  Ultrafast photocurrent measurements are performed on individual carbon
nanotube PN junction photodiodes. The photocurrent response to sub-picosecond
pulses separated by a variable time delay {\Delta}t shows strong photocurrent
suppression when two pulses overlap ({\Delta}t = 0). The picosecond-scale decay
time of photocurrent suppression scales inversely with the applied bias VSD,
and is twice as long for photon energy above the second subband E22 as compared
to lower energy. The observed photocurrent behavior is well described by an
escape time model that accounts for carrier effective mass.
