Uncovering Polar Domains in the Rashba Semiconductor BiTeI Through Piezoresponse Force and Second Harmonic Generation Microscopy

Nonlinear optics has proven a powerful probe for understanding the intrinsic crystal structure and phase of material. Recently, it has been shown that the 2D class of vdW materials exhibit remarkable nonlinear optical properties. Among the exciting family of 2D materials that are being investigated for their nonlinear optical properties, the Rashba family of semiconductors, including BiTeI, has emerged as an excellent candidate for studying the origins of optical nonlinearities in polar semiconductors. BiTeI and related materials have a large Rashba spin-orbit coupled state, and their large polarity is expected to persist down to the monolayer limit. Here, we used second harmonic generation (SHG) microscopy to show that BiTeI is an extremely nonlinear material using layer-dependent studies and extracting its nonlinear coefficients. This then enabled us to exploit SHG microscopy to image polar domains in BiTeI and correlate it this with piezoresponse force microscopy (PFM), providing key insights into the origin of the domains.