# Anisotropic Charge Carrier and Coherent Acoustic Phonon Dynamics of Black Phosphorus Studied by Transient Absorption Microscopy

## TL;DR

This study investigates the anisotropic dynamics of charge carriers and coherent acoustic phonons in black phosphorus using polarization-resolved transient absorption microscopy, revealing thickness-dependent decay and anisotropic damping.

## Contribution

It provides new insights into the anisotropic and thickness-dependent behaviors of charge carriers and phonons in black phosphorus, highlighting the dominant deformation potential mechanism for phonon generation.

## Key findings

- Charge carrier decay dynamics are anisotropic and thickness-dependent.
- Coherent acoustic phonon damping is anisotropic due to polarization-dependent absorption.
- Sound velocity measurements from oscillation frequency agree with acoustic echo analysis.

## Abstract

Due to its corrugated hexagonal lattice structure, Black phosphorus (BP) has unique anisotropic physical properties, which provides an additional freedom for designing devices. Many body interactions, including interactions with phonon, is crucial for heat dissipation and charge carrier mobility in device. However, the rich properties of the coherent acoustic phonon, including anisotropy, propagation and generation were not fully interrogated. In this paper, the polarization-resolved transient absorption microscopy was conducted on BP flakes to study the dynamics of photoexcited charge carriers and coherent acoustic phonon. Polarization-resolved transient absorption images and traces were recorded and showed anisotropic and thickness-dependent charge carriers decay dynamics. The damping of the coherent acoustic phonon oscillation was found to be anisotropic, which was attributed to the polarization-dependent absorption length of the probe pulse. From the analysis of initial oscillation amplitude and phase of coherent acoustic phonon oscillation, we proposed that the direct deformation potential mechanism dominated the generation of coherent acoustic phonons in our experiment. Besides, we obtained the sound velocity of the coherent acoustic phonon from the oscillation frequency and the acoustic echo, respectively, which agreed well with each other. These findings provide significant insights into the rich acoustic phonon properties of BP, and promise important application for BP in polarization-sensitive optical and optoelectronic devices.

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Source: https://tomesphere.com/paper/1901.04101