Electric field-induced second-harmonic generation in silicon nanocrystals embedded in a SiO_x matrix

SiO_x films of varying stoichiometry were deposited by plasma-enhanced chemical vapor deposition (PECVD). Applying an annealing process at 1100°C, silicon nanocrystals formed within these films. Subsequently, free-space electric field-induced second-harmonic (EFISH) measurements with a fundamental wavelength of 1030 nm were carried out, demonstrating a voltage-controllable second-order nonlinearity approaching χ⁽²⁾ values of classic nonlinear crystals, such as KDP (<i>χ</i><i>z</i><i>y</i><i>x</i>(2)=0.84<i>p</i><i>m</i>/<i>V</i>). Moreover, the EFISH data were evaluated to extract the voltage-independent third-order nonlinearities of the film materials. We observed an increase of more than an order of magnitude of the third-order nonlinearities of the films due to the annealing process. Furthermore, the third-order nonlinearities of as-deposited and annealed films can be strongly enhanced by material composition reaching a maximum of <i>χ</i>⁽³⁾ = 2.9 × 10^-20<i>m</i>²<i>V</i>^-2 for the films with the highest refractive index. To determine the contribution of the nanocrystals to the EFISH effect, we compare the EFISH response of nanocrystal-containing films with that of as-deposited SiO_x films without nanocrystals but of the same refractive index. We find that the nanocrystals do not significantly influence the third-order nonlinearities but an increased refractive index does.