Ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate crystals

TL;DR

This paper explores the ultrafast behavior of coherent phonons and phonon-polaritons in lithium niobate crystals using femtosecond spectroscopy, revealing their coexistence, coupling dynamics, and potential for high-speed electro-optic modulation.

Contribution

It provides the first detailed experimental observation of phonon-polariton modes near 3.8 and 14.9 THz and elucidates their coupling dynamics in lithium niobate.

Findings

Coexistence of phonon-polariton E-modes near 3.8 and 14.9 THz confirmed.

Discovered a 0.2 ps time lag indicating phonon-polariton coupling.

LiNbO₃ shows potential for ultrafast electro-optic phonon modulators over 10 THz.

Abstract

This study investigates the ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate (LiNbO$_{3}$) crystals using reflective pump-probe spectroscopy with 25-fs time resolution. In addition to several coherent optical phonon modes, the electro-optic sampling measurements explored the coexistence of phonon-polariton E-modes near 3.8 and 14.9 THz, which agrees considerably with the theoretical phonon-polariton dispersion curves. We also discovered that a time lag ($\Delta$t $\approx$ 0.2 ps) between the coherent optical phonon and phonon-polariton originates from the coupling time for propagating phonon-polariton. Our findings provide a possible application of LiNbO$_{3}$ for ultrafast electro-optic phonon modulators with bandwidths greater than 10 THz.