  Nonlinear couplings between photons and electrons in new materials give rise
to a wealth of interesting nonlinear phenomena. This includes frequency mixing,
optical rectification or nonlinear current generation, which are of particular
interest for generating radiation in spectral regions that are difficult to
access, such as the terahertz gap. Owing to its specific linear dispersion and
high electron mobility at room temperature, graphene is particularly attractive
for realizing strong nonlinear effects. However, since graphene is a
centrosymmetric material, second-order nonlinearities a priori cancel, which
imposes to rely on less attractive third-order nonlinearities. It was
nevertheless recently demonstrated that dc-second-order nonlinear currents as
well as ultrafast ac-currents can be generated in graphene under optical
excitation. The asymmetry is introduced by the excitation at oblique incidence,
resulting in the transfer of photon momentum to the electron system, known as
the photon drag effect. Here, we show broadband coherent terahertz emission,
ranging from about 0.1-4 THz, in epitaxial graphene under femtosecond optical
excitation, induced by a dynamical photon drag current. We demonstrate that, in
contrast to most optical processes in graphene, the next-nearest-neighbor
couplings as well as the distinct electron-hole dynamics are of paramount
importance in this effect. Our results indicate that dynamical photon drag
effect can provide emission up to 60 THz opening new routes for the generation
of ultra-broadband terahertz pulses at room temperature.
