Observation of photoinduced polarons in semimetal 1T-TiSe 2 .

In this work, ultrafast carrier dynamics of mechanically exfoliated 1T-TiSe 2 flakes from the high-quality single crystals with self-intercalated Ti atoms are investigated by femtosecond transient absorption spectroscopy. The observed coherent acoustic and optical phonon oscillations after ultrafast photoexcitation reveal the strong electron-phonon coupling in 1T-TiSe 2 . The ultrafast carrier dynamics probed in both visible and mid-infrared regions indicate that some photogenerated carriers localize near the intercalated Ti atoms and form small polarons rapidly within several picoseconds after photoexcitation due to the strong and short-range electron-phonon coupling. The formation of polarons leads to a reduction of carrier mobility and a long-time relaxation process of photoexcited carriers for several nanoseconds. The formation and dissociation rates of the photoinduced polarons are dependent on both the pump fluence and the thickness of TiSe 2 sample. This work offers new insights into the photogenerated carrier dynamics of 1T-TiSe 2 , and emphasizes the effects of intercalated atoms on the electron and lattice dynamics after photoexcitation.