Nonlinear microscopy of lead iodide nanosheets

Lead iodide (PbI₂) is a layered material with unique optical and electrical properties, including direct bandgap in the bulk and a layered crystalline structure, consisting of close-packed Pb atoms sandwiched between two layers of Iodine atoms [1]. Compared to the widely-studied TMDCs, 2D PbI2 is a new type of halide semiconductors with a relatively larger visible bandgap (E_g ∼ 2.4 eV) which endows its distinct optical properties. Despite being studied for decades and being used as a precursor for perovskite materials, the recently developed PbI₂ nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors [2] and nanolasers [3]. Such 2D nanosheets also show a great potential for low-dimensional nonlinear optical devices, However, their nonlinear properties are still unexplored, while novel applications of PbI2 nanosheets require careful characterization of their crystalline structure, thickness and nonlinearity. Here we demonstrate the nonlinear microscopy of PbI2 nanosheets using the polarization and thickness dependence of the second harmonic generation (SHG) and third harmonic generation (THG) from solution-grown nanosheets. Our measurements allow to precisely determine their thickness and crystalline orientation with a non-invasive optical technique.