Dynamics of coherent optical phonons in chalcogenide compounds

Summary form only given. Crystalline chalcogenide compounds such as Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃ are intriguing materials because it is not only the best known thermoelectric materials but also recognized as topological insulators [1]. Even though its phonon properties have been studied extensively by using Raman spectroscopy, the dynamical investigations are lacking [2,3]. We present ultrafast time-resolved measurements of coherent optical phonons with aim to reveal their dynamics (amplitude, damping rate, frequency and initial phase).The measurements were carried out by a reflection-type pump-probe technique at room temperature. The light source was a mode locked Ti:sapphire laser operating at a wavelength of 800 nm with a pulse duration of 40 fs. We used an electro-optic (EO) sampling technique due to measure anisotropic phonon modes. The reflectivity change was recorded as a function of the time delay between the pump and the probe pulses. The Bi2Se3, Bi2Te3, and Sb2Te3 single crystals are grown via a modified Bridgman process [1]. By subtracting the non-oscillatory background modulation from the reflectivity change, a beat structure is found in the oscillation. Figure 1 (a) and (b) show the time-resolved reflection change of Bi2Te3 and Sb2Te3, respectively. Anisotropic Eg2 phonons have been observed the EO sampling in addition to isotropic (A1g1 and A1g2) phonons in Bi2Te3 and Sb2Te3 in common with Bi2Se3 [4]. The observed frequencies agree well with Raman spectroscopy [5]. We analyzed the obtained oscillations in the transient reflectivity using three damped oscillations.We clearly observed anisotropic Eg modes by using an EO sampling technique. The amplitude of Sb2Te3 is slightly larger than those in either Bi2Te3 or Bi2Se3. We revealed that the life time of Bi2Te3 is longer than other samples, and the phonons with the higher frequency decay faster.