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.

The thinnest beryllium nanowires with high strength and uniformity are theoretically constructed of connected Be octahedron units. Based on this, Ca- and Mg-doped beryllium nanowires are successfully constructed and researched. They are unusual all-metal charge transfer salts Ca(Be)Mg (n = 1-7), and they surprisingly display considerable second-order nonlinear optical (NLO) responses (β = 1.05 × 10-1.12 × 10 au). This is because the effect of doping Ca and Mg atoms brings great increase in β. In addition, more notably, the effect of the nanowire length on β revealed that the β value gradually and rapidly increases with the increase in the number of Be 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(-ω; ω, 0) at ω = 0.005 au is slightly larger than the corresponding value of β. Significantly, the effect of the nanowire length on β(-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.