  In order to exploit the intriguing optical properties of graphene it is
essential to gain a better understanding of the light-matter interaction in the
material on ultrashort timescales. Exciting the Dirac fermions with intense
ultrafast laser pulses triggers a series of processes involving interactions
between electrons, phonons and impurities. Here we study these interactions in
epitaxial graphene supported on silicon carbide (semiconducting) and iridium
(metallic) substrates using ultrafast time- and angle-resolved photoemission
spectroscopy (TR-ARPES) based on high harmonic generation. For the
semiconducting substrate we reveal a complex hot carrier dynamics that
manifests itself in an elevated electronic temperature and an increase in
linewidth of the $\pi$ band. By analyzing these effects we are able to
disentangle electron relaxation channels in graphene. On the metal substrate
this hot carrier dynamics is found to be severely perturbed by the presence of
the metal, and we find that the electronic system is much harder to heat up
than on the semiconductor due to screening of the laser field by the metal.
