Ultrafast dynamics of coherent phonon vibration in thermoelectric materials

Dynamics of phonons and excited carriers are key parameters for the performance of technologically important devices. The excited carriers (electrons and holes), which are either pumped optically or electrically, will relax back to the ground states by scattering with phonons, boundaries, impurities, and so on. Time-resolved optical measurement is a unique way to study specific optical phonon mode as ultrafast pulses generate predominantly the A 1g coherent phonon vibration. Compared with the Raman scattering technique, the time-resolved measurement offers real-time investigation of phonon dynamics with better signal-noise ratio and bandwidth resolution. This study presents the ultrafast dynamics of coherent phonon vibration in thermoelectric material such as Bi, Bi2Te3, and Sb 2Te3 films through time resolved reflectivity measurements. Ultrafast time-resolved reflectivity of a bismuth thin film pumped by 800 nm/400 nm wavelength pulses was measured to investigate the coherent phonons in bismuth. It is found that the pulse with a wavelength of 400 nm is more efficient to excite the free carriers and coherent phonon than the 800 nm wavelength pulse does. By controlling the coherent phonon vibration through a double-pulse femtosecond pulse train generated from a temporal pulse shaper, energy coupling from laser pulse to coherent phonons in Bi is investigated. It is found that the increase of bismuth temperature is dependent on the separation time between the two laser pulses. Using a numerical fitting, which considers the effect of convolution between the incident pulses and the materials response, the measured temperature increases using different pulse-to-pulse separations allow quantitative determination of the amount of laser energy coupled from excited electrons to coherent phonon vibration. Time-resolved reflectivity measurements of Bi2Te3 show a distinct second harmonic vibration around 3.68 THz at room temperature caused by the nonlinearity of coherent phonon itself. The scattering rate between A1g coherent phonon and room temperature incoherent phonons is derived by measuring the pump-fluence-dependent scattering rate of A 1g coherent phonon. It is observed that the energy coupling from photoexcited carriers to lattice through coherent phonon vibration is more efficient and faster at higher pump fluence. By measuring the time-resolved reflectivity at long time delay, the thermal conductivity and effective absorption depth were derived for Bi2Te3 film. Compared with Bi 2Te3, lattice heating by both coherent optical phonon vibration and electron-phonon coupling is less efficient in Sb2Te3. The coherent acoustic phonon generation in Bi2Te3 and Sb2Te3 films is also discussed