Colored ultrafast acoustics: from fundamentals to applications.

The aim of this paper is to review the various laser-wavelength effects reported in the field of ultrafast acoustics (UA). First observed by chance in 1999, a wavelength change can indeed have a strong effect on the signal detected in UA. After the physical origin of the effect was clarified and from a systematic exploration we established that all the opto-acoustic mechanisms acting in UA are influenced by the laser-wavelength. From that we suggested original applications of UA to fundamental and applied physics. So emerged a new field, now referred as Colored Picosecond Acoustics or APiC.

High-acoustic-impedance tantalum oxide layers for insulating acoustic reflectors.

This work describes the assessment of the acoustic properties of sputtered tantalum oxide films intended for use as high-impedance films of acoustic reflectors for solidly mounted resonators operating in the gigahertz frequency range. The films are grown by sputtering a metallic tantalum target under different oxygen and argon gas mixtures, total pressures, pulsed dc powers, and substrate biases. The structural properties of the films are assessed through infrared absorption spectroscopy and X-ray diffraction measurements. Their acoustic impedance is assessed by deriving the mass density from X-ray reflectometry measurements and the acoustic velocity from picosecond acoustic spectroscopy and the analysis of the frequency response of the test resonators.

Tunability of aluminum nitride acoustic resonators: a phenomenological approach.

A phenomenological approach is developed to identify the physical parameters causing the dc-voltage-induced tunability of aluminum nitride (AlN) acoustic resonators, widely used for RF filters. The typical resonance frequency of these resonators varies from 2.038 GHz at -200 V to 2.062 GHz at +200 V. This indicates, based on these RF measurements versus dc bias and the model used, that the AlN stiffness variation versus dc bias is the prominent effect because both resonance and antiresonance experience a similar variation, respectively, 24 MHz and 19 MHz at 400 V. Picosecond ultrasonics were also used to prove independently that the acoustic velocity (and therefore AlN stiffness) is sensitive to dc bias and that the variation induced is comparable to that extracted from the resonance measurements. It turned out that the stiffness relative variation for an electric field of 1 V/µm extracted from picosecond ultrasonics is 54 ppm-µm/V. This is in good agreement with the value extracted from the RF measurements, namely 57.2 ppm-µm/V. The overall tunability of these AlN resonators reaches 1.1%, which is an interesting figure, although probably not high enough for genuine applications.

On the existence of subresonance generated in a one-dimensional chain of identical spheres.

In this paper, the propagation of short compressional pulses through a one-dimensional chain of identical spherical beads is analyzed. First, a single sphere is studied. Then, an infinite chain of identical spheres is considered. Finally, a finite linear chain can be seen as a particular case of an infinite chain. It shows the existence of allowed and forbidden frequency bands. In the case of a finite chain of spheres, discrete resonance frequencies are excited and may be identified as "subresonance" of modes observed with a single sphere. We also analyze how the coupling between two adjacent spheres and the number of beads in the chain, modify the degeneracy of the stationary states. Finally, the theoretical results are qualitatively verified with experimental data obtained with the "resonant sphere technique."