Manipulation of lattice vibration by ultrafast spectroscopy

Abstract

The ultrafast pump-probe spectroscopy allows us to make movies of the dynamics of the carriers and vibrational excitations on the timescales shorter than the typical scattering time. In general, the temporal evolution of the reflectivity change is comprised of the oscillatory and the non-oscillatory components. The former corresponds to the coherent lattice vibration, while the latter is related to the complex cooling process of the hot carriers. To investigate the dynamics of the hot carrier and the lattice vibration, it is necessary to decouple the two parts in the detected signal. Comparatively, the manipulation of the coherent lattice vibration is easier in spite of its super-high frequency and subatomic vibration amplitude. In this work, the behavior of the coherent lattice vibration in Bi2Te3 single crystalline film with a thickness of 100 nm is studied by using the double pump-single probe ultrafast spectroscopy. Firstly, the coherent lattice vibration with the subatomic amplitude and a frequency of about 1.856 THz is simulated by a femtosecond pump pulse, and its damped oscillation signal is detected by the reflectivity change of a probe pulse. Compared with the Raman spectrum, this vibration is confirmed to be the coherent optical phonon with A1g1 symmetric vibration mode. To manipulate this lattice vibration, a pulse shaper is then installed in the pump-beam arm to generate double pump pulses with the different separation times and the intensity ratios. The resulting reflectivity change is found to be a superposition of the pulse train: the oscillation amplitude is enhanced when the separation time is matched to the period of the oscillation; if the separation time is the odd times the half-period of the oscillation, the A1g1 vibration mode can be completely cancelled out after adjusting the intensity ratio. Finally, by maintaining the same intensity ratio, the amplitudes of the oscillation signals after the second pump pulse are measured with different separation times. The results agree well with the theoretical predictions: the amplitude of the oscillation after the second pump pulse shows a cosine function of separation time with a period of about 1080 fs, which is the twice the period of the oscillation illuminated by a single pump pulse. This work suggests that the lattice vibration can be optically manipulated, thus provides an effective way to disentangle the lifetimes of the phonons and the interactions with the excited carriers in the ultrafast energy relaxation process in semiconductor, which is extremely important for a number of interesting phenomena such as the non-thermal melting and the insulator-to-metal transition.