Strong Terahertz Emission and Coherent Phonon Dynamics in 2D Ferroelectrics

Recently two-dimensional (2D) materials have dominated the focus of the study of terahertz (THz) wave generation and detection due to their strong light-matter interactions, high mobility, and ultrafast carrier dynamics [1]. In particular, 2D ferroelectrics show exceptionally strong optical nonlinearity, which makes it an intriguing candidate for exploring nonlinear THz optical emission phenomena. In this talk, I will present our characterization and mechanism understanding of THz emission properties of a recently discovered 2D ferroelectric semiconductor under ultrafast laser excitation. By conducting THz emission spectroscopy, we found it exhibits strong THz emission with a generation efficiency even higher than other bulk conventional nonlinear crystals. By employing both below-bandgap and above-bandgap excitation, we clarified the THz emission contributions from resonant and non-resonant optical rectification processes as well as surface-induced transient photocurrents. Specifically, the wavelength-dependent anisotropy and enhanced emission characteristics help differentiate between these processes. Furthermore, we observe collective phonon oscillations corresponding to the symmetric bending vibrations of the Nb-I bond along the polar axis, which induce coherent oscillatory photocurrents in the THz range. This provides a new approach for experimentally detecting and controlling low-energy modes via THz emission in photoinduced states. Overall, our results reveal a new understanding of THz dynamics in 2D vdW ferroelectrics and demonstrate great application potential for high-performance THz optoelectronics.

Y. Wu, et al. “Emerging probing perspective of two-dimensional materials physics: terahertz emission spectroscopy”. Light Sci Appl 13, 146 (2024).