Fig. 1. TEM images showing (a) a few-layer MoSe2 flake and (b) a monolayer MoSe2 flake. The scale bar is 50 nm in (a) and 5 nm in (b). (c) Raman spectrum of the few-layer MoSe2 flakes. (d) AFM image displaying the thickness of a large number of MoSe2 flakes in a ∼5  μm×5  μm area. (e) Statistical thickness distribution of the MoSe2 flakes in as-prepared dispersions.

Fig. 2. Z-scan results of few-layer MoSe2. The linear absorption coefficients are (a) 5.22  cm−1 and (b) 6.51  cm−1, respectively, which are shown in the insets. The measurements were carried out under irradiation of increasing laser intensity. (c) Schematic of an open-aperture Z-scan. The laser pulses are at a center wavelength of 800 nm, with duration of ∼100  fs and a repetition rate of 100 kHz from a Ti: sapphire mode-locked laser (Coherent, RegA 9000).

Fig. 3. Experimental (scatters) and fitting (solid lines) degenerate pump-probe traces of few-layer MoSe2 based on an 800 nm laser with pulse duration of ∼100  fs and repetition rate of 100 kHz. The inset in (c) shows the relaxation processes of the excited carriers. The inset in (d) shows degenerate pump-probe setup. An intense beam is employed to pump the materials, while another beam with relatively low intensity, which is delayed by a motorized linear translation stage, is used for probing the excited carriers. These two beams are modulated by an optical chopper at 733 Hz and 422 Hz, respectively. A half-wave plate and a polarizer are utilized to eliminate the coherent spikes.

Fig. 4. NLO performance of few-layer-MoSe2 analyzed by a slow-saturable absorber model. (a) Experimental (scatters) and fitting (solid lines) transmission as a function of intensity. (b) Corresponding differential absorption converted from (a).