Optical parametric amplification is a second-order nonlinear process whereby an optical signal is amplified by a pump via the generation of an idler field¹. This mechanism is inherently related to spontaneous parametric down-conversion, which currently constitutes the building block for entangled photon pair generation², a process that is exploited in modern quantum technologies. Here we demonstrate single-pass optical parametric amplification at the ultimate thickness limit; using semiconducting transition metal dichalcogenides^3,4, we show that amplification can be attained over propagation through a single atomic layer. Such a second-order nonlinear interaction at the two-dimensional limit bypasses phase-matching requirements⁵ and achieves ultrabroad amplification bandwidths. In agreement with first-principle calculations, we observe that the amplification process is independent of the in-plane polarization of signal and pump fields. By the use of AA-stacked multilayers, we present a clear pathway towards the scaling of conversion efficiency. Our results pave the way for the development of atom-sized tunable sources of radiation with potential applications in nanophotonics and quantum information technology.

光学参量放大是二阶非线性过程，通过该光学参量通过泵产生的空转场¹来放大光信号。这种机制与自发的参数下转换具有内在的联系，自发的参数下转换目前构成纠缠光子对生成²的构建块，这种纠缠光子对在现代量子技术中得到了利用。在这里，我们展示了极限厚度极限下的单通光学参量放大；使用半导体过渡金属二卤化物^3,4，我们表明可以通过单个原子层的传播实现放大。这种在二维极限处的二阶非线性相互作用会绕过相位匹配要求⁵并实现超宽放大带宽。与第一性原理计算一致，我们观察到放大过程与信号场和泵浦场的面内极化无关。通过使用AA堆叠的多层，我们提出了一条清晰的途径，可以扩大转换效率。我们的研究结果为开发具有原子尺寸的可调辐射源铺平了道路，并有望在纳米光子学和量子信息技术中得到应用。