## Abstract

The prominent third-order nonlinear optical properties of ${\mathrm{WTe}}_{2}$ films are studied through the Z-scan technique using a femtosecond pulsed laser at 1030 nm. Open-aperture (OA) and closed-aperture (CA) Z-scan measurements are performed at different intensities to investigate the nonlinear absorption and refraction properties of ${\mathrm{WTe}}_{2}$ films. OA Z-scan results show that ${\mathrm{WTe}}_{2}$ films always hold a saturable absorption characteristic without transition to reverse saturable absorption. Further, a large nonlinear absorption coefficient $\beta $ is determined to be $-3.37\times {10}^{3}\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{cm}/\mathrm{GW}$ by fitting the OA Z-scan curve at the peak intensity of $15.603\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{GW}/{\mathrm{cm}}^{2}$. In addition, through the slow saturation absorption model, the ground state absorption cross section, excited state absorption cross section, and absorber’s density were found to be $1.4938\times {10}^{-16}\text{\hspace{0.17em}}{\mathrm{cm}}^{2}$, $1.2536\times {10}^{-16}\text{\hspace{0.17em}}{\mathrm{cm}}^{2}$, and $6.2396\times {10}^{20}\text{\hspace{0.17em}\hspace{0.17em}}{\mathrm{cm}}^{-3}$, respectively. CA Z-scan results exhibit a classic peak–valley shape of the CA Z-scan signal, which reveals a self-defocusing optical effect of ${\mathrm{WTe}}_{2}$ films under the measured environment. Furthermore, a considerable nonlinear refractive index value ${n}_{2}$ can be obtained at $-1.629\times {10}^{-2}\text{\hspace{0.17em}\hspace{0.17em}}{\mathrm{cm}}^{2}/\mathrm{GW}$. Ultimately, the values of the real and imaginary parts of the third-order nonlinear susceptibility of ${\mathrm{WTe}}_{2}$ are roughly estimated to be $-5.93\times {10}^{-9}$ and $-1.01\times {10}^{-8}\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{esu}$ with the help of first-principles calculations. ${\mathrm{WTe}}_{2}$ possesses sizeable third-order nonlinear coefficients compared with other materials, which suggests that it has great potential to improve performance in nonlinear optical devices.

© 2019 Chinese Laser Press

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