Optical Nonlinearity Mapping and Upconversion Excitation of Defect Photoluminescence in Epitaxial-Grown Boron Nitride Layers

Boron nitride (BN) van der Waals crystals have been shown to exhibit intriguing optical properties, including ultraviolet (UV) lasing [1] and second harmonic generation (SHG) [2]. By tailoring the BN crystal structure through epitaxial growth, both intrinsic band-edge transitions and defect-related photoluminescence (PL) can be harnessed [3]. Here, we investigate how the structural polytypism in epitaxially grown BN layers influences second-order nonlinear optical responses such as SHG and sum-frequency generation (SFG) while also enabling new upconversion excitation pathways for short-wavelength PL. The sp2-bonded BN layers were grown on sapphire substrates using continuous-flow metal-organic vapor phase epitaxy at 1050°C (sample A) and 1300°C (sample B), shown in Fig. 1a. Raman spectroscopy confirmed the characteristic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{E}_{2_{g}}$</tex> phonon mode near 1367 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>, validating successful BN growth, Fig. 1b. During the growth, various mid-gap defect states arise, producing broad PL emissions across the bandgap [3]. Differences in crystal stacking order (e.g., Bernal and rhombohedral) result in non-centrosymmetric domains that exhibit strong <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\chi^{(2)}$</tex> optical nonlinearity. Near-infrared-excited SHG and SFG maps, shown in Fig. 1c, illustrate a diverse “landscape” of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\chi^{(2)}$</tex> response-exhibiting sample A, which correlated with the observation of ultraviolet anti-Stokes PL recorded under 400–550 nm wavelength (ca. 3.1-2.3 eV) excitation. We find that reabsorption of the SHG at higher energies may be responsible for the subsequent excitation of defect-related PL features, tentatively attributed to C2-dimer centers (Fig. 1d). These results emphasize the role of polytypism in engineering new excitation pathways for UV and violet luminescence in ultra-thin BN layers. By refining epitaxial growth protocols to optimize upconverted PL intensity, this approach may foster BN-based optoelectronic devices operating in the ultraviolet range.