Abstract

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.

© 2019 IEEE