Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.Low-energy coherent charge-density wave excitations are investigated in blue bronze (K$_{0.3}$MoO$_{3}$) and red bronze (K$_{0.33}$MoO$_{3}$) by femtosecond pump-probe spectroscopy. A linear gapless, acoustic-like dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K$_{0.33}$MoO$_{3}$. The amplitude mode exhibits a weak (optic-like) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.
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