Terahertz-driven parametric excitation of Raman-active phonons in LaAlO$_{3}$

TL;DR

This work demonstrates terahertz-driven parametric excitation of a Raman-active E_g phonon in LaAlO$_3$, revealing a distinct subharmonic component (~0.32 THz) arising from a parametric drive mediated by pairs of acoustic phonons. The authors combine THz time-domain spectroscopy with polarization- and fluence-dependent measurements and support their interpretation with a microscopic model in which the THz field excites two acoustic modes that couple to the Raman mode via acousto-optic interactions, effectively modulating the Raman mode’s frequency in a Mathieu-like fashion. Phonon dispersion calculations show strong coupling between the Raman mode and upper acoustic branches in the 50–133 cm$^{-1}$ range, yielding an average two-phonon energy around $\overline{\Omega} \approx 0.7$ THz and explaining the observed subharmonic via $2\overline{\Omega}-\Omega_R$. The findings establish a pathway for coherent parametric control of phonons (phononics) using THz fields, with potential implications for sensors, energy harvesting, and information processing technologies.

Abstract

Achieving parametric excitation in an oscillating physical system involves periodically adjusting one of its parameters to modulate the oscillator's natural frequency. This phenomenon has been observed in numerous systems within physics and engineering, profoundly transforming modern science and technology. Despite rapid progress, the parametric control of collective excitations, such as phonons, remains a challenge while promising to generate novel and intriguing effects in a largely unexplored field. Here, we investigate the terahertz (THz) field-induced dynamics of Raman-active phonons in LaAlO$_3$ (LAO), an archetypal quantum material for its electronic and structural properties. Utilizing intense THz pulses, we demonstrate a novel mechanism of parametric phonon excitation marked by substantial subharmonic components. Theoretical analysis can successfully capture the hallmarks of the observed phenomena in a physical scenario with the THz field inducing a parametric coupling between the Raman mode and pairs of acoustic phonon excitations.

Figures (8)

• Figure 1: Sketches of the experiment and of LaAlO$_{3}$ unit cell and phonon dynamics.(a) Schematic of the pump-probe setup used in this study. (b) LaAlO$_{3}$ crystal structure in the rhombohedral phase and atomic displacements involved in the E$_g$ mode
• Figure 2: Experimentally detected polarization rotation in LaAlO$_{3}$ following near-infrared or THz pumps.Polarization rotation of a transmitted 800 nm probe through a LaAlO$_{3}$ crystal, following the excitation by a near-infrared (1300nm) (orange curve) or a broadband THz excitation (blue curve), in (a) time and (b) frequency domains. Measurements are performed at 8 K. The FFT is evaluated in the range of 0-50 ps. The sample response is normalized by the pump fluence. Inset in panel (a): time-domain dynamics at longer timescales.
• Figure 3: Theoretical modeling of parametric phonon dynamics in LaAlO$_{3}$. Spectral intensity of the Raman amplitude $|Q_R|$ (solid black line) as a function of frequency. The inset shows the phonon dispersions: the Raman-active optical branch (blue) and three acoustic branches (shades of red). The up-conversion mechanism is illustrated, where two acoustic phonons (red dots) convert into a Raman-active phonon (blue dot).
• Figure S1: THz-TDS on LaAlO$_{3}$ [100] sample, the incident field is the same as the pumping field of Fig. \ref{['fig2']}. The field amplitude is 550kV/cm.
• Figure S2: Sample's response to broadband THz pump as a function of the incoming probe polarizations state, i.e. S (blue curve), P (green curve), and 45deg (orange curve). The rotation of the probe polarization is reported in the time domain (a) and in the frequency domain (b)