Coherent acoustic phonons in semiconductor superlattices

We investigate the excitation of coherent acoustic phonons in GaAs/AlAs sup erlattices with femtosecond laser pulses. Zone-folded acoustic modes of dif ferent order are observed in the time-domain. The excitation of the modes i s based on an impulsive stimulated Raman process. The coherent phonon ampli tudes can be traced over more than 100 ps which allows us to distinguish pr opagation and decay of the phonons. The corresponding linewidths of the tim e-domain data are below <0.2 cm(-1) in the frequency domain. This is a new approach for high resolution spectroscopy of low energy phonons in semicond uctors.