Saturable Absorption Properties of ReS2 Films and Mode-Locking Application Based on Double-Covered ReS2 Micro Fiber

ReS
₂
films with different thickness are fabricated by a vacuum filtration from liquid exfoliated ReS
₂
suspension. The saturable absorption properties of these films are investigated by an open-aperture Z-scan technique with a femtosecond laser. The saturable absorption coefficients αNL and the imaginary part of the third-order optical susceptibility Imχ
⁽³⁾
are from -61.5 to -171.8 cm/GW and from -2.24 x 10
^-11
to -6.27 x 10
^-11
esu with the change of thickness. The figure of merit increases with the increasing of the thickness of ReS
₂
films, which suggests the possible layer-coupling influence to the nonlinear optical properties. A double-covered ReS
₂
microfiber as a saturable absorber is then fabricated. We obtain stable mode-locking pulses with highdamage threshold (410 mW), which are centered at 1563.3 nm with the duration of 3.8 ps and the fundamental repetition of 1.78 MHz. Our results provide complementary information for the reliable ReS
₂
film fabrication with good nonlinear optical property for the mode-locking fiber lasers.