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Recently, metasurfaces have been proven to be effective and compact devices for the design of arbitrary wavefronts. Metasurfaces are planar metamaterials with a subwavelength thickness that allows wavefront shaping by introducing in-plane variations, namely, gradients, in the spatial wave response of these flat structures. Here we report a new class of acoustic gradient-index (GRIN) metasurfaces engineered from soft graded-porous silicone rubber with a high acoustic index for broadband ultrasonic three-dimensional wavefront shaping in water. The functionalities of these soft flat lenses are illustrated through various experiments, which demonstrate beam steering and beam focusing, as well as vortex beam generation in free space. These new GRIN metasurfaces may have important applications in various domains using designed ultrasonic fields (biomedical imaging, industrial non-destructive testing, contactless particle manipulation), since their fabrication is very straightforward with common polymer science engineering. Here, the authors report an approach for creating various acoustic wavefronts in free space by using soft gradient-index porous metasurfaces. These flat lenses generate steered planar, focused spherical and helical ultrasonic beams in water by manipulating the spatial distribution of the material porosity.

Negative refraction of acoustic waves is demonstrated through underwater experiments conducted at ultrasonic frequencies on a 3D locally resonant acoustic metafluid made of soft porous silicone-rubber micro-beads suspended in a yield-stress fluid. By measuring the refracted angle of the acoustic beam transmitted through this metafluid shaped as a prism, we determine the acoustic index to water according to Snell’s law. These experimental data are then compared with an excellent agreement to calculations performed in the framework of Multiple Scattering Theory showing that the emergence of negative refraction depends on the volume fraction Φ of the resonant micro-beads. For diluted metafluid ( Φ = 3 % ), only positive refraction occurs whereas negative refraction is demonstrated over a broad frequency band with concentrated metafluid ( Φ = 17 % ).

Numerous luminophores may be encapsulated into silica nanoparticles (< 100 nm) using the reverse microemulsion process. Nevertheless, the behaviour and effect of such luminescent molecules appear to have been much less studied and may possibly prevent the encapsulation process from occurring. Such nanospheres represent attractive nanoplatforms for the development of biotargeted biocompatible luminescent tracers. Physical and chemical properties of the encapsulated molecules may be affected by the nanomatrix. This study examines the synthesis of different types of dispersed silica nanoparticles, the ability of the selected luminophores towards incorporation into the silica matrix of those nanoobjects as well as the photophysical properties of the produced dye-doped silica nanoparticles. The nanoparticles present mean diameters between 40 and 60 nm as shown by TEM analysis. Mainly, the photophysical characteristics of the dyes are retained upon their encapsulation into the silica matrix, leading to fluorescent silica nanoparticles. This feature article surveys recent research progress on the fabrication strategies of these dye-doped silica nanoparticles.

Soft acoustic metamaterials that consist of a concentrated suspension of macroporous microbeads and that show a broadband negative acoustic refractive index are now demonstrated. Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . One of the challenges at present is to extend that field beyond electromagnetism by realizing three-dimensional (3D) media with negative acoustic indices 9 . We report a new class of locally resonant ultrasonic metafluids consisting of a concentrated suspension of macroporous microbeads engineered using soft-matter techniques. The propagation of Gaussian pulses within these random distributions of ‘ultra-slow’ Mie resonators is investigated through in situ ultrasonic experiments. The real part of the acoustic index is shown to be negative (up to almost − 1) over broad frequency bandwidths, depending on the volume fraction of the microbeads as predicted by multiple-scattering calculations. These soft 3D acoustic metamaterials open the way for key applications such as sub-wavelength imaging and transformation acoustics, which require the production of acoustic devices with negative or zero-valued indices.

One way to obtain materials with a negative index is to achieve media with doubly negative constitutive parameters. We show that the double negativity required for perfect (non-dissipative) media is not necessary for real passive (lossy) metamaterials, for which single negativity of one of the two constitutive parameters may be sufficient. The analysis is presented through original diagrammatic representations involving both the real and imaginary parts of the dynamic constitutive parameters for isotropic media. A valid demonstration for acoustic and electromagnetic metamaterials is supported by considering an actual soft 3D locally-resonant ultrasonic metafluid having a negative acoustic index while being a single-negative medium. The crucial role of the imaginary parts of the constitutive parameters is then discussed through its impact on the figure of merit. Finally, the analysis is extended to an example of an electromagnetic negative-index metamaterial exhibiting both single and double negativity.

Fluorescent silica nanoparticles are widely used for various applications from mechanical reinforcement to biology. In many cases, their surface has to be tailored. Herein fluorescent silica nanoparticles are synthesized by a reverse micro-emulsion process and functionalized by silane coupling agents owning amino and thiol groups. The functionalization is then characterized by macroscopic well-known methods (zeta potential, hydrophilic to hydrophobic ratio, etc.) and an original method based onto TEM observations of the contrast between the silica core and the metallic ions chelated by the functional groups grafted onto the surface is also introduced. This method reveals that the functionalization is effective and that it occurs by “nano domains.” It is therefore possible to characterize the functionalization by in situ observations. Finally, the characterized nanoparticles are incorporated into a PMMA thin film. The fluorescence of the nanoparticles allows the monitoring of the level of dispersion of the nanoparticles within the polymer and confirms all the other characterizations.

We describe a method for the fabrication of acoustic metasurfaces, which is based on soft porous polymer materials. The materials are obtained using an emulsion templating technique, which allows for the fabrication of soft porous polymers with fully controlled porosity values between 0 and 30%. Our approach involves the polymerization of water-in-silicone emulsions with controlled water volume fractions. The obtained wet solid monolith samples are dried using three different methods. Due to the softness of the polymer matrix, and like in polyHIPE hydrogels or silica aerogels, the first method—regular air drying—leads to a collapse of the material and we present a complete experimental study of the observed kinetics as well as a model to account for the observed results. We show that this model can catch the kinetics characteristics. Then, using two alternative drying techniques, H 2 O 2 -assisted and supercritical drying, we show that it is possible to obtain materials with fully controlled porosities. The speed of sound—or equivalently the material acoustic index—inside the material being dependent on its porosity, we obtain a gradient-index acoustic material by spatially controlling the porosity distribution along the two dimensions of these metasurfaces. Their ability in terms of wavefront shaping is then demonstrated through a deflecting experiment performed in water with a sample having a thickness five times smaller than the incident acoustic wavelength at ultrasonic frequencies.

We consider theoretically the scattering of an incident evanescent plane wave by a spherical particle. The scattering problem is treated in a classic way by applying the T-matrix formalism and the resulting field is expressed on the basis of the different vibration modes of the particle. Compared to the case of a homogeneous plane incident wave, additional azimuthal scattered modes are excited and their contribution provokes a symmetry breaking of the field. Importantly, if a mode is preferentially excited by choosing the corresponding reduced frequency, the scattered radiation exhibits a spiral structure. The scattered field has a rotating phase around the scatterer which comes from the formation of spiral scattered waves and this effect is accentuated by increasing the evanescence degree of the incident wave. These results could have important implications for the contactless manipulation of objects with acoustic radiation forces and torques.

Based on micro-Raman spectroscopy (μRS) and X-ray photoelectron spectroscopy (XPS), we study the structural damage incurred in monolayer (1L) and few-layer (FL) graphene subjected to atomic-layer deposition of HfO 2 and Al 2 O 3 upon different oxygen plasma power levels. We evaluate the damage level and the influence of the HfO 2 thickness on graphene. The results indicate that in the case of Al 2 O 3 /graphene, whether 1L or FL graphene is strongly damaged under our process conditions. For the case of HfO 2 /graphene, μRS analysis clearly shows that FL graphene is less disordered than 1L graphene. In addition, the damage levels in FL graphene decrease with the number of layers. Moreover, the FL graphene damage is inversely proportional to the thickness of HfO 2 film. Particularly, the bottom layer of twisted bilayer (t-2L) has the salient features of 1L graphene. Therefore, FL graphene allows for controlling/limiting the degree of defect during the PE-ALD HfO 2 of dielectrics and could be a good starting material for building field effect transistors, sensors, touch screens and solar cells. Besides, the formation of Hf-C bonds may favor growing high-quality and uniform-coverage dielectric. HfO 2 could be a suitable high-K gate dielectric with a scaling capability down to sub-5-nm for graphene-based transistors.

Gadolinium oxide nanoparticles are more and more used. They can notably provide interesting fluorescence properties. Herein they are incorporated into a non-aqueous-based polymer, the poly(methyl methacrylate). Their dispersion within the polymer matrix is the key to improve the composite properties. As-received gadolinium oxide nanopowders cannot be homogeneously dispersed in such a polymer matrix. Two surface treatments are, therefore, detailed and compared to achieve a good stability of the nanoparticles in a non-aqueous solvent such as the 2-butanone. Then, once the liquid suspensions have been stabilized, they are used to prepare nanocomposites with homogeneous particles dispersion. The two approaches proposed are an hybrid approach based on the growth of a silica shell around the gadolinium oxide nanoparticles, and followed by a suitable silane functionalization; and a non-hybrid approach based on the use of surfactants. The surface treatments and formulations involved in both methods are detailed, adjusted and compared. Thanks to optical methods and in particular to the use of a ‘home made’ confocal microscope, the dispersion homogeneity within the polymer can be assessed. Both methods provide promising and conclusive results.