Ab initio Study of Nonlinear Optical Susceptibilities in Silicon Nanowires

TL;DR

This study uses density functional theory to show that silicon nanowires have significantly enhanced nonlinear optical susceptibilities, promising strong second harmonic generation and potential for optoelectronic applications.

Contribution

It provides the first ab initio calculations of second and third order susceptibilities in silicon nanowires, highlighting surface and strain effects on nonlinear optical properties.

Findings

$hi^{(2)}$ up to 200 pm/V indicating strong SHG

$hi^{(3)}$ comparable to bulk silicon, with enhancements in certain orientations

Surface termination and strain can further enhance nonlinear responses

Abstract

Using Time Independent Density Functional Theory (TIDFT) it is shown that the 2nd order optical susceptibilities of narrow (1nm-2nm) Silicon Nanowires (SiNW) are enhanced due to surface termination. The value of $\chi^{(2)}$ is enhanced up to 200 pm/V which is promising a strong Second Harmonic Generation (SHG) in SiNWs. For [100], [110] and [111] SiNWs, yxx component of $\chi^{(2)}$ tensor is 81, 225 and 81 pm/V, respectively. These are in close agreement with $\chi^{(2)}$ values reported for strained silicon waveguides in experiments. The 3rd order susceptibility, $\chi^{(3)}$, is within the range of (0.1-12)x$10^{-18} \frac{m^{2}}{V^{2}}$ which is close to the experimental values of bulk silicon (0.1-0.2)x$10^{-18} \frac{m^{2}}{V^{2}}$ for [110] and [100] SiNWs and it is 100 times better for [111] SiNW. This study suggests possibilities of enhancing SHG in SiNWs through symmetry breaking via strain and surface termination/reconstruction as well as suitability of this DFT-based method in predicting nonlinear optical susceptibilities of nano structures.