  We manipulate the transient terahertz (THz) waves emitted from metal-graphite
interfaces, where potential barriers were formed because of work function
differences. To adaptively control the phase of the THz waves, two distinct
groups of metals were evaporated on n-type doped highly oriented pyrolytic
graphite (HOPG): group A, which consisted of Pt, Au, and Ag with work functions
larger than that of HOPG and group B, which consisted of Al and Ti with work
functions smaller than that of HOPG. The phase of the transient THz lineshapes
from group A was opposite to that of group B under infrared laser excitation,
which is indicative of opposite band bending and concomitant interfacial doping
for ambipolar transport at the metal-graphite junctions. The amplitude of the
THz waves could be further substantiated by the work function differences and
modified minority carrier mobilities at the depletion regions.
