Giant second harmonic generation in bulk monolayer MoS2 thin films

Monolayer molybdenum disulfide (MoS2) features exceptional second-order nonlinear optical (NLO) susceptibility, while being atomically thin limits its efficiency in second harmonic generation (SHG). The naturally existing 2H-phase MoS2 may offer a larger optical cross section in its bulk form but is inactive for SHG due to the restored centrosymmetry. Here, we report a thickness- and area-scalable bulk monolayer MoS2 (BM-MoS2) thin film for highly efficient SHG. The solution-assembled centimeter-scale BM-MoS2 consists of alternating monolayer MoS2 crystals and organic molecular layers that prevent interlayer coupling, thus preserving monolayer-like physical properties while achieving increased optical cross sections. The SHG studies demonstrate a giant SHG in BM-MoS2 that is 126 times higher than monolayer MoS2 and 21 times higher than gallium arsenide (GaAs), a material with the highest second-order NLO susceptibility among known bulk semiconductors. The facile assembly of BM-MoS2 thin films with efficient SHG offers a scalable pathway for developing ultrathin, efficient, and cost-effective NLO devices.