We have demonstrated an all - optical , fast , and non - invasive method to resolve , with unprecedented resolution , the crystalline integrity of atomically thin WS 2 crystals via experimentally probing and theoretically interpreting their nonlinear optical properties . In particular , we showed that PSHG raster - scanning imaging microscopy enables highresolution pixel - by - pixel ( pixel size of 120 × 120 nm 2 ) mapping of the armchair crystal axis orientation , which can be used to provide firstorder information on crystallographic imperfections stemming from inhomogeneities in the distribution of armchair orientations . Moreover , the measured pixel - by - pixel PSHG data were fit to a new theoretical model based on the nonlinear optical response of such crystals , which was utilized as a second - order filter that effectively enhances the optical contrast and provides quantification of the crystal quality in terms of the standard deviation of the calculated armchair orientational distributions . Consequently , we demonstrate an ultrahigh - resolution nonlinear optical method that can be used to identify small variations in the crystal armchair direction . This information with such a high resolution is unattainable by conventional SHG imaging as well as by Raman and photoluminescence mapping .