Femtosecond Third-Order Nonlinear Electronic Responses of 2D Metallic NbSe2

This manuscript reports on the third-order nonlinear optical responses of two-dimensional metallic NbSe2 suspended in acetonitrile (ACN). The standard Z-scan technique was employed with 190 fs optical pulses at 790 nm, a repetition rate of 750 Hz, and an intensity ranging from 30 to 300 GW/cm2. A self-focusing nonlinear refractive index (NLR), n2=+(1.8±0.1)×10−15 cm2/W, and a nonlinear absorption (NLA) coefficient, α2=+(3.5±0.2)×10−2 cm/GW, were measured, with the NLA arising from a two-photon process. Aiming to further understand the material’s electronic nonlinearities, we also employed the Optical Kerr Gate (OKG) to evaluate the material’s time response and measure the NLR coefficient in an optical intensity range different from the one used in the Z-scan. For optical pulses of 170 fs at 800 nm and a repetition rate of 76 MHz, the modulus of the NLR coefficient was measured to be n2=4.2±0.5×10−14 cm2/W for intensities up to 650 MW/cm2, with the material’s time response limited by the pulse duration. The ultrafast time response and electronic optical nonlinearities are explained based on the material’s 2D structure.