Giant room-temperature nonlinearities from a monolayer Janus topological semiconductor

TL;DR

This paper reports giant room-temperature nonlinear optical responses in a monolayer Janus topological semiconductor, revealing potential for optoelectronic applications and advancing understanding of topological effects in nonlinear optics.

Contribution

It demonstrates large nonlinearities in a 2D Janus topological material, linking topological band properties to enhanced nonlinear optical responses.

Findings

>50 times higher harmonic generation than 2H MoS2

>20 times higher terahertz emission than 2H MoS2

Colossal nonlinear responses due to topological band mixing and symmetry breaking

Abstract

Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and topological properties, such as Berry curvature and the quantum metric tensor, have stimulated great interest. Here, we report giant room-temperature nonlinearities in an emergent non-centrosymmetric two-dimensional topological material, the Janus transition metal dichalcogenides in the 1T' phase, which are synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently reveal orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities of 1T' MoSSe (e.g., > 50 times higher than 2H MoS$_2$ for 18th order harmonic generation; > 20 times higher than 2H MoS$_2$ for terahertz emission). It is elucidated that such colossal nonlinear optical responses come from topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and preludes the applications of topological materials in optoelectronics down to the monolayer limit. This two-dimensional form of topological materials also constitute a unique platform for examining origin of the anomalous high-harmonic generation, with potential applications as building blocks for scalable attosecond sources.