Tunable nonlinear refractive index of two-dimensional MoS2, WS2, and MoSe2 nanosheet dispersions [Invited]

Liquid-phase-exfoliation technology was utilized to prepare layered MoS2, WS2, and MoSe2 nanosheets in cyclohexylpyrrolidone. The nonlinear optical response of these nanosheets in dispersions was investigated by observing spatial self-phase modulation（SSPM） using a 488 nm continuous wave laser beam. The diffraction ring patterns of SSPM were found to be distorted along the vertical direction right after the laser traversing the nanosheet dispersions. The nonlinear refractive index of the three transition metal dichalcogenides dispersions n2 was measured to be 10-7cm2W-1, and the third-order nonlinear susceptibility χ（3）10-9 esu. The relative change of effective nonlinear refractive index Δn2e∕n（2e） of the MoS2, WS2, and MoSe2 dispersions can be modulated 0.012–0.240, 0.029–0.154, and 0.091–0.304, respectively, by changing the incident intensities. Our experimental results imply novel potential application of two-dimensional transition metal dichalcogenides in nonlinear phase modulation devices.

Nonlinear optical performance of few-layer molybdenum diselenide as a slow-saturable absorber

Two-dimensional transition metal dichalcogenides are considered promising materials for next-generation photonics and nano-optical devices. Although many previous reports have shown saturable absorption of molybdenum diselenide(Mo Se2), these nonlinear optical(NLO) properties of Mo Se2 were measured in separate works and under different conditions with their hot-carrier relaxations. Here, we conducted a series of coherent studies on the NLO properties of few-layer Mo Se2 via open-aperture Z-scan and degenerate pump-probe techniques. These measurements were taken to test the materials' capabilities as a slow-saturable absorber. A slowabsorber model was employed to analyze the NLO measurements, and the results show that the NLO modulation depth was modeled to be 7.4% and 15.1% for the linear absorption coefficients of 5.22 cm-1 and 6.51 cm-1,respectively. The corresponding saturated intensities were modeled to be 39.37 MW∕cm2 and 234.75 MW∕cm2,respectively. The excitation carrier recovery time of few-layer Mo Se2 was measured by degenerate pump-probe techniques to be ～220 ps. These nonlinear optical performances make it a promising slow-saturable absorber for passive mode locking in femtosecond lasers.