Phase manipulation of terahertz waves by work function engineering in   metal-graphite structures
  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. 
Muhammad Irfan, Soo Kyung Lee, Jong-Hyuk Yim, Yong Tak Lee and   Young-Dahl Jho
cond-mat.mtrl-sci physics.optics