Generation and Propagation of a Picosecond Acoustic Pulse at a Buried Interface: Time-Resolved X-Ray Diffraction Measurements

We report on the propagation of coherent acoustic wave packets in $(001)$ surface oriented ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}/\mathrm{GaAs}$ heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along $[001]$ and $[00\overline{1}]$ directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.

Figure 1

Static x-ray diffraction trace from a single heterostructure sample consisting of a $1.5\mu \mathrm{m}$ (001) ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ eptiatxial film grown on (001) GaAs. The two individual 004 Bragg peaks are clearly resolved.

Figure 2

Intensity of the x-ray diffraction (measured relative to before excitation and normalized to the maximum) following femtosecond laser excitation above the gap in the GaAs substrate. Three different angles near the 004 Bragg peak are shown: (a) 27.546°, on the high angle side of the ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ peak; (b) 27.5415°, on the low angle side of the ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ peak; and (c) 27.5690°, on the low angle side of the GaAs peak.

Figure 3

Simulation of the strain as a function of depth at four different time delays following laser excitation where the energy is deposited in a thin region of the GaAs substrate. A simple thermoelastic model is used in which the strain is due to differential thermal expansion and diffusion is slow. An elastic response is generated which travels at the longitudinal speed of sound. The transmission coefficient for the pulse at the heterostructure interface is unity and the reflection coefficient at the free surface is $-1$. Notice that a unipolar compression pulse propagates through the ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ film, reflects off the free surface becoming a tensile pulse, and later transmits into the GaAs substrate.

Figure 4

Time and angle resolved x-ray diffraction from the ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ film showing the propagation of a short acoustic pulse that was generated in the GaAs substrate. Experimental data are on the left and a simulation in which strain is incorporated into dynamical diffraction theory is on the right.