Second harmonic generation in defective hexagonal boron nitride.

Determining the role of defects in materials can be an important task both for the fundamental understanding of their influence on material properties and for future applications. In this work, we studied the influence of defects on the second harmonic generation (SHG) in hexagonal boron nitride (h-BN). We characterized the sample by photoluminescence imaging and spectroscopy, showing strong and sharp photoluminescence emission at visible range from h-BN flakes due to single defect states. By doing second harmonic imaging, we found strong emission from the h-BN flakes that correlates spatially with the photoluminescence imaging. By doing polarization-resolved SHG, we found deviations from the expected polarization pattern in pristine h-BN samples. We also characterized the nonlinear optical susceptibility of h-BN with defects with a value of one order of magnitude larger than for pristine h-BN, which highlights the role of defects in the efficiency of SHG. Therefore defect engineering could be used as a potential tool for nonlinear optical signal enhancement.