Strong optical nonlinearity of the nonstoichiometric silicon carbide

Abstract

Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric Si_xC_1−x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH₄]/[CH₄ + SiH₄] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm⁻¹ contributed by transverse and longitudinal optical modes of Si–C bonds are simultaneously enhanced in stoichiometric and C-rich Si_xC_1−x samples, whereas the Si-associated Raman peak at 520 cm⁻¹ decreases with increasing C/Si composition ratio. The C-rich Si_xC_1−x film shows additional Raman scattering peaks at 1330 and 1580 cm⁻¹, which are individually contributed by the diamond-like C–C bonds and the G peak of the graphite-like C–C bonds, respectively. The nonlinear refractive index increases to 1 ± 0.1 × 10⁻¹¹ cm² W⁻¹ upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the Si_xC_1−x film and enhances its nonlinear refractive index effectively. In comparison with the sp³-orbital C–C bond, the sp²-orbital C–C bond in Si_xC_1−x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the Si_xC_1−x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si–Si bonds which favor two-photon absorption, the sp²-orbital C–C bonds with delocalized π-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp²-orbital C–C bonds in C-rich Si_xC_1−x films.