In this work we perform ultrafast time-resolved reflectivity measurements to study the symmetry breaking in the charge-density wave (CDW) phase of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. By extracting the coherent phonon spectrum in the CDW phase of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, we discover close phonon pairs near 1.3 THz and 3.1 THz, as well as a new mode at 1.84 THz. The 1.3 THz phonon pair and the 1.84 THz mode are observed up to the CDW transition temperature. Combining density-functional theory calculations, we show that these phonon pairs arise from the coexistence of Star of David and inverse Star of David distortions combined with sixfold rotational symmetry breaking. An anisotropy in the magnitude of transient reflectivity change is also revealed at the onset of CDW order. Our results thus indicate broken sixfold rotational symmetry in the charge-density wave state of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, along with the absence of nematic fluctuation above $T_{\text{CDW}}$. Meanwhile, the measured coherent phonon spectrum in the CDW phase of ${\mathrm{CsV}}_3{\mathrm{Sb}}_{5-x}{\mathrm{Sn}}_x$ with $x=0.03$–0.04 matches the staggered inverse Star of David with interlayer $\pi$ phase shift. This CDW structure contrasts with undoped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ and explains the evolution from a phonon pair to a single mode at 1.3 THz by $x=0.03$–0.04 Sn doping.

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• Charge density waves
• Kagome lattice
• Density functional theory
• Ultrafast pump-probe spectroscopy