The recent progress on black phosphorus makes it a promising candidate material for broadband nanophotonic devices, especially operating in the mid-infrared spectral region. Here, the excited carrier dynamics and nonlinear optical response of unoxidized black phosphorus nanosheets and their wavelength dependence were systematically studied from 800 nm to 2.1 μm. The wavelength-dependent relaxation times of black phosphorus nanosheets are determined to be 360 fs to 1.36 ps with photon energies from 1.55 to 0.61 eV. In a comparative study with graphene, we found that black phosphorus has a faster carrier relaxation in near- and mid-infrared region. With regard to nonlinear optical absorption, the response of black phosphorus significantly increases from near- to mid-infrared, and black phosphorus is also confirmed to be better as saturable absorber to MoS2 in infrared region.

Wide-spectral saturable absorption (SA) has been experimentally demonstrated in two-dimensional (2D) nanomaterials with outstanding performance, such as low saturation intensity, deep modulation depth, and fast recovery time of excited carriers. Hence, 2D nanomaterials can be utilized as saturable absorbers for mode-locking or Q-switching to generate laser pulses with short duration and high repetition rate. Here, the SA properties of graphene, layered transition metal dichalcogenides, Group-V elements, and other 2D nanomaterials are reviewed by summarizing their slow-or fastsaturable absorption behavior using the modified Frantz-Nodvik model or the steady-state solution of Hercher's rate equations. The dependence of SA in 2D nanomaterials on excitation wavelength, linear absorption coefficient, and pulse duration is also explained. Finally, the applications of these 2D nanomaterials in a range of pulsed lasers are summarized. Figure 2. a-c) Schemes showing the principle of open-aperture (a), closed-aperture Z-scans (b), and I-scan (c).

Carrier interactions in 2D nanostructures are of central importance not only in condensed‐matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe2) through ultrafast time‐resolved pump–probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe2. These results imply that few‐layer PtSe2 has a semiconductor‐like carrier relaxation instead of a metal‐like one. The relaxation follows a triple‐exponential decay process and exhibits thickness‐dependent relaxation times. This occurs along with a band‐filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many‐body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

Liquid-phase-exfoliation technology was utilized to prepare layered MoS 2 , WS 2 , and MoSe 2 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 n 2 was measured to be ∼10 −7 cm 2 W −1 , and the third-order nonlinear susceptibility χ 3 ∼ 10 −9 esu. The relative change of effective nonlinear refractive index Δn 2e ∕n 2e of the MoS 2 , WS 2 , and MoSe 2 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.