Coherent phonons in a $Bi_2Se_3$ film generated by an intense single-cycle THz pulse

TL;DR

This paper demonstrates the generation of coherent phonons in a $Bi_2Se_3$ film using intense single-cycle THz pulses, revealing specific phonon modes and their nonlinear excitation mechanisms.

Contribution

It reports the first observation of multiple coherent phonons in $Bi_2Se_3$ driven by single-cycle THz pulses and explains their nonlinear generation processes.

Findings

Largest signal at 4.05 THz corresponding to $E_g^2$ mode

Resonant excitation of Raman mode by second harmonic of THz-driven oscillator

Generation of other phonons explained by lattice anharmonicity

Abstract

We report an observation of coherent phonons of $E_g^1$, $E_u^1$, $A_{1g}^1$, and $E_g^2$ symmetry generated in a single-crystal film of $Bi_2Se_3$ by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the $E_g^2$ mode. The generation of $E_g^2$ phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear $E_u^1$ oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of $E_g^2$. The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of $E_g^1$ (1.1 THz) and $A_{1g}^1$ (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in $Bi_2Se_3$.