## 3 Results and Discussions
### Low Frequency Mini-Brillouin-Zone Mode
#### 3.2.4 Indications of Stimulated Subharmonic Decay of MBZ-Center Mode
\begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multirow{2}{*}{**Experiment**} & \multirow{2}{*}{\(\Delta B\) (GHz)} & \multirow{2}{*}{\(f_{c}\),\(f_{e}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} & \multirow{2}{*}{\(f_{fx}\) (GHz)} \\  & & & & & & & & \\ \hline \multirow{2}{*}{15.1} & \multirow{2}{*}{2.1} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{Lowest} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{Lowest} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{Lowest} & \multirow{2}{*}{31.4} \\  & & & U 16.90 & & & & & \\ \hline \multirow{2}{*}{32.0} & \multirow{2}{*}{2.2} & \multirow{2}{*}{\(k=0\)} & \multirow{2}{*}{\(p=-1\)} & \multirow{2}{*}{31.56} & \multirow{2}{*}{\(q=0\)} & \multirow{2}{*}{\(p=-1\)} & \multirow{2}{*}{31.20} & \multirow{2}{*}{\(f_{\text{B}}\) in} \\  & & & U 32.18 & & & U 32.41 & & \\ \hline \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{-} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(30.8\)} \\  & & & U 49.07 & & & & \\ \hline \multirow{2}{*}{63.1} & \multirow{2}{*}{2.2} & \multirow{2}{*}{\(k=0\)} & \multirow{2}{*}{\(p=-2\)} & \multirow{2}{*}{63.82} & \multirow{2}{*}{\(k=0\)} & \multirow{2}{*}{\(p=-2\)} & \multirow{2}{*}{63.67} & \multirow{2}{*}{\(f_{\text{B}}\) in} \\  & & & U 64.35 & & & U 64.814 & & \\ \hline \multirow{2}{*}{78.3} & \multirow{2}{*}{3.4} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(k=\pi/d_{DR}\)} & \multirow{2}{*}{\(31.8\)} \\  & & & U 81.24 & & & & \\ \hline \multirow{2}{*}{95.1} & \multirow{2}{*}{2.2} & \multirow{2}{*}{\(k=0\)} & \multirow{2}{*}{\(p=-3\)} & \multirow{2}{*}{95.97} & \multirow{2}{*}{\(k=097.22\)} & \multirow{2}{*}{\(p=-3\)} & \multirow{2}{*}{96.18} & \multirow{2}{*}{\(p=+3\)} \\  & & & U 96.53 & & & & \\ \hline \multirow{2}{*}{110.1} & \multirow{2}{*}{1.8} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=0\)} & \multirow{2}{*}{\(p=-3\)} & \multirow{2}{*}{96.18} & \multirow{2}{*}{\(p=+3\)} \\  & & & U 96.53 & & & & \\ \hline \multirow{2}{*}{110.1} & \multirow{2}{*}{1.8} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=\pi/d_{SL}\)} & \multirow{2}{*}{\(k=113.41\)} & \multirow{2}{*}{\(k=0\)} \\  & & & U 113.40 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Acoustic phonon spectrum in the low frequency region. Our second approach to find additional support for the hypothesis of the stimulated subharmonic decay process is to monitor the time dependence of the \(\sim\)15 GHz mode and compare it to the other modes of interest, in particular, the mode at \(\sim\)32 GHz. We analyze the low-frequency part of the oscillations starting from the delay time of 400 ps, where the signal coming from the detection of high-amplitude high-frequency combs in MQWs stops dominating (see Figure 2a). The idea is to extract amplitudes of the individual spectral lines as a function of the time delay from a short-time FFT result, shown in the right-hand-side of Figure 8a. The outcome, i.e., a rising amplitude of the 15 GHz oscillation and simultaneously decaying amplitude of 32 GHz oscillation over time delay, as plotted in Figure 8b, supports the argument that the excitation of the 15 GHz edge mode could stem from the stimulated decay of the \(\sim\)32 GHz center mode. The analysis of the decay of the \(\sim\)32 GHz mode amplitude presented in Figure 8b demonstrates that the attenuation of the mode in the interval from 400 ps to about 450 ps is amplitude-dependent and diminishing with time. Only in the time-delay interval after around 450 ps can the curve be fitted by an exponent with a constant decay time.