Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3

TL;DR

This study investigates the GHz-range acoustic phonon dynamics in Bi2Se3 thin films, revealing how film thickness influences phonon modes and their transition from standing waves to Lamb waves, with implications for understanding topological insulator properties.

Contribution

It provides a detailed analysis of how acoustic phonon modes evolve with film thickness in Bi2Se3, highlighting the transition from FBAWR to Lamb wave regimes and their relation to elastic properties.

Findings

Oscillation frequency increases from ~35 to ~70 GHz as film thickness decreases from 40 to 15 nm.

Transition from traveling-acoustic-waves to FBAWR modes occurs around 40 nm thickness.

Rapid decay of oscillation amplitude below 15 nm due to intersurface coupling switching phonon regimes.

Abstract

Transient reflectivity traces measured for nanometer-sized films of the topological insulator Bi2Se3 revealed GHz-range oscillations driven within the relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV photon energy. These oscillations have been suggested to result from acoustic phonon dynamics, including coherent longitudinal acoustic phonons in the form of standing acoustic waves. An increase of oscillation frequency from ~35 to ~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the interplay between two different regimes employing traveling-acoustic-waves for films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR) modes for films thinner than 40 nm. The amplitude of oscillations decays rapidly for films below 15 nm thick when the indirect intersurface coupling in Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The frequency range of coherent longitudinal acoustic phonons is in good agreement with elastic properties of Bi2Se3.