Free induction decay processes of coherent folded longitudinal acoustic phonons in GaAs/AlAs finite size superlattices: control through the design of phonon dispersion curves

We present the concept on coherence that, in finite-size systems, free induction decay time is controllable through appropriately designing frequency dispersion curves as function of wave vector. We confirmed the present concept from numerical calculation of free induction decay time of coherent folded longitudinal acoustic (LA) phonons that are generated through impulsive stimulated Raman scattering processes in fine-size GaAs/AlAs superlattices. It is well-known that, in finite-size superlattices, the wave-vector (momentum) conservation is relaxed; therefore, the width of the folded LA phonon Raman band is broadened. This is the preset focusing viewpoint. We found that a ratio of GaAs-thickness to AlAs-layer thickness, which governs the line shape of the phonon dispersion curve, influences the free induction decay time of the coherent folded LA phonon. The above-mentioned ratio also causes a change in the zone-centre gap frequency of the folded LA phonon dispersion. The zone-centre gap relates to the free induction decay time. The coherent folded LA phonon is an ensemble of the in-phase oscillating folded LA phonons. Since the finite-effect causes the relaxation of the wave-vector conservation, the ensemble of the in-phase oscillating folded LA phonons are composed of the phonons with various frequencies. The variety in frequency of the constituent folded LA phonons results in the appearance of the free induction decay depending on the zone-centre gap frequency depending on the phonon dispersion curve.