  We report on the charge dynamics of kagome FeGe, an antiferromagnet with a
charge density wave (CDW) transition at $T_{\mathrm{CDW}} \simeq 105$ K, using
polarized infrared spectroscopy and band structure calculations. We reveal a
pronounced optical anisotropy, various excitations associated with flat bands
and van Hove singularities (VHSs), and a moderate level of electronic
correlations. Notably, there are two types of remarkable spectral weight (SW)
redistributions for above and below $T_{\mathrm{CDW}}$. The former involves a
transfer between incoherent and coherent excitations driven by the magnetic
splitting-induced elevation of flat bands. The latter manifests itself as a
sudden change of SW from low to high energies for both $a$ and $c$ directions,
suggesting a first-order transition and the three-dimensional nature of CDW.
These anomalies in SW significantly differ from those observed in other kagome
metals like CsV$_3$Sb$_5$, where the nesting of VHSs results in a pronounced
CDW gap feature. Instead, our findings can be accounted for by the jump of VHSs
relative to the Fermi energy via a first-order structural transition involving
large partial Ge1-dimerization. Our study thus unveils a complex interplay
among structure, magnetism, electronic correlations, and charge order in FeGe,
offering valuable insights for a comprehensive understanding of CDW order in
kagome systems.
