Optics Seminar

Nonlinear optical response in layered boron nitride (BN) is strongly governed by polytypism and vertical heterostructure design. In this seminar, I will discuss two manifestations of this effect in MOVPE-grown van der Waals BN layers. First, I will show how the spectral profile of second harmonic generation (SHG) in hybrid hBN/rBN stacks can be reshaped through the interplay of stacking-dependent optical interference and the dispersive second-order nonlinear susceptibility of rhombohedral BN, leading in particular to ultraviolet-enhanced SHG in selected heterostructure geometries. Second, I will turn to defect-related ultraviolet luminescence and demonstrate how nonlinear up-conversion enables excitation of the C300 emission band under sub-bandgap visible pumping. A comparison between centrosymmetric hBN and non-centrosymmetric rBN shows that while two-photon absorption dominates in hBN, SHG provides an additional and more efficient excitation pathway in rBN, extending access to UV defect luminescence toward longer wavelengths. Together, these results identify polytypism as a practical handle on ultraviolet nonlinear optics in epitaxial BN.