Femtosecond study of A1g phonons in the strong 3D topological insulators: From pump-probe to coherent control

TL;DR

This study uses femtosecond spectroscopy to observe and control A1g phonons in topological insulators Bi2Te3 and Bi2Se3, revealing strong carrier-phonon coupling and enabling selective phonon amplitude manipulation.

Contribution

It demonstrates optical control of A1g phonons in 3D topological insulators, highlighting phonon chirp and selective amplitude modulation without affecting frequency or lifetime.

Findings

Observation of coherent A1g phonons with well-defined properties

Detection of phonon chirp indicating strong carrier-phonon coupling

Selective phonon amplitude control without changing frequency or coherence

Abstract

Fully symmetric A1g phonons are expected to play a dominant role in electron scattering in strong topological insulators (TIs), thus limiting the ballistic transport of future electronic devices. Here, we report on femtosecond time-resolved observation of a pair of A1g coherent phonons and their optical control in two strong 3D TIs, Bi2Te3 and Bi2Se3, by using a second pump pulse in ultrafast spectroscopy measurements. Along with well-defined phonon properties such as frequency and lifetime, an obvious phonon chirp has been observed, implying a strong coupling between photo-carriers and lattices. The coherent phonon manipulation, on the other hand, allows us to change the phonon amplitude selectively but does not affect either the frequency or coherence lifetime of the chosen mode.