  The absence of mirror symmetry, or chirality, is behind striking natural
phenomena found in systems as diverse as DNA and crystalline solids. A
remarkable example occurs when chiral semimetals with topologically protected
band degeneracies are illuminated with circularly polarized light. Under the
right conditions, the part of the generated photocurrent that switches sign
upon reversal of the light's polarization, known as the circular photogalvanic
effect, is predicted to depend only on fundamental constants. The conditions to
observe quantization are non-universal, and depend on material parameters and
the incident frequency. In this work, we perform terahertz emission
spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral
topological semimetal CoSi. We identify a large longitudinal photocurrent
peaked at 0.4 eV reaching $\sim$ 550 $\mu A/V^{2}$, which is much larger than
the photocurrent in any chiral crystal reported in the literature. Using
first-principles calculations we establish that the peak originates from
topological band crossings, reaching 3.3$\pm$0.3 in units of the quantization
constant. Our calculations indicate that the quantized CPGE is within reach in
CoSi upon doping and increase of the hot-carrier lifetime. The large
photo-conductivity suggests that topological semimetals could potentially be
used as novel mid-infrared detectors.
