  Semi-metals might offer a stronger interaction and a better confinement for
terahertz wave than semiconductors, while preserve tunability. Particularly,
graphene-based materials are envisioned as terahertz modulators, filters and
ultra-broadband sources. However, the understanding of terahertz generation
from those materials is still not clear, thus limits us recognizing the
potential and improving device performances. Graphite, the mother material of
graphene and a typical bulk semi-metal, is a good system to study semi-metals
and graphene-based materials. Here we experimentally modulate and maximize the
terahertz signal from graphite surface, thus reveal the mechanism - surface
field driving photon induced carriers into transient current to radiate
terahertz wave. We also discuss the differences between graphite and
semiconductors; particularly graphite shows no temperature dependency from room
temperature to 80C. Above knowledge will help us understand terahertz
generations, achieve maximum output and electric modulation, in semi-metal or
graphene based devices.
