Optimized optical generation and detection of superlattice acoustic phonons

We present a device for the optimization of the folded acoustic phonon signal in femtosecond pump-probe experiments through the spatial decoupling of the generation and detection processes in two different GaAs/AlAs superlattices separated by a thick GaAs layer. A thickness gradient in one of them allows a fine tuning of the generation and detection spectral responses. In the optimal overlap condition, a clear enhancement of the signal is observed at a time delay that matches the travel time of acoustic phonons through the intermediate layer. This nanostructure provides a sensitive tool for studying terahertz phonon propagation over long distances.