Dispersion of nonlinear refractive index in layered WS₂ and WSe₂ semiconductor films induced by two-photon absorption

Both the nonlinear absorption and nonlinear refraction properties of WS₂ and WSe₂ semiconductor films have been characterized by using Z-scan technique with femtosecond pulses at the wavelength of 1040 nm. It is found that these films have two-photon absorption response with the nonlinear absorption coefficient of ∼10³  cm GW^-1, and a dispersion of nonlinear refractive index in the WS₂ films that translated from positive in the monolayer to negative in bulk materials.