Photoacoustic 3-D imaging of polycrystalline microstructure improved with transverse acoustic waves

Abstract

Non-invasive fast imaging of grain microstructure of polycrystalline ceria with sub-micrometric spatial resolution is performed via time-domain Brillouin scattering. The propagation of a nanoacoustic pulse is monitored down to 8 μm deep in a 30 × 30 μm 2 area. Grains boundaries are reconstructed in three-dimensions via a two-step processing method, relying on the wavelet synchro-squeezed transform and the alphashape algorithm. Imaging contrast is improved by taking advantage of stronger sensitivity to anisotropy of transverse acoustic waves, compared with longitudinal waves. Utilization of transverse waves in the image processing reveals additional boundaries, confirmed by an electron backscattering diffraction pattern but not discerned using longitudinal waves. A buried inclined interface between differently oriented grains is identified by monitoring changes in amplitude (phase) of the portion of the signal associated with transverse (longitudinal) waves. Estimates of the inclination angle of this interface prove the sensitivity of our laser ultrasonic method to image inclined boundaries.

Picosecond acoustics: a new way to access elastic properties of materials at pressure and temperature conditions of planetary interiors

“…For an absolute determination of the sound velocity at high pressure, the refractive index variation has to be known or modeled. Nevertheless, due to their sensitivity, these measurements can be used either to obtain a 3D image of the sample [6], or to get the values of the sound velocity at their extremes by frequency dispersion studies [8], even if the n 0 variation is unknown.…”

“…Temporal, imaging and movie-mode measurements have been performed at λ= 800 nm on a commercially available (Optowise) single-crystal of MgO oriented along the [001] direction 6 , with a surface of 2x2 mm 2 and a nominal thickness of 100(20) µm. MgO is transparent at this wavelength, as such it needs to be coated at the pump side to allow the absorption of the laser and to generate an acoustic wave.…”

“…Since its invention in 1984 [5] this technique has been extended to measurements at high pressure and high temperature and used to experimentally determine elastic constants, sound velocities and equations of state (EOS). Recently, 3D-echography imaging measurements of samples in a diamond anvil cell (DAC) have also been reported [6].…”

Towards acoustic microscopy at the nanoscale by coupling atomic force microscopy with picosecond ultrasonics

“…Since picosecond acoustics was developed by Thomsen et al [1,2], time-resolved pump-probe spectroscopy has proven to be a very effective way to probe and image nanoscale elastic properties of a wide variety of materials. Employed both, in industrial and fundamental research, it has recently been used to perform elasticity imaging of various systems such as biological cells [3][4][5][6][7][8][9][10] and polycristalline materials [11][12][13]. The generation of acoustic waves with frequencies up to a couple hundred GHz enables achievement of very good in-depth resolution [14].…”

Time-domain Brillouin imaging of sound velocity and refractive index using automated angle scanning

“…The latter technique, known as picosecond ultrasonic interferometry or TDBS (Time-Domain Brillouin Scattering), relies on the photoelastic effect, and has been used for depth profiling and three-dimensional (3D) imaging of the longitudinal sound velocity in transparent samples, including biological cells, assuming a constant refractive index distribution [17] , [18] , [19] , [20] , [21] , [22] , [23] . In addition, various other applications have been suggested for TDBS, such as monitoring polycrystalline aggregates, ion implantation profiles, internal stress distributions, and temperature gradients [24] , [25] , [26] . TDBS has also been used to profile both the refractive index and sound velocity by multiple probe-light incidence angle measurements, but the proposed apparatus is not automated, leading to arduous data acquisition procedures [11] , [27] , [28] .…”