Schematic diagram of the picosecond ultrasonic experiment. The acoustic pulse was excited by an optical pump pulse and was monitored by the interference behavior of a reflected probe pulse between the interfaces of the sample and the region where refractive index was modulated by the acoustic pulse.
The measured optical reflection change as a function of time delay for the sample of . is the time at which the acoustic pulse entered the (111) GaAs substrate. The oscillations at the frequencies of 27.0 and resulted from the propagation of the acoustic pulse in InGaAs and GaAs layers were observed. The complete trace is shown in the inset.
(a) The measured oscillation frequencies of the optical reflection changes and (b) the calculated indices of refraction for the samples with different In contents.
Longitudinal sound velocity of along the  orientation vs the composition . The measured values are denoted as solid dots, and the calculated ones obtained from the literature values of elastic constants are represented as solid triangles (Ref. 26) and open circles (Ref. 10). The solid line is a quadratic fitting curve according to the measured values of and the calculated sound velocity of InAs. The linear-interpolation curve of the sound velocity is shown in dashed line, and the open rhombuses are the calculated values by assuming linear compositional dependence of elastic constants of .
Longitudinal sound velocities of along the  direction for different In compositions. The calculated sound velocities along  were derived using the literature values of elastic constants and mass densities according to .
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