Ultracompact nonlinear platforms based on van der Waals semiconductors for classical and quantum states of light (Invited Paper)

Nonlinear optics lies at the heart of classical and quantum light generation. Standard nonlinear crystals exhibit moderate second-order nonlinearities (χ(2) = 1-30 pm/V) but achieve high nonlinear conversion efficiencies due to their large thickness (millimeter/centimeter), limiting miniaturization. Here we realize ultracompact nonlinear structures based on non-centrosymmetric van der Waals (i.e., layered) materials, e.g., 3R-MoS2, which exhibit high nonlinearity (χ(2)~100 pm/V) and easy system integration, making them highly promising for frequency conversion over ultrathin pathlengths[1], entangled photon pair generation[2], and efficiency scaling with thickness of nonlinear processes like second harmonic generation (SHG)[3]. To achieve high efficiencies, comparable to bulk nonlinear crystals, we realize periodically-poled microscopic structures for quasi-phase matched up- and down-conversion [4], and nonlinear metasurfaces [5] which bypass phase-matching constraints. van der Waals periodically poled structures and nonlinear metasurfaces provide macroscopic nonlinear conversion efficiencies over microscopic thicknesses and open new pathways towards the realization of efficient, on-chip integrated nonlinear devices with ultracompact footprints, for generating and manipulating both classical and quantum states of light. References [1] C. Trovatello et al. "Tunable Optical Nonlinearities in Layered Materials" ACS Photonics 11, 2860-2873 (2024). [2] M. A. Weissflog et al. “A tunable transition metal dichalcogenide entangled photon-pair source” Nat. Commun. 15, 7600 (2024). [3] X. Xu et al. “Towards compact phase-matched and waveguided NLO in layered semiconductors” Nat. Photonics 16, 698-706 (2022). [4] C. Trovatello et al. "Quasi-phase-matched up- and down-conversion in PP layered semiconductors" Nat. Photonics (2025). [5] Z.H. Peng, M. Cotrufo et al. 3R-stacked transition metal dichalcogenide non-local metasurface for efficient second-harmonic generation. Nat. Photonics (2025).