The absorption of ballistic phonons with frequencies in the 100-GHz ra
nge by a two-dimensional electron gas (2DEG) has been studied in a qua
ntizing magnetic field. The 2DEG was formed at the interface of a GaAs
/AlxGa1-x As heterojunction. Acoustic phonons were created by heating
the substrate locally with a focused laser beam. The phonons traveled
ballistically through the crystal and were partially absorbed by the 2
DEG. This led to a transfer of momentum into the 2DEG (phonon-drag eff
ect) resulting in phonon-induced voltages and currents. These quantiti
es gave detailed information about the interaction between acoustic ph
onons and the 2DEG as a function of both the incident angle of the abs
orbed phonons and the magnetic field. We observed that the dependence
of the phonon-drag signal on the angles of incidence was neither affec
ted by the magnetic field nor by the phonon spectrum. The absolute int
ensity of the phonon-drag signal, however, oscillated in phase with th
e Shubnikov-de Haas oscillations. These results could be explained wit
h a simple microscopic theory of the electron-phonon interaction toget
her with a macroscopic model for the response of the 2DEG on the absor
ption of ballistic phonons.