晶格振动的超快光谱调控

采用飞秒激光抽运脉冲激发了Bi_2Te_3薄膜频率为1.856 THz的声子相干振动,并用探测光测量得到了其阻尼振动信号.结合Raman光谱,确定该振动为A~1_(1g)对称振动模式的相干光学声子.为了实现该模式振动的调控,在抽运光路上安装了脉冲整形器,进而控制生成具有不同时间间隔和能量比的两束脉冲激光.研究表明,当两束脉冲的间隔时间为相干光学声子振动半周期的奇数倍时,调整两束脉冲的能量比值,可以实现A~1_(1g)模式振动的完全消除.继而将两束脉冲的能量比值保持不变,得到了振幅随间隔时间的变化曲线,与理论分析符合.结果进一步证实了用超快光谱调控特定晶格振动的可行性,从而为研究材料内部超快能量传递过程提供了有效手段.

Manipulation of lattice vibration by ultrafast spectroscopy

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 A_1g¹ 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 A_1g¹ 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.