Effects of the nanowire length on large second-order nonlinear optical responses: a theoretical investigation of the thinnest doped beryllium nanowires with IR and UV working wavebands

Abstract

The thinnest beryllium nanowires with high strength and uniformity are theoretically constructed of connected Be₆ octahedron units. Based on this, Ca- and Mg-doped beryllium nanowires are successfully constructed and researched. They are unusual all-metal charge transfer salts Ca²⁺(Be₆)_n⁴⁻Mg²⁺ (n = 1–7), and they surprisingly display considerable second-order nonlinear optical (NLO) responses (β₀^e = 1.05 × 10⁴–1.12 × 10⁵ au). This is because the effect of doping Ca and Mg atoms brings great increase in β₀^e. In addition, more notably, the effect of the nanowire length on β₀^e revealed that the β₀^e value gradually and rapidly increases with the increase in the number of Be₆ octahedron units (n). Thus, these doped beryllium nanowires are a new class of NLO nanowires. Fortunately, these NLO nanowires possess working wavebands in the infrared (IR, >2800 nm) and ultraviolet (UV, <200 nm) regions. Then, these doped beryllium NLO nanowires could also be used as new hot IR and UV NLO materials. Considering the dispersion effect, the frequency-dependent value of the electro-optical Pockels effect (EOPE) β^e(−ω; ω, 0) at ω = 0.005 au is slightly larger than the corresponding value of β₀^e. Significantly, the effect of the nanowire length on β^e(−ω; ω, 0) is also displayed. Obviously, a new design strategy of enhancing NLO responses by increasing n was obtained. Noticeably, the nanowires display Janus electronic properties of both stronger electron-donating and electron-withdrawing behaviors. This work predicts that novel metal nanowires may be applied in new hot IR and UV NLO materials as well as molecular electronic devices.