Second Harmonic Generation and Bulk Photovoltaic Effect in Boron Nitride Nanotubes: A Systematic Ab Initio Investigation.

Boron nitride nanotubes (BNNTs) are promising for nonlinear optics, yet their nonlinear optical properties (NLOPs), particularly for small diameters (<20 Å), remain insufficiently understood. Using density functional theory, we systematically investigate one-dimensional BNNTs and their two-dimensional counterpart, hexagonal boron nitride (h-BN), focusing on their structures, electronic properties, and second-order responses. We find that BNNTs exhibit a significantly enhanced second-harmonic generation (SHG) and bulk photovoltaic effect (BPVE) compared to h-BN, with strong dependence on diameter and chirality. Zigzag and chiral BNNTs show pronounced enhancement, whereas armchair BNNTs display a negligible response due to centrosymmetry. The enhancement is attributed to curvature-induced hybridization and quantum confinement. Through analysis of valence band contributions and density of states, we elucidate the underlying electronic mechanisms and propose a synergistic model for the regulation of NLOPs by chirality and diameter. Our results establish a clear structure-property relationship, providing a theoretical framework for designing low-dimensional optoelectronic and nonlinear photonic devices.