Polarization dependence of coherent phonon generation and detection in the 3D topological insulator Bi2Te3

TL;DR

This study investigates how the polarization of light affects the generation and detection of coherent phonons in the topological insulator Bi2Te3, revealing symmetry properties and the origin of specific phonon modes.

Contribution

It provides the first detailed polarization-dependent analysis of coherent phonons in Bi2Te3, clarifying phonon symmetry and identifying the 3.6 THz mode as crystalline Te.

Findings

All phonon modes are symmetry-independent in isotropic detection.

Eg symmetry modes are detectable but weaker in anisotropic detection.

The 3.6 THz mode is attributed to crystalline Te from tellurium segregation.

Abstract

We have studied the polarization dependence of coherent phonons in the topological insulator Bi2Te3. Using polarization-dependent femtosecond pump-probe spectroscopy, we measured coherent phonons as a function of angle when the pump and probe polarizations were fixed, and the crystal orientation was rotated. For isotropic detection, depending on the spot position, oscillations either from only low- and high-frequency phonons of A1g symmetry, or in addition from the mode at 3.6 THz were observed. All the modes were found to be independent of the orientation of electric field vector with respect to the crystal axes testifying to their full symmetry while no modes of lower symmetry appeared in any polarization geometry. For anisotropic detection both modes of Eg symmetry could be detected, but their amplitudes were considerably smaller than those of A1g symmetry. To clarify the coherent phonon assignment and the process of coherent phonon generation in Bi2Te3, the time-domain measurements were complemented by spontaneous Raman scattering. The comparison of frequency- and time-domain results and the polarization dependence suggest that the 3.6 THz mode belongs to crystalline Te arising due to tellurium segregation. A discrepancy between the time- and frequency domain data is discussed.