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18 May 1998

Volume 72, Issue 20, pp. 2499-2618

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Influence of boundary conditions on time-reversal focusing through heterogeneous media

Mickaël Tanter, Jean-Louis Thomas, and Mathias Fink

Appl. Phys. Lett. 72, 2511 (1998); http://dx.doi.org/10.1063/1.121403 (3 pages) | Cited 5 times

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This letter presents a way to overcome ultrasonic focusing degradations through strongly diffracting nondissipative layers. A first set of experiments shows that, using finite aperture transducer arrays, neither the time-reversal technique, nor other focusing techniques are able to achieve proper focusing through this kind of aberrator. These experimental results show the limits of a finite aperture time-reversal mirror compared to the theoretical behavior of a time-reversal cavity. To simulate a time-reversal cavity, totally reflecting walls are set between the time-reversal mirror and the aberrator. The experiments and numerical simulations presented in this letter show that as soon as we introduce these reflecting boundaries, the time-reversal focusing becomes optimal and the spatial resolution is strongly improved. © 1998 American Institute of Physics.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.38.Ar Transducing principles, materials, and structures: general

A sonochemical approach to the surface synthesis of cadmium sulfide nanoparticles on submicron silica

N. Arul Dhas and A. Gedanken

Appl. Phys. Lett. 72, 2514 (1998); http://dx.doi.org/10.1063/1.120624 (3 pages) | Cited 42 times

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A sonochemical procedure for the preparation of cadmium sulfide (CdS) nanoparticles coated on submicron spherical silica particles has been described. Ultrasonic irradiation of a slurry of silica microspheres, cadmium sulfate, and thiourea in an aqueous medium for 3 h under ambient air yields cadmium sulfide–silica (CSS) composite. Heating the initial amorphous CSS nanocomposite at 150 °C for 1 h under a N2 atmosphere yields diffraction peaks assignable to the CdS phase. The transmission electron microscopy image of CSS shows that the CdS nanoparticles homogeneously coated on the silica (SiO2) carrier. The infrared spectroscopy illustrated the structural changes that occur when the amorphous SiO2 is coated with CdS nanoparticles ultrasonically. The photoluminescence spectrum of the CdS and CSS shows a broadband with a maximum centered around 560 nm, which is similar to that of quantum CdS particles. The sharp onset of a CSS emission band compared to that of the CdS emission band in the red region indicates that the SiO2 carrier provides a medium for controlling the aggregation of CdS nanoparticles. © 1998 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.05.Dz II-VI semiconductors
62.65.+k Acoustical properties of solids
81.07.-b Nanoscale materials and structures: fabrication and characterization
78.30.Fs III-V and II-VI semiconductors
78.55.Et II-VI semiconductors
81.20.-n Methods of materials synthesis and materials processing
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
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