Phonon-modulated Kerr nonlinearity in ultrathin 2H-MoTe2

TL;DR

This paper introduces a low-power, phase-sensitive nonlinear spectroscopy technique to monitor and control coherent phonons in ultrathin 2H-MoTe2, revealing coupled electronic and phonon dynamics with high sensitivity.

Contribution

It demonstrates a novel low-power nonlinear spectroscopic method for real-time control of phonons and Kerr nonlinearity in 2D materials, combining experimental and theoretical approaches.

Findings

Achieved displacive excitation of coherent phonons at low laser power

Demonstrated spectral broadening and oscillations in probe spectrum

Controlled phonon modes and Kerr nonlinearity via dual-pump scheme

Abstract

Controlling nonequilibrium responses in optically driven quantum materials is essential for advancing applications in energy conversion, ultrafast electronics, and quantum computation. Nonlinear optical spectroscopy serves as a powerful tool to investigate ultrafast electron and phonon dynamics in these systems; however, conventional nonlinear approaches often require intense laser pulses (> 10 GW/cm2) and typically encounter a strong background. Here, we introduce a phase-sensitive nonlinear spectroscopic technique that operates at low laser powers (~ 10 kW/cm2, pulse energies ~ 10 pJ) and enables real-time monitoring and active control of coherent phonons in a few-layer (three to five) thick 2H-MoTe2. Upon excitation with ultrashort (~ 10 fs) pump pulses, we achieve displacive excitation of coherent phonons, which periodically modulate the Kerr nonlinearity of the material, leading to cross-phase modulation (XPM) of a delayed probe pulse. This phase modulation induces spectral broadening and oscillations in the center of mass (COM) of the probe spectrum in time, enabling the detection of subtle nonlinear optical responses in a background-free manner. The nonlinear response can be selectively amplified or attenuated by adjusting the strength of the pump pulse, which controls the distribution of photoexcited carriers in the electronic bands. By combining two-color nondegenerate pump-probe measurements and time-dependent density-functional theory (TDDFT) calculations, we directly resolve the coupled nonequilibrium electronic and phonon dynamics. A dual-pump pulse scheme enables precise control of phonon oscillations, allowing selective activation or suppression of specific phonon modes and correspondingly the modulation of the Kerr nonlinearity.