  When two lasers are applied to a non-centrosymmetric material, light can be
generated at the difference of the incoming frequencies $\Delta\omega$, a
phenomenon known as difference frequency generation (DFG), well characterized
in semiconductors. In this work, we derive a general expression for DFG in
metals, which we use to show that the DFG in chiral topological semimetals
under circular polarized light is quantized in units of $e^3/h^2$ and
independent of material parameters, including the scattering time $\tau$, when
$\Delta\omega \gg \tau^{-1}$. In this regime, DFG provides a simpler
alternative to measure a quantized response in metals compared to previous
proposals based on single frequency experiments. Our general derivation
unmasks, in addition, a free-carrier contribution to the circular DFG beyond
the semiclassical one. This contribution can be written as a Fermi surface
integral, features strong frequency dependence, and oscillates with a $\pi/2$
shift with respect to the quantized contribution. We make predictions for the
circular DFG of chiral and non-chiral materials using generic effective models,
and ab-initio calculations for TaAs and RhSi. Our work provides a complete
picture of the DFG in the length gauge approach, in the clean, non-interacting
limit, and highlights a plausible experiment to measure topologically
quantizated photocurrents in metals.
