We report time-resolved measurement by phonon absorption in the fracti
onal quantum HalI regime. Experiments have been conducted on an n-type
heterojunction grown on a 1.8 mm GaAs wafer. The start of the absorpt
ion signal is consistent with the ballistic phonon time-of-flight. The
initial rise time is consistent with the time constant of our device
and external circuits. This is followed by unusually slow rise and dec
ay times.