Terahertz-induced tunnel ionization drives coherent Raman-active phonon in Bismuth

TL;DR

This paper reveals a novel mechanism where intense THz pulses induce tunnel ionization in bismuth, directly exciting Raman-active phonons and enabling ultrafast control of material properties.

Contribution

It introduces a new THz-driven tunnel ionization mechanism for exciting coherent phonons, distinct from conventional anharmonic coupling or sum frequency processes.

Findings

THz pulses at 1 THz excite A1g phonon at 2.9 THz in bismuth

Tunnel ionization acts as a displacive driving force for phonon excitation

Mechanism enables ultrafast control of electronic and structural properties

Abstract

Driving coherent lattice motion with THz pulses has emerged as a novel pathway for achieving dynamic stabilization of exotic phases that are inaccessible in equilibrium quantum materials. In this work, we present a previously unexplored mechanism for THz excitation of Raman-active phonons. We show that intense THz pulses centered at 1 THz can excite the Raman-active $A_{1g}$ phonon mode at 2.9 THz in a bismuth film. We rule out the possibilities of the phonon being excited through conventional anharmonic coupling to other modes or via a THz sum frequency process. Instead, we demonstrate that the THz-driven tunnel ionization provides a plausible means of creating a displacive driving force to initiate the phonon oscillations. Our work highlights a new mechanism for exciting coherent phonons, offering potential for dynamic control over the electronic and structural properties of semimetals and narrow-band semiconductors on ultrafast timescales.