Origin of the significantly enhanced optical transitions in layered boron nitride

It is normally expected that an excellent optical material should have p→s-like transitions at the absorption edge. This is because the strength of p→s-like transitions usually is much stronger than those of p→p transitions, especially in those ionic semiconductors where the electronic states are more localized and behave as atomic characters. Here, we demonstrate an exception that the luminescence intensity of hexagonal boron nitride (h-BN) could be at least two orders of magnitude greater than that of AlN, despite the dominated atomic p→p transitions at the absorption edge of h-BN. Using group theory analysis and first-principles calculations, we show that the strong optical transitions in h-BN originate from the unusually strong p→p-like transitions and its "two-dimensional" nature. As learned from h-BN, we demonstrate that one can dramatically increase the absorption or luminescence intensity at the fundamental absorption edge of an optical material by confining its thickness into a few layers, which is much more effective than the commonly used superlattice technology.