Resonances of coherent acoustic phonons were excited and detected by femtosecond light pulses for determining the normal elastic constant of ultrathin platinum films. The elastic constant increases with the decrease of the film thickness, exceeds the bulk value at the thickness near 5 nm, and significantly increases at low temperatures. It shows a correlation with the normal lattice distance. Thus, this Letter provides evidence of the stiffness enhancement in ultrathin films caused by lattice anharmonicity.

All independent components of the elastic constants, internal friction, and piezoelectric coefficients of synthetic ␣-quartz have been simultaneously determined by resonance ultrasound spectroscopy coupled with laser-Doppler interferometry. Seventeen crystals obtained from Z and X regions with various infrared-absorption values were used; for each crystal, a complete set of the coefficients was determined using 72 resonance frequencies, which were measured by a needle-transducer tripod in a vacuum at a constant temperature with frequency-error limit of 0.001%. The infrared-absorption value and grown region did not strongly affect the material coefficients. Among the eight ͑elastic plus piezoelectric͒ coefficients, piezoelectric coefficients were significantly different from previously reported values. The six independent internal-friction components showed a positive correlation with the temperature derivatives of the corresponding elastic constants, indicating that the dominant damping mechanism was the phonon-phonon scattering. Our values at 30°C are

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This paper presents advanced techniques to determine all independent elastic-stiffness coefficients Cij, the associated internal friction Qij -1 , and piezoelectric coefficients eij of monocrystal langasite (La3Ga5SiO14) using a single rectangular parallelepiped specimen. Langasite's crystal structure belongs to the trigonal system with point group 32, thus six independent Cij, two eij, and two dielectric coefficients e ij. All of the elastic and piezoelectric coefficients affect the mechanical resonance frequencies of the solid specimen, and measuring them very accurately permits one to determine the Cij and eij with known density, dimensions, and e ij. We developed a piezoelectric tripod to support the specimen upward and measured the free-vibration resonance frequencies with minimum load from its own weight. This weak and stable acoustic coupling ensures high accuracy of the frequency measurement better than 10 -5 , being enough to determine the reliable coefficients.Our Cij fall in the range of results measured with previous (conventional) methods. Our e11 is smaller than the reported values by 1.2-13%, and e14 is larger than those by 44-97%. For the internal friction measurement, we used a solenoid coil to vibrate the specimen without any contact. The longitudinal-wave internal friction considerably exceeds the shear-wave internal friction, which can be interpreted as phonon-phonon interactions.

We studied the relationship between elastic constants and microstructure in sputtered vitreous SiO 2 thin films using pump-probe picosecond laser ultrasound. The delayed probe light pulse is diffracted by the acoustic wave excited by the pump light pulse, inducing Brillouin oscillations, seen as reflectivity change in the probe pulse, whose frequency can be used to extract the sound velocity and elastic moduli. Theoretical calculations were made to explain the asymmetric response of Brillouin oscillations and to predict the possible error limit of the determined elastic constants. The thin films containing defects exhibited lower elastic constant. A micromechanics modeling was developed to evaluate defect porosity and attenuation caused by scattering was able to predict the defect size. Elastic moduli of the defect-free specimens increased with increasing sputtering power, eventually exceeding the bulk value, and correlated with phonon frequencies, indicating that the decrease in the Si-O-Si bond angle of the tetrahedral structure increased the stiffness.