  Weyl semimetals exhibit exotic Fermi-arc surface states, which strongly
affect their electromagnetic properties. We derive analytical expressions for
all components of the composite density-spin response tensor for the surfaces
states of a Weyl-semimetal model obtained by closing the band gap in a
topological insulating state and introducing a time-reversal-symmetry-breaking
term. Based on the results, we discuss the electromagnetic susceptibilities,
the current response, and other physical effects arising from the density-spin
response. We find a magnetoelectric effect caused solely by the Fermi arcs. We
also discuss the effect of electron-electron interactions within the random
phase approximation and investigate the dispersion of surface plasmons formed
by Fermi-arc states. Our work is useful for understanding the electromagnetic
and optical properties of the Fermi arcs.
