Nonlinear optics lies at the heart of classical and quantum light generation. The invention of periodic poling revolutionized nonlinear optics and its commercial applications by enabling robust quasi-phase-matching in crystals such as lithium niobate. However, reaching useful frequency conversion efficiencies requires macroscopic dimensions, effectively limiting on-chip integration with ultracompact footprints.

Here we realize a periodically poled van der Waals semiconductor (3R-MoS₂). Due to its exceptional nonlinearity, we achieve macroscopic frequency conversion efficiency (0.01%-0.1%) over a microscopic thickness of only 3μm, 10−100× thinner than current systems with similar performances.

Further, we report the generation of photon pairs at telecom wavelengths via quasi-phase-matched spontaneous parametric down-conversion. This work opens the new and unexplored field of phase-matched nonlinear optics with microscopic van der Waals crystals, unlocking new applications that require simple, ultracompact technologies such as on-chip entangled photon-pair sources for integrated quantum circuitry and sensing.

2312. Trovatello et al. “Quasi-phase-matched up- and down-conversion in periodically poled layered semiconductors” https://arxiv.org/abs/2312.05444 (2023)