  Chiral multifold fermions are quasiparticles described by higher spin
generalizations of the Weyl equation, and are realized as low energy
excitations near symmetry protected band crossings in certain chiral crystals.
In this work we calculate the linear optical conductivity of all chiral
multifold fermions. We show that it is enhanced with respect to that of Weyl
fermions with the same Fermi velocity, and features characteristic activation
frequencies for each multifold fermion class, providing an experimental
fingerprint to detect them. We calculate the conductivity for realistic chiral
multifold semimetals by using lattice tight-binding Hamiltonians that match the
effective models of multifold fermions at low energies, for space groups 199
and 198. The latter includes RhSi, for which we give quantitative predictions,
and also CoSi and AlPt. Our predictions can be tested in absorption or
penetration depth measurements, and are necessary to extract the recently
proposed quantized photocurrents from experiments.
