  Utilizing ultrafast time-resolved pump-probe spectroscopy measurements, we
investigate the photoinduced quasiparticle dynamics of the topological
materials LaSbTe and CeSbTe. In LaSbTe, the relaxation of quasiparticles is
dominated by two different mechanisms: electron-phonon coupling, and
phonon-assisted electron-hole recombination. Significantly, the amplitude of
photoinduced reflectivity related to the former one shows two pronounced peaks
at 156 K and 263 K, indicating the occurrence of two charge density wave (CDW)
phase transitions. The ultrafast responses of CeSbTe share a lot of
similarities with LaSbTe, and an additional CDW phase transition at 154 K is
revealed in CeSbTe. However, the slower relaxation of CeSbTe exhibits an exotic
behavior that deviates from the typical phonon-assisted electron-hole
recombination process, probably due to the imbalance between the electron- and
hole-type carriers. Unlike LaSbTe, the relaxation times of CeSbTe vary slightly
with the pump power, inferring the possible participation of 4$f$ electron in
the decay process. In addition, two oscillation modes around 1 THz and 3 THz
are identified in both LaSbTe and CeSbTe, which are mostly likely to be
coherent phonon modes. These findings unravel the existence of multiple CDW
orders in LaSbTe and CeSbTe, offering insights into the underlying physics of
these systems.
