Ultrafast giant enhancement of second harmonic generation through level occupation engineering

TL;DR

This paper demonstrates an ultrafast 40% enhancement of second harmonic generation in NiPS3 by optically controlling electron occupation levels, combining theoretical modeling and experimental validation on a femtosecond timescale.

Contribution

It introduces a novel method to ultrafastly enhance SHG via level occupation engineering, advancing photonic circuit modulation techniques.

Findings

Achieved 40% SHG enhancement within 500 femtoseconds.

Confirmed theoretical predictions with temperature-dependent measurements.

Discussed potential applications in transition metal thiophosphates.

Abstract

Optical nonlinearity, especially the second harmonic generation (SHG), is generally weak in materials but has the potential to be applied in high-speed optical computers and energy-efficient artificial intelligence systems. In order to program such photonic circuits, electrical and all-optical modulation mechanisms of optical nonlinearity have been proposed. Among them the electrical methods are bottlenecked by speed, while optical methods like Floquet engineering provides a fast heat-free route, but has only been experimentally shown to suppress SHG. Here we theoretically and experimentally demonstrated an ultrafast enhancement of SHG by 40% on a timescale of $\sim$ 500 femtosecond in van der Waals NiPS$_3$. We performed single-ion model calculations to show that by optically control the electron occupation of different energy levels, the SHG can be enhanced due to different electronic states involved in the SHG process. We then performed temperature-dependent time-resolved measurements in both linear and nonlinear optics, which confirm our calculations. We also discussed the implications for other materials in the transition metal thiophosphates (MPX$_3$) family.