Role of polarization force for coherent phonon generation within the Bi₂Te₃ thin film

To date, the generation mechanism of coherent phonon within the topological insulators is an important but still unconfirmed topic. In this work, using double pulse technique, the dynamic properties and possible mechanisms of the coherent phonon(A_1g¹ , A_1g², E_g²) in Bi₂Te₃ were investigated. Among these three phonon modes, decreasing intensity and frequency as increasing temperature was observed, which can be attributed to the reducing temperature gradient, leading to thermal force generation mechanism. However, by analyzing the temporal evolution of the phonon frequency, the frequency of E_g² was gradually shifted to higher frequency rather than red shift trend observed in the A_1g¹ and A_1g². This indicates that an additional way for generating E_g² mode with nature of an-harmonicity might be inevitable. Here, the polarization force mechanism for E_g² mode was proposed and discussed in detail.