Uncovering polar domains in the Rashba semiconductor BiTeI through piezoresponse force and second harmonic generation microscopy

Nonlinear optics has proven to be a powerful probe for understanding the intrinsic crystal structure and phase of materials. Recently, it has been shown that the 2D class of vdW materials exhibits remarkable nonlinear optical properties. Among this exciting family of 2D materials, the Rashba family of semiconductors, including BiTeI, has emerged as an excellent candidate for studying the origins of optical nonlinearities in polar materials. 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 SHG emission intensity with piezoresponse force microscopy (PFM), providing key insights into the origin of the domains.