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Linear and nonlinear optical properties of silicon suboxide Si O x films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24 at.%⁠. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80 eV in the nanosecond regime and at 1.50 eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index (n 2∼− 10− 8 cm 2∕ W) and nonlinear absorption coefficient (β∼ 10− 6 cm∕ W)⁠. Under femtosecond …