Spectroscopic identification of distinct nonlinear photocurrents unveils quantum geometric properties of electron wave functions and the momentum-space topological structures. This is especially interesting, but still puzzling, for chiral topological semimetals with possibilities of hosting giant quantized circular photogalvanic effect. Here we report a comprehensive terahertz (THz) emission spectroscopic analysis of nonlinear photoconductivity of chiral multifold CoSi at 0.26 ∼ 1 eV. We find a large linear shift conductivity ($17\mu \mathrm{A}/{\mathrm{V}}^2$), and confirm a giant injection conductivity ($167\mu \mathrm{A}/{\mathrm{V}}^2$) as a consequence of strongly interfered nonquantized contributions from the vicinity of multifold nodes with opposite chiralities. The bulk injection current excited by the pump field with a complex wave vector is shown to carry both longitudinal and transverse components. Symmetry analyses further unveil weak nonlocal photon drag effect in addition to the photogalvanic effect. This work not only highlights chiral transition-metal monosilicides for midinfrared photovoltaic applications via various nonlinear optical channels, but also consolidates the THz spectroscopy for quantitative photovoltaic research.

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• Optical & microwave phenomena
• Photocurrent
• Topological materials
• Weyl semimetal
• Resistivity measurements
• Terahertz techniques