Several anomalous symmetry-breaking states have been reported at temperatures well below the charge-density wave (CDW) transition in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Whether these states are concomitant with an electronic structure change remains elusive. Here, we present a study of the optical properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ in the CDW state. In addition to the CDW gap, which opens below $T_{\text{CDW}}$ = 91 K, a conspicuous suppression of the optical conductivity develops around 200 ${\text{cm}}^{-1}$ below $T^{*}\simeq 50\mathrm{K}$, indicating the opening of another low-energy gap well below the CDW transition. A detailed Drude-Lorentz analysis manifests that, similar to the CDW gap, this low-energy gap also opens in the saddle-point bands with $\mathrm{V}\text{−}3d$ character near the $M$ point. Our experimental results not only reveal a Fermi surface reconstruction coinciding with the emergence of the anomalous symmetry-breaking states, but also underline the importance of the $\mathrm{V}\text{−}3d$ orbitals in driving these anomalous states in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

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• Electronic structure
• Fermi surface
• Optical conductivity
• Infrared spectroscopy