  We argue that a range of strong metallic electro-optic (EO) effects can be
naturally realized from non-Drude dynamics of free carriers in metals. In
particular, in clean metals we identify skew-scattering and a "Snap"
(third-order derivative of velocity) dominating the Pockels and Kerr EO
behavior of metals in the clean limit. Strikingly, we find that both Pockels
and Kerr EO in metals play critical roles in metallic EO phenomena: for
instance, metallic Pockels and Kerr EO effectively compete to produce a
field-activated birefringence that is non-reciprocal in applied DC fields.
Similarly, both contribute to sizeable field-induced modulations to
transmission and reflection across a range of frequencies. We find metallic EO
effects can be naturally realized in layered 2D materials such as gapped
bilayer graphene producing pronounced values of EO coefficients in the
terahertz -- an interesting new metallic platform for terahertz electro-optic
modulation.
