Nonlinear Optical Susceptibilities of [100], [110] and [111] Silicon Nanowires: A DFT Study

Using time-independent density functional theory (TIDFT), we demonstrate enhancement of the 2nd order optical susceptibilities of narrow (1 nm – 2 nm) unstrained Silicon Nanowires (SiNW) due to surface termination. It is shown that
$\mathrm{x}^{(2)}$
is enhanced up to 200 pm/V which promises a strong second harmonic generation (SHG) in SiNWs. For [100, 110] and [111] SiNWs, the
$yxx$
component of
$\mathrm{x}^{(2)}$
tensor is 81, 225 and 81 pm/V, respectively. These are in close agreement with values reported for strained silicon waveguides in experiments. For [110] and [100] nanowires, the 3rd order nonlinear optical susceptibility,
$\mathrm{x}^{(3)}$
, is within the range of
$(0.1-12)\times 10^{-18}$
m
²
/V
²
which is close to the experimental values for bulk silicon. For [111] nanowires this is 100 times better than bulk silicon. This study shows methods of enhancing SHG in SiNWs through symmetry breaking via strain and/or surface termination and reconstruction. It also showcases the suitability of this fast DFT-based method in predicting the nonlinear optical susceptibilities of nano structures.