Effect of spatial inhomogeneity on coherent control of phonons

We investigated coherent phonon dynamics in the charge-density wave (CDW) of 1T-TiSe 2 using pump-pump-probe spectroscopy at 4.8 K. For the observed optical and zone-folded phonon modes, the amplitude and initial phase were periodically modulated depending on the pump-pump separation time, as expected from interference between damped harmonic oscillations. By contrast, the CDW amplitude mode exhibited periodic modulations not only in its amplitude and initial phase but also in its decay constant and frequency. Our numerical simulations suggest that the observed modulations originate from the inhomogeneous distribution of photogenerated carriers owing to the Gaussian pump beam.