Boron arsenide nanotubes as multi-resonant nonlinear optical nanostructures: radius-dependent linear response and third-harmonic generation

This study presents the theoretical investigation into the linear and third-order nonlinear optical properties of Boron Arsenide Nanotubes (BAsNTs). Employing a fifth-nearest-neighbor tight-binding Hamiltonian fitted to density-functional theory and a density-matrix formalism, the dielectric response, quadratic electro-optic coefficients, two-photon absorption and third-harmonic generation are systematically calculated for different zigzag nanotubes as a function of tube radius. The results show that BAsNTs, characterized by a moderate and chirality-independent band gap, exhibit pronounced optical responses from the visible to the near-infrared region. In the infrared, the absorption peaks display a clear red shift and enhanced intensity with increasing radius, while the visible-range spectra feature robust, radius-independent resonances at approximately 2.7 eV and 3.1 eV, serving as stable optical features of these nanostructures. The static polarizability increases nearly linearly with tube radius and the quadratic electro-optic response reveals strong family-dependent behavior near the band edge. The nonlinear spectra further exhibit multiphoton resonances below the single-photon gap, with third-harmonic generation intensities that grow and red-shift as the tube radius increases, displaying distinct family dependence. A comparative analysis with Carbon Nanotubes (CNTs) shows that while CNTs may exhibit higher peak intensities, BAsNTs offer a greater number of optical resonances within specific energy ranges. Altogether, these findings establish BAsNTs as promising candidates for infrared and near-infrared photonics, frequency conversion, and nonlinear optoelectronic applications.