J.-F. Robillard

Tunable magnetoelastic phononic crystals

The feasibility of tuning the band structure of phononic crystals is demonstrated by employing magnetostrictive materials and applying an external magnetic field. Band structures are calculated with a plane wave expansion method that accounts for coupling between the elastic behavior and the magnetic field through the development of elastic, piezomagnetic, and magnetic permeability effective tensors. We show the contactless tunability of the absolute band gaps of a two-dimensional phononic crystal composed of an epoxy matrix and Terfenol-D inclusions. The tunable phononic crystal behaves like a transmission switch for elastic waves when the magnitude of an applied magnetic field crosses a threshold.

Band gap tunability of magneto-elastic phononic crystal

The possibility of control and tuning of the band structures of phononic crystals offered by the introduction of an active magnetoelastic material and the application of an external magnetic field is studied. Two means to obtain large elastic properties variations in magnetoelastic material are considered: Giant magnetostriction and spin reorientation transition effects. A plane wave expansion method is used to calculate the band structures. The magnetoelastic coupling is taken into account through the consideration of an equivalent piezomagnetic material model with elastic, piezomagnetic, and magnetic permeability tensors varying as a function of the amplitude and orientation of the applied magnetic field. Results of contactless tunability of the absolute bandgap are presented for a two-dimensional phononic crystal constituted of Terfenol-D square rod embedded in an epoxy matrix.

Complete thin film mechanical characterization using picosecond ultrasonics and nanostructured transducers: experimental demonstration on SiO2

Complete mechanical measurements are performed in submicron films using the picosecond ultrasonic technique. The Al layer deposited on the top of the sample acting as a transducer is replaced with a nanostructured Al film. Using an usual picosecond ultrasonic setup we can excite and detect high-frequency longitudinal and surface acoustic waves. From this we can deduce Young’s modulus and Poisson’s ratio of any isotropic thin film. Experimental results obtained for a thin silica layer on silicon are in very good agreement with literature.

Band structures tunability of bulk 2D phononic crystals made of magneto-elastic materials

The feasibility of contactless tunability of the band structure of two-dimensional phononic crystals is demonstrated by employing magnetostrictive materials and applying an external magnetic field. The influence of the amplitude and of the orientation with respect to the inclusion axis of the applied magnetic field are studied in details. Applications to tunable selective frequency filters with switching functionnality and to reconfigurable wave-guides and demultiplexing devices are then discussed.

Phase-controlling phononic crystal

We report on a phononic crystal (PC) consisting of a square array of cylindrical polyvinylchloride inclusions in air that can be used to control the relative phase of two incident acoustic waves with different incident angles. The phase shift between waves propagating through the crystal depends on the angle of incidence of the incoming waves and the PC length. The behavior of the PC is analyzed using the finite-difference-time-domain method. The band structure and equifrequency contours calculated via the plane wave expansion method show that the distinctive phase controlling properties are attributed to noncollinear wave and group velocity vectors in the PC as well as the degree of refraction.

Phase-controlling phononic crystals: realization of acoustic Boolean logic gates.

A phononic crystal (PC) consisting of a square array of cylindrical polyvinylchloride inclusions in air is used to construct a variety of acoustic logic gates. In a certain range of operating frequencies, the PC band structure shows square-like equi-frequency contours centered off the gamma point. This attribute allows for the realization of non-collinear wave and group velocity vectors in the PC wave vector space. This feature can be utilized to control with great precision, the relative phase between propagating acoustic waves in the PC. By altering the incidence angle of the impinging acoustic beams or varying the PC thickness, interferences occur between acoustic wave pairs. It is recognized that information can be encoded with this mechanism (e.g., wave amplitudes/interference patterns) and accordingly to construct a series of logic gates emulating Boolean functions. The NAND, XOR, and NOT gates are demonstrated with finite-difference time-domain simulations of acoustic waves impinging upon the PC.

A converging route towards very high frequency, mechanically flexible, and performance stable integrated electronics

The ability to realize flexible circuits integrating sensing, signal processing, and communicating capabilities is of central importance for the development of numerous nomadic applications requiring foldable, stretchable, and large area electronics. A key challenge is, however, to combine high electrical performance (i.e., millimeter wave, low noise electronics) with mechanical flexibility required for chip form adaptivity in addition to highly stable electrical performance upon deformation. Here, we describe a solution based on ultimate thinning and transfer onto a plastic foil of high frequency CMOS devices initially processed on conventional silicon-on-insulator wafers. We demonstrate a methodology relying on neutral plane engineering to provide high performance stability upon bending, by locating the active layer, i.e., the transistor channel, at the neutral fiber of the flexible system. Following this strategy, record frequency performance of flexible n-MOSFETs, featuring fT/fMAX of 120/145 GHz, is ...

Phase-control in two-dimensional phononic crystals

A theoretical model is developed to ascertain the necessary band structure and equi-frequency contour (EFC) features of two-dimensional phononic crystals (PCs) for the realization of phase control between propagating acoustic waves. Two different PCs, a square array of cylindrical polyvinylchloride inclusions in air and a triangular array of cylindrical steel inclusions in methanol, offer band structures and EFCs with highly dissimilar features. We demonstrate that PCs with EFCs showing non-collinear wave and group velocity vectors are ideal systems for controlling the phase between propagating acoustic waves. Finite-difference time-domain simulations are employed to validate theoretical models and demonstrate the control of phase between propagating acoustic waves in PC structures.

Low work function thin film growth for high efficiency thermionic energy converter: Coupled Kelvin probe and photoemission study of potassium oxide

Recent researches in thermal energy harvesting have revealed the remarkable efficiency of thermionic energy converters comprising very low work function electrodes. From room temperature and above, this kind of converter could supply low power devices such as autonomous sensor networks. In this type of thermoelectric converters, current injection is mainly governed by a mechanism of thermionic emission at the hot electrode which explains the interest for low work function coating materials. In particular, alkali metal oxides have been identified as excellent candidates for coating converter electrodes. This paper is devoted to the synthesis and characterization of potassium peroxide K2O2 onto silicon surfaces. To determine optimal synthesis conditions of K2O2, we present diagrams showing the different oxides as a function of temperature and oxygen pressure from which phase stability characteristics can be determined. From the experimental standpoint, we present results on the synthesis of potassium oxide under ultra high vacuum and controlled temperature. The resulting surface is characterized in situ by means of photoemission spectroscopy (PES) and contact potential difference (CPD) measurements. A work function of 1.35 eV is measured which and the expected efficiency of the corresponding converter is discussed. It is generally assumed that the decrease of the work function in the alkali/oxygen/silicon system, is attributed to the creation of a surface dipole resulting from a charge transfer between the alkali metal and oxygen.

2D–3D Phononic Crystals

This chapter presents a comprehensive description of the properties of phononic crystals ranging from spectral properties (e.g., band gaps) to wave vector properties (refraction) and phase properties. These properties are characterized by experiments and numerical simulations.