We study terahertz (THz) emission from GaAs as a function of photon energy and electric field. THz radiation arises from transport of photogenerated charge in an electric field and by hot carrier diffusion (the photo-Dember effect). These mechanisms can be separated by experiments in which either the electric field or the kinetic energy of the carriers is varied. For electric fields E∼4 kV/cm, we find that the electric field controls THz emission for carrier temperatures kBTC⩽0.1 eV, while hot-carrier diffusion dominates for kBTC≈1 eV. Both mechanisms contribute at intermediate fields and carrier temperatures. Our results are consistent with estimates of the relative magnitudes of these two effects.

Many ultrasonic devices, among which are surface and bulk acoustic wave devices and ultrasonic transducers, are based on multilayers of heterogeneous materials, i.e., piezoelectrics, dielectrics, metals, and conducting or insulating fluids. We introduce metal and fluid layers and half spaces into a numerically stable scattering matrix model originally proposed for solving the problem of plane wave propagation in piezoelectric and dielectric multilayers. The method is stable for arbitrary thicknesses of the layers. We discuss how the surface Green's functions can be computed for an arbitrary stack of homogeneous materials with plane interfaces. Aditionnally, we set up a backscattering algorithm to compute the distribution of electromechanical fields at any point in the stack. The model is assessed by considering some well-known examples.

International audienceThe development of new surface acoustic wave devices exhibiting complicated electrode patterns or layered excitation transducers has been favored by an intense innovative activity in this area. For instance, devices exhibiting interdigital transducers covered by piezoelectric or dielectric layers have been fabricated and tested, but the design of such structures requires simulation tools capable to accurately take into account the actual shape of the wave guide elements. A modeling approach able to address complicated surface acoustic wave periodic structures (defined in the saggital plane) exhibiting any geometry then has been developed and implemented. It is based on the combination of a finite element analysis and a boundary element method. A first validation of the computation is reported by comparison with standard surface wave devices. Surface transverse wave resonators covered by amorphous silica have been built and consequently used for theory/experiment assessment. Also the case of recessed electrodes has been considered. The proposed model offers large opportunities for modeling any two-dimensional periodic elastic wave guide

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International audienceAs layer transfer techniques have been notably improved in the past years, lithium niobate (LiNbO3) appears as a candidate for the next generation of ultrawide band radio frequency (rf) filters. Depending on the crystalline orientation, LiNbO3 can achieve electromechanical coupling factors Kt2 more than six times larger than those of sputtered aluminum nitride films. In this letter, a process based on direct bonding, grinding, polishing, and deep reactive ion etching is proposed to fabricate a single crystal LiNbO3 film bulk acoustic resonator. From the fabricated test vehicles, Kt2 of 43% is measured confirming the values predicted by theoretical computation

Responses of partially immersed elastic structures using a symmetric formulation for coupled boundary element and finite element methods

The possibility to excite and detect acoustic waves in fluids using capacitive transducers built on silicon using surface micromachining offers attractive opportunities in the manufacturing of high quality low cost imaging probes. As in the case of standard probe transducers, simulation codes are required to accurately design such devices. The periodic structures extensively used for these capacitive transducers has to be accounted for. In this work, a two-dimensional finite element analysis of capacitive micromachined ultrasonic transducers (cMUT) is proposed, taking into account periodicity and radiation in fluids. The convergence of the calculation is verified using different computation approaches. It is then shown that the periodic computations provide a rapid and precise analysis of the cMUT compared to non periodic calculations. The mutual displacements are deduced from the periodic harmonic calculation, providing an efficient estimation of cross-talk phenomena arising for cMUT radiating in water. The capability of cMUT operating under such conditions to generate a low velocity wave guided at the fluid/silicon interface is theoretically pointed out.

International audienceWe report on the observation of elastic waves propagating in a two-dimensional phononic crystal composed of air holes drilled in an aluminum nitride membrane. The theoretical band structure indicates the existence of an acoustic band gap centered around 800 MHz with a relative bandwidth of 6.5% that is confirmed by gigahertz optical images of the surface displacement. Further electrical measurements and computation of the transmission reveal a much wider attenuation band that is explained by the deaf character of certain bands resulting from the orthogonality of their polarization with that of the source