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

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.

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• Ultrafast carrier-lattice interactions and interlayer modulations of Bi2Se3 by X-ray free electron laser diffraction
• A supervised learning algorithm for interacting topological insulators based on local curvature
• Picosecond volume expansion drives a later-time insulator-metal transition in a nano-textured Mott Insulator
• Phase diagram and phonon-induced backscattering in topological insulator nanowires
• Multiphoton-pumped UV-Vis transient absorption spectroscopy of 2D materials: basic concepts and recent applications
• Two-photon IR pumped UV-Vis transient absorption spectroscopy of Dirac fermions in the 2D and 3D topological insulator Bi2Se3
• Ultrafast transient absorption spectroscopy of 2D semiconductors: a review