Rapid Wave Velocity Measurement by Brillouin Scattering Using Coherent Phonons Induced by ScAlN Piezoelectric Thin-Film Transducer.

It is difficult to perform 2-D imaging of elastic properties using the Brillouin scattering technique because the weak thermal phonon signal in the sample leads to low measurement accuracy and long measurement times. To improve the phonon signal, we artificially induced acoustic phonons using a ScAlN thin-film piezoelectric transducer, which has a giant piezoelectricity. The film was grown using RF magnetron sputtering of a ScAl alloy target on a silica glass bar sample. Using a microwave probe, the electric power applied to the film was 1 mW at 875 MHz. We obtained the enhancement of the Brillouin scattering signal in the silica glass bar sample due to the induced phonons. Compared with and without the induced phonons from the ScAlN film transducer, the peak intensity improved by nearly 3 orders of magnitude. This technique can significantly shorten the time required for the Brillouin scattering measurements.

Wave velocities in articular cartilage measured by micro-Brillouin scattering technique.

Micro-Brillouin scattering was used to measure gigahertz ultrasonic wave velocities in the articular cartilage of a bovine femur. Velocities propagating parallel to the surface of the subchondral bone were 3.36-3.83 × 103 m/s in a dry cartilage sample. Anisotropy measurements were also performed in a 10-µm-diameter local area of the cartilage matrix. A weak velocity anisotropy reflected characteristics of the layers. The velocity also depended on the water content. In the middle layer, the velocity in the dry sample was 3.58 × 103 m/s, whereas that for a fully wet sample was 2.04 × 103 m/s.

Acoustic Wave Velocities and Refractive Indices in an m-Plane GaN Single Crystal Plate and c-Axis Oriented ScAlN Films Measured by Brillouin Scattering Techniques.

We have experimentally investigated wave velocities and refractive indices in bulk and film samples [a GaN single crystal plate and c-axis-oriented ScxAl(1-x)N (x = 0.00-0.63) films] by Brillouin scattering. All of the piezoelectrically unstiffened elastic constants and the ordinary refractive index of the GaN single crystal plate were determined from the reflection induced A (RIA) scattering geometry and the combination of 90R and 180° scattering geometries. The uncertainties of the measured wave velocities were approximately 0.17% (RIA) and 2.5% (combination technique). In addition, the longitudinal wave velocities of ScxAl(1-x)N films propagating in the normal direction were obtained by the combination technique. The maximum uncertainty was approximately 3.3%. The shear wave velocities and refractive indices of ScxAl(1-x)N films were also investigated by the 90R scattering geometry using velocities measured by high-overtone bulk acoustic resonators. The softening trends of the elasticity were obtained from the measured longitudinal and shear wave velocities, although there were large uncertainties in the Brillouin measurement system owing to thermal instability.

Gigahertz acoustic wave velocity measurement in GaN single crystals considering acousto-electric effect.

The resistivity-frequency characteristics of longitudinal wave velocities propagating parallel to the c-axis in a GaN single crystal were theoretically estimated by considering the piezoelectric acousto-electric effect. The temperature and frequency dependences of longitudinal and shear wave velocities in conductive and semiconductive GaN single-crystal samples were experimentally investigated by Brillouin scattering. The temperature dependence of longitudinal and shear wave velocities had a linear tendency in the conductive sample, whereas in the semiconductive sample, those had a similar tendency to the predicted velocity changes resulting from the piezoelectric stiffening effect. However, the temperature dependence of shear wave velocity, which does not possess piezoelectric coupling, had a tendency similar to that of the longitudinal wave in the semiconductive sample, unexpectedly. The frequency dependence of longitudinal wave velocities in the semiconductive sample had a tendency similar to the predicted velocity changes resulting from the piezoelectric stiffening effect.

Application of a micro-Brillouin scattering technique to characterize bone in the GHz range.

The evaluation of elastic properties of bone matrix has been investigated using several techniques such as nanoindentation and scanning acoustic microscopy (SAM). These techniques make use of good spatial resolution, which can prevent effects due to microstructures at the level of several hundreds of microns. In this paper, micro-Brillouin scattering (µ-BR) is introduced as another possible technique to characterize the elastic properties of bone. This technique is well known as a non-contact and non-destructive method to evaluate viscoelastic properties of transparent materials in the GHz range. Using thin, translucent bone specimens with thicknesses of around 100 µm, and the reflection induced optical geometry, ultrasonic wave velocities in the GHz range were obtained. Because this technique optically measures thermal phonons in the specimen, we can easily measure in-plane anisotropy of wave velocities by rotating the specimen. In a single trabecula, the site matched data between SAM and µ-BR showed good correlation, revealing the applicability of this technique to characterize material properties of bone. Some recent results on the anisotropy in a trabecula and the elasticity evaluation of newly and matured bones are also introduced.

Comparative investigation of elastic properties in a trabecula using micro-Brillouin scattering and scanning acoustic microscopy.

Micro-Brillouin scattering (µ-BR) and a 200 MHz scanning acoustic microscope (SAM) with similar spatial resolutions were applied to evaluate tissue elastic properties in two directions in a trabecula. Acoustic impedance measured by SAM was in the range of 5-9 Mrayl. Wave velocities determined by µ-BR were in the range of (4.75-5.11) × 10(3) m/s. Both exhibited a similar trend of variation across the trabecula and were significantly correlated (R(2) = 0.63-0.67, p < 0.01). µ-BR is useful for the evaluation of tissue stiffness within a trabecula. Combined with SAM or nanoindentation, it can provide additional information to assess elastic anisotropy at the micro-scale.

Micro-Brillouin scattering measurements in mature and newly formed bone tissue surrounding an implant.

The evolution of implant stability in bone tissue remains difficult to assess because remodeling phenomena at the bone-implant interface are still poorly understood. The characterization of the biomechanical properties of newly formed bone tissue in the vicinity of implants at the microscopic scale is of importance in order to better understand the osseointegration process. The objective of this study is to investigate the potentiality of micro-Brillouin scattering techniques to differentiate mature and newly formed bone elastic properties following a multimodality approach using histological analysis. Coin-shaped Ti-6Al-4V implants were placed in vivo at a distance of 200 µm from rabbit tibia leveled cortical bone surface, leading to an initially empty cavity of 200 µm×4.4 mm. After 7 weeks of implantation, the bone samples were removed, fixed, dehydrated, embedded in methyl methacrylate, and sliced into 190 µm thick sections. Ultrasonic velocity measurements were performed using a micro-Brillouin scattering device within regions of interest (ROIs) of 10 µm diameter. The ROIs were located in newly formed bone tissue (within the 200 µm gap) and in mature bone tissue (in the cortical layer of the bone sample). The same section was then stained for histological analysis of the mineral content of the bone sample. The mean values of the ultrasonic velocities were equal to 4.97×10(-3) m/s in newly formed bone tissue and 5.31×10(-3) m/s in mature bone. Analysis of variance (p=2.42×10(-4)) tests revealed significant differences between the two groups of measurements. The standard deviation of the velocities was significantly higher in newly formed bone than in mature bone. Histological observations allow to confirm the accurate locations of the velocity measurements and showed a lower degree of mineralization in newly formed bone than in the mature cortical bone. The higher ultrasonic velocity measured in newly formed bone tissue compared with mature bone might be explained by the higher mineral content in mature bone, which was confirmed by histology. The heterogeneity of biomechanical properties of newly formed bone at the micrometer scale may explain the higher standard deviation of velocity measurements in newly formed bone compared with mature bone. The results demonstrate the feasibility of micro-Brillouin scattering technique to investigate the elastic properties of newly formed bone tissue.