Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3

TL;DR

This study uses spatially resolved femtosecond pump-probe techniques to investigate carrier and phonon dynamics in Bi2Se3, revealing ultrafast thermalization, phonon oscillations, and carrier diffusion properties.

Contribution

It provides new insights into ultrafast carrier and phonon behaviors in Bi2Se3 with spatial resolution, including diffusion coefficients and relaxation times.

Findings

Carrier thermalization and relaxation are sub-ps.

Two distinct phonon oscillations observed.

Carrier diffusion coefficient estimated at ~500 cm²/s.

Abstract

Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies, and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 square centimeters per second. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 square centimeters per second.