflectivity and topography imagery of depth-resolved microscopic surfaces

We have constructed an interference microscope that produces, in real time, reflectivity and topography images of surfaces with depth discrimination better than 1>mu;m . Intensity and phase images are obtained at the rate of 50 per second by use of a multiplexed lock-in detection and MMX assembler-optimized calculation routines. With a wavelength of 0.84microm , depth discrimination of 0.7microm and lateral resolution of 0.3microm were demonstrated, in good agreement with theory. Two-dimensional cross-sectional reflectivity and topography images taken at different depths in an integrated circuit are presented.     

Exploiting the time-reversal operator for adaptive optics, selective focusing, and scattering pattern analysis

We report on the experimental measurement of the backscattering matrix of a weakly scattering medium in optics, composed of a few dispersed gold nanobeads. The decomposition of the time-reversal operator is applied to this matrix and we demonstrate selective and efficient focusing on individual scatterers, even through an aberrating layer. Moreover, we show that this approach provides the decomposition of the scattering pattern of a single nanoparticle. These results open important perspectives for optical imaging, characterization, and selective excitation of nanoparticles.     

Nombre de representations d'une permutation comme produit de deux cycles de longueurs donnees

We consider the Ramsey number r(sK₂, G) where sK₂(s?1) denotes a set of s disjoint edges and G is an arbitrary finite simple graph with no isolated vertices. We obtain upper and lower bounds in the general case. Exact results are obtained for certain classes of graphs.     

Photothermal displacement spectroscopy: An optical probe for solids and surfaces

We present a sensitive technique for determining the optical and thermal properties of solids, surfaces and thin films. This technique, photothermal displacement Spectroscopy, is based on the detection of the thermal expansion of a sample upon absorption of electromagnetic radiation. The technique is well suited for in situ ultrahigh vacuum studies and for experiments where wide temperature ranges are required. We show that surface and bulk optical absorption can be distinguished and that surface absorptions of αL=10^?6/W of incident power can be measured. The theoretical basis of the signal generation is given, and excellent experimental and theoretical agreement is demonstrated. The implications of our findings to imaging and microscopy are discussed.