3R-stacked transition metal dichalcogenide non-local metasurface for efficient second-harmonic generation

Monolayer transition metal dichalcogenides are van der Waals semiconductors that exhibit exceptionally high second-order nonlinear susceptibilities χ⁽²⁾ = 100–1,000 pm V⁻¹, but limited conversion efficiency ∝[χ(2)]2z2≈10−10, due to their atomic thickness z. Contrary to the naturally occurring hexagonal crystal phase, which possesses inversion symmetry in samples with an even number of layers, the non-centrosymmetric rhombohedral phase (3R) enables much larger second-order nonlinear signals in bulk samples. However, at increased thicknesses (~200 nm), phase mismatch becomes relevant, limiting the maximum efficiency to ~10⁻⁶. Quasi-phase-matched 3R-MoS₂ stacks have recently pushed conversion efficiencies beyond 10⁻⁴ (0.01%), over thicknesses of a few micrometres. Here we bypass phase-matching constraints by patterning subwavelength 3R-MoS₂ flakes to realize non-local optical resonances, characterized by a field profile that is highly localized along the transverse direction but largely delocalized in the metasurface plane. By leveraging the large field confinement and high quality factors offered by our metasurface design, we are able to achieve two orders of magnitude (140×) second-harmonic generation enhancement compared with unpatterned 3R-MoS₂ flakes with the same thickness, enabling single-pass second-harmonic conversion efficiencies of ~10⁻⁴ over only 160-nm-thick metastructures at relevant telecom wavelengths. This work opens new pathways towards the realization of efficient, on-chip-integrable nonlinear devices with compact footprints based on layered semiconductors, particularly relevant for integrated photonic circuitry and with potential applications in the field of quantum photonics.