Nonequilibrium A1g longitudinal optical phonon with a frequency of 1.84 THz in bismuth telluride (Bi2Te3) is coherently excited by ultrafast pulses. Time-resolved reflectivity measurements show a distinct second harmonic vibration around 3.68 THz at room temperature caused by the nonlinearity of coherent phonon potentially related to the favorable crystal structure of Bi2Te3. The scattering rate between A1g coherent phonon and room temperature incoherent phonon is derived from the pump-fluence-dependent scattering rate of A1g coherent phonon. It is also observed that energy coupling from photoexcited carriers to lattice through coherent phonon vibration is more efficient and faster at higher pump fluence.
Ultrafast dynamics of photoexcited coherent phonon in Bi2Te3 thin films