We investigated the generation and detection of coherent acoustic phonons in GaAs/AlAs superlattices under resonant excitation of confined electronic states. Pump-probe experiments were performed on high-quality samples [1], varying the excitation photon energy around the interband transitions e1hh2, e1lh1, and e2hh2. By tuning the excitation energy close to these transitions, strong resonance effects were observed in both phonon generation and photoelastic detection.

Experiments using picosecond pulses revealed well-resolved resonances with linewidths of 4-7 meV, allowing selective excitation of individual electronic states. Modeling of the pump–probe response, using a unified approach to describe the complex refractive index and the resonant photoelastic function based on a Brillouin–Raman electronic density model [2,3], successfully reproduced the experimental spectra. The extracted parameters describe the excitonic transitions and the optical coupling of the quantum wells within the superlattice, providing quantitative insight into the resonant generation of coherent acoustic phonons.

Power-dependent measurements revealed saturation effects in the −1 CZ mode intensity when the excitation was resonant with the e1hh1 transition, while the e2hh2 resonance increased linearly up to the highest pump powers. A phenomenological population-dynamics model, based on rate equations for the confined electronic levels, explains these saturation effects as a consequence of state filling and carrier relaxation dynamics associated with the acoustic phonon generation process.

These results highlights the capability of resonant picosecond ultrafast spectroscopy to probe electron–phonon coupling in semiconductor superlattices, enabling the extraction of electronic, optical, and elastic parameters with sub-meV precision.

References
[1] B. Jusserand et al., Phys. Rev. B 85, 041302(R) (2012).
[2] B. Jusserand et al., Appl. Phys. Lett. 103, 093112 (2013).
[3] B. Jusserand et al., Phys. Rev. Lett. 115, 267402 (2015).