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

TL;DR

The study addresses how intense THz fields can coherently drive Raman-active phonons in Bi. It identifies THz-induced tunnel ionization as a rapid, threshold-activated mechanism that injects carriers and provides a displacive driving force for the A1g phonon, with a quantitative TI-based model reproducing the observed field dependence. The experiments show a clear threshold around 200 kV/cm and a fast carrier-rise near 200 fs, yielding a 2.9 THz phonon whose amplitude tracks TI predictions. This work reveals a new, low-heating route to dynamic lattice control in semimetals and narrow-gap materials, with potential implications for ultrafast manipulation of correlated and topological states in moiré systems and beyond.

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.

Figures (13)

• Figure 1: (a) Schematic of THz pump optical probe spectroscopy. (b) THz-induced reflectivity change $\Delta R/R$ of a 50 nm thick bismuth film as a function of pump-probe delay time at 400 kV/cm. We use a cosine function to fit the phonon oscillation isolated by a polynomial fit. The initial phase of the phonon oscillation is found to be (0.035 $\pm$ 0.009)$\pi$.
• Figure 2: (a) The THz pulse width is defined by the full width at half maximum (FWHM) of the square of the Hilbert transform of the THz field. The inset shows THz field measured by electro-optic sampling. (b) Spectral magnitude of phonon oscillations (red) and THz pulse (blue) obtained by Fourier transform.
• Figure 3: (a) Time-resolved reflectivity change $\Delta R/R$ for two representative THz pump field strength of 166 and 290 kV/cm. (b) Average reflectivity change in the time interval of 0 to 0.4 ps labelled by the black dashed lines as a function of THz field strength. (c) Phonon oscillation amplitude as a function of THz field strength. It deviates from the quadratic field dependence (gray curve). Below 200 kV/cm, no phonon oscillations are observed (hatched area).
• Figure 4: (a) Schematic for band structure of bismuth near the L points. $\mathcal{E}_g$ is the band gap and $\mathcal{E}_F$ is the Fermi energy. (b) The temporal profile of the transient tunneling rate $dn/dt$ and the transient injected carrier density $\Delta n$ at a field strength of 400 kV/cm. For comparison, the waveform of THz pulse (gray curve) in time domain is also plotted. (c) Phonon dynamics at 400 kV/cm simulated by the tunnel ionization (TI) model. The experimental phonon oscillation at the same field strength is also plotted with offset for comparison. (d) Simulated phonon amplitude as a function of THz field strength plotted together with the phonon amplitude determined by our experiments.
• Figure S1: (Color online) The schematic of our THz pump optical probe setup.