Nonlinear Optical Responses of Thin Nanowire Assemblies of Drum-Shaped Boron Clusters

The stability of drum-shaped boron clusters and their feasible modification for metal doping render these nanomaterials potential candidates for nonlinear optics. In the present study, systematic theoretical calculations are performed to study the possible formation of finite-size nanowire assemblies by stacking B₁₄ or B₁₄M (M = Fe, Co) building blocks. The size evolution of structure, electronic, static, and dynamic nonlinear optical (NLO) properties of (B₁₄)_n, (B₁₄Fe)_n, and (B₁₄Co)_n with n = 1–6 are investigated. Although the drum-shaped structure of the building blocks is retained in most cases, however, in larger sizes of assemblies, the small expansion of building blocks in the middle and the compression at the terminals are observed. Our results highlight that the energy gap of boron nanowire assemblies can be finely tuned by altering their length. This is also inspiring for the modulation of the first hyperpolarizability by varying the number of stacked units. Among all of the examined systems, the highest hyperpolarizability (β_tot = 1.35 × 10⁵ a.u.) is observed for (B₁₄Fe)₆ owing to the reduced energy gap and increased charge transfer. To study the dynamic NLO response, the frequency-dependent properties in terms of electro-optical Pockels effects (EOPE), second harmonic generation (SHG), hyper-Rayleigh scattering (HRS), dc-Kerr coefficients, electric field-induced second harmonic generation (ESHG), and nonlinear refractive index (n₂) are evaluated. In most cases, the SHG process has a stronger NLO response than EOPE and HRS at the incident wavelength of Nd:YAG laser. In the case of third-order properties, high-frequency-induced ESHG up to 3.4 × 10⁷ a.u. is computed for the designed nanowires. The present research inspires experimental exploration for the synthesis of boron-based nanowires and highlights the potential application of these materials for second harmonic generators.