  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.
