Exciton Engineering of Nonlinear Optics in a Two-Dimensional Janus System

Nonlinear optical (NLO) responses are vital for lasers, sensing, and quantum technologies. In two-dimensional systems, many-body excitonic effects can profoundly modify NLO responses, giving rise to novel exciton-mediated NLO phenomena. However, our understanding of these effects in real materials remains limited, particularly regarding controllable manipulation of NLO responses via excitons. Here, we report strong excitonic second-harmonic generation (SHG) in an experimentally synthesized, ground-state-stable Janus monolayer, RhSeCl. Combining density functional theory and many-body perturbation theory, we reveal that its giant SHG arises not only from intrinsic symmetry breaking but also crucially from an exciton-assisted double resonance mechanism. In bilayers, distinct stacking configurations enable giant SHG modulation, accompanied by a switch in the dominant contribution from intralayer to interlayer excitons. Vertical electric fields further enhance tunability through precise control of hybridization between interlayer and intralayer excitons. Our finding establishes Janus materials as a promising excitonic NLO platform, enabling the systematic design of nonlinear quantum light sources through controlled exciton engineering.