Optical nonlinearity induced UV luminescence in sp2-bonded boron nitride films

Polytypism, arising from the different arrangements of successive two-dimensional (2D) atomic planes, strongly influences various physical properties of sp2-bonded layered boron nitrides (BNs), including optical nonlinearity and high-frequency light generation through upconversion of longer wavelengths, by altering the crystal symmetry. Rhombohedral BN (rBN), one such polytype, stacks its 2D atomic sheets in an ABC sequence and, unlike AA′-stacked hexagonal BN (hBN), lacks inversion symmetry. This breaking of centrosymmetry enables second-harmonic generation (SHG), which extends into the UV-C spectral region due to its wide bandgap. Here, we demonstrate that this UV-C SHG can be generated in metal-organic vapor-phase epitaxy-grown rBN films and effectively used to stimulate various defect-related photoluminescence (PL). We focus on the PL of the C300 color center (CBCN defect) at 4.1 eV, notable for its bright emission and single-photon purity. By qualitatively comparing the spectral features of this luminescence in rBN with those recorded for centrosymmetric reference hBN specimens that cannot demonstrate SHG but can exhibit two-photon absorption (TPA), we highlight the distinct roles of SHG in rBN and TPA in hBN in generating the 4.1 eV PL, and further emphasize the superior efficiency of SHG as a pathway for inducing the defect emission, using far longer-wavelength photons.